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https://github.com/c64scene-ar/llvm-6502.git
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daf6c541b2
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47704 91177308-0d34-0410-b5e6-96231b3b80d8
1728 lines
44 KiB
Plaintext
1728 lines
44 KiB
Plaintext
//===---------------------------------------------------------------------===//
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// Random ideas for the X86 backend.
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//===---------------------------------------------------------------------===//
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Missing features:
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- Support for SSE4: http://www.intel.com/software/penryn
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http://softwarecommunity.intel.com/isn/Downloads/Intel%20SSE4%20Programming%20Reference.pdf
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- support for 3DNow!
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- weird abis?
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//===---------------------------------------------------------------------===//
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CodeGen/X86/lea-3.ll:test3 should be a single LEA, not a shift/move. The X86
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backend knows how to three-addressify this shift, but it appears the register
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allocator isn't even asking it to do so in this case. We should investigate
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why this isn't happening, it could have significant impact on other important
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cases for X86 as well.
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//===---------------------------------------------------------------------===//
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This should be one DIV/IDIV instruction, not a libcall:
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unsigned test(unsigned long long X, unsigned Y) {
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return X/Y;
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}
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This can be done trivially with a custom legalizer. What about overflow
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though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
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//===---------------------------------------------------------------------===//
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Improvements to the multiply -> shift/add algorithm:
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http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
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//===---------------------------------------------------------------------===//
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Improve code like this (occurs fairly frequently, e.g. in LLVM):
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long long foo(int x) { return 1LL << x; }
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http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
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http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
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http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html
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Another useful one would be ~0ULL >> X and ~0ULL << X.
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One better solution for 1LL << x is:
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xorl %eax, %eax
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xorl %edx, %edx
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testb $32, %cl
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sete %al
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setne %dl
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sall %cl, %eax
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sall %cl, %edx
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But that requires good 8-bit subreg support.
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Also, this might be better. It's an extra shift, but it's one instruction
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shorter, and doesn't stress 8-bit subreg support.
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(From http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01148.html,
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but without the unnecessary and.)
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movl %ecx, %eax
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shrl $5, %eax
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movl %eax, %edx
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xorl $1, %edx
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sall %cl, %eax
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sall %cl. %edx
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64-bit shifts (in general) expand to really bad code. Instead of using
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cmovs, we should expand to a conditional branch like GCC produces.
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//===---------------------------------------------------------------------===//
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Compile this:
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_Bool f(_Bool a) { return a!=1; }
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into:
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movzbl %dil, %eax
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xorl $1, %eax
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ret
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(Although note that this isn't a legal way to express the code that llvm-gcc
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currently generates for that function.)
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//===---------------------------------------------------------------------===//
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Some isel ideas:
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1. Dynamic programming based approach when compile time if not an
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issue.
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2. Code duplication (addressing mode) during isel.
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3. Other ideas from "Register-Sensitive Selection, Duplication, and
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Sequencing of Instructions".
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4. Scheduling for reduced register pressure. E.g. "Minimum Register
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Instruction Sequence Problem: Revisiting Optimal Code Generation for DAGs"
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and other related papers.
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http://citeseer.ist.psu.edu/govindarajan01minimum.html
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//===---------------------------------------------------------------------===//
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Should we promote i16 to i32 to avoid partial register update stalls?
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//===---------------------------------------------------------------------===//
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Leave any_extend as pseudo instruction and hint to register
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allocator. Delay codegen until post register allocation.
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Note. any_extend is now turned into an INSERT_SUBREG. We still need to teach
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the coalescer how to deal with it though.
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//===---------------------------------------------------------------------===//
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It appears icc use push for parameter passing. Need to investigate.
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//===---------------------------------------------------------------------===//
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Only use inc/neg/not instructions on processors where they are faster than
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add/sub/xor. They are slower on the P4 due to only updating some processor
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flags.
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//===---------------------------------------------------------------------===//
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The instruction selector sometimes misses folding a load into a compare. The
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pattern is written as (cmp reg, (load p)). Because the compare isn't
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commutative, it is not matched with the load on both sides. The dag combiner
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should be made smart enough to cannonicalize the load into the RHS of a compare
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when it can invert the result of the compare for free.
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//===---------------------------------------------------------------------===//
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How about intrinsics? An example is:
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*res = _mm_mulhi_epu16(*A, _mm_mul_epu32(*B, *C));
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compiles to
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pmuludq (%eax), %xmm0
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movl 8(%esp), %eax
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movdqa (%eax), %xmm1
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pmulhuw %xmm0, %xmm1
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The transformation probably requires a X86 specific pass or a DAG combiner
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target specific hook.
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//===---------------------------------------------------------------------===//
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In many cases, LLVM generates code like this:
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_test:
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movl 8(%esp), %eax
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cmpl %eax, 4(%esp)
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setl %al
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movzbl %al, %eax
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ret
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on some processors (which ones?), it is more efficient to do this:
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_test:
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movl 8(%esp), %ebx
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xor %eax, %eax
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cmpl %ebx, 4(%esp)
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setl %al
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ret
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Doing this correctly is tricky though, as the xor clobbers the flags.
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//===---------------------------------------------------------------------===//
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We should generate bts/btr/etc instructions on targets where they are cheap or
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when codesize is important. e.g., for:
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void setbit(int *target, int bit) {
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*target |= (1 << bit);
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}
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void clearbit(int *target, int bit) {
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*target &= ~(1 << bit);
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}
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//===---------------------------------------------------------------------===//
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Instead of the following for memset char*, 1, 10:
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movl $16843009, 4(%edx)
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movl $16843009, (%edx)
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movw $257, 8(%edx)
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It might be better to generate
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movl $16843009, %eax
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movl %eax, 4(%edx)
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movl %eax, (%edx)
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movw al, 8(%edx)
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when we can spare a register. It reduces code size.
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//===---------------------------------------------------------------------===//
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Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently
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get this:
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define i32 @test1(i32 %X) {
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%Y = sdiv i32 %X, 8
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ret i32 %Y
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}
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_test1:
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movl 4(%esp), %eax
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movl %eax, %ecx
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sarl $31, %ecx
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shrl $29, %ecx
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addl %ecx, %eax
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sarl $3, %eax
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ret
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GCC knows several different ways to codegen it, one of which is this:
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_test1:
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movl 4(%esp), %eax
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cmpl $-1, %eax
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leal 7(%eax), %ecx
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cmovle %ecx, %eax
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sarl $3, %eax
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ret
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which is probably slower, but it's interesting at least :)
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//===---------------------------------------------------------------------===//
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We are currently lowering large (1MB+) memmove/memcpy to rep/stosl and rep/movsl
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We should leave these as libcalls for everything over a much lower threshold,
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since libc is hand tuned for medium and large mem ops (avoiding RFO for large
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stores, TLB preheating, etc)
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//===---------------------------------------------------------------------===//
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Optimize this into something reasonable:
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x * copysign(1.0, y) * copysign(1.0, z)
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//===---------------------------------------------------------------------===//
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Optimize copysign(x, *y) to use an integer load from y.
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//===---------------------------------------------------------------------===//
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%X = weak global int 0
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void %foo(int %N) {
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%N = cast int %N to uint
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%tmp.24 = setgt int %N, 0
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br bool %tmp.24, label %no_exit, label %return
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no_exit:
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%indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ]
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%i.0.0 = cast uint %indvar to int
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volatile store int %i.0.0, int* %X
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%indvar.next = add uint %indvar, 1
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%exitcond = seteq uint %indvar.next, %N
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br bool %exitcond, label %return, label %no_exit
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return:
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ret void
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}
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compiles into:
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.text
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.align 4
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.globl _foo
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_foo:
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movl 4(%esp), %eax
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cmpl $1, %eax
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jl LBB_foo_4 # return
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LBB_foo_1: # no_exit.preheader
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xorl %ecx, %ecx
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LBB_foo_2: # no_exit
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movl L_X$non_lazy_ptr, %edx
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movl %ecx, (%edx)
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incl %ecx
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cmpl %eax, %ecx
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jne LBB_foo_2 # no_exit
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LBB_foo_3: # return.loopexit
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LBB_foo_4: # return
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ret
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We should hoist "movl L_X$non_lazy_ptr, %edx" out of the loop after
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remateralization is implemented. This can be accomplished with 1) a target
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dependent LICM pass or 2) makeing SelectDAG represent the whole function.
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//===---------------------------------------------------------------------===//
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The following tests perform worse with LSR:
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lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor.
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//===---------------------------------------------------------------------===//
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We are generating far worse code than gcc:
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volatile short X, Y;
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void foo(int N) {
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int i;
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for (i = 0; i < N; i++) { X = i; Y = i*4; }
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}
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LBB1_1: # entry.bb_crit_edge
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xorl %ecx, %ecx
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xorw %dx, %dx
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LBB1_2: # bb
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movl L_X$non_lazy_ptr, %esi
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movw %cx, (%esi)
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movl L_Y$non_lazy_ptr, %esi
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movw %dx, (%esi)
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addw $4, %dx
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incl %ecx
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cmpl %eax, %ecx
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jne LBB1_2 # bb
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vs.
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xorl %edx, %edx
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movl L_X$non_lazy_ptr-"L00000000001$pb"(%ebx), %esi
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movl L_Y$non_lazy_ptr-"L00000000001$pb"(%ebx), %ecx
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L4:
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movw %dx, (%esi)
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leal 0(,%edx,4), %eax
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movw %ax, (%ecx)
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addl $1, %edx
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cmpl %edx, %edi
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jne L4
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This is due to the lack of post regalloc LICM.
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//===---------------------------------------------------------------------===//
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Teach the coalescer to coalesce vregs of different register classes. e.g. FR32 /
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FR64 to VR128.
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//===---------------------------------------------------------------------===//
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mov $reg, 48(%esp)
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...
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leal 48(%esp), %eax
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mov %eax, (%esp)
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call _foo
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Obviously it would have been better for the first mov (or any op) to store
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directly %esp[0] if there are no other uses.
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//===---------------------------------------------------------------------===//
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Adding to the list of cmp / test poor codegen issues:
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int test(__m128 *A, __m128 *B) {
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if (_mm_comige_ss(*A, *B))
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return 3;
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else
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return 4;
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}
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_test:
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movl 8(%esp), %eax
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movaps (%eax), %xmm0
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movl 4(%esp), %eax
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movaps (%eax), %xmm1
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comiss %xmm0, %xmm1
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setae %al
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movzbl %al, %ecx
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movl $3, %eax
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movl $4, %edx
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cmpl $0, %ecx
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cmove %edx, %eax
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ret
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Note the setae, movzbl, cmpl, cmove can be replaced with a single cmovae. There
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are a number of issues. 1) We are introducing a setcc between the result of the
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intrisic call and select. 2) The intrinsic is expected to produce a i32 value
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so a any extend (which becomes a zero extend) is added.
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We probably need some kind of target DAG combine hook to fix this.
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//===---------------------------------------------------------------------===//
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We generate significantly worse code for this than GCC:
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150
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http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701
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There is also one case we do worse on PPC.
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//===---------------------------------------------------------------------===//
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If shorter, we should use things like:
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movzwl %ax, %eax
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instead of:
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andl $65535, %EAX
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The former can also be used when the two-addressy nature of the 'and' would
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require a copy to be inserted (in X86InstrInfo::convertToThreeAddress).
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//===---------------------------------------------------------------------===//
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Another instruction selector deficiency:
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void %bar() {
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%tmp = load int (int)** %foo
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%tmp = tail call int %tmp( int 3 )
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ret void
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}
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_bar:
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subl $12, %esp
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movl L_foo$non_lazy_ptr, %eax
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movl (%eax), %eax
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call *%eax
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addl $12, %esp
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ret
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The current isel scheme will not allow the load to be folded in the call since
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the load's chain result is read by the callseq_start.
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//===---------------------------------------------------------------------===//
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For this:
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int test(int a)
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{
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return a * 3;
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}
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We currently emits
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imull $3, 4(%esp), %eax
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Perhaps this is what we really should generate is? Is imull three or four
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cycles? Note: ICC generates this:
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movl 4(%esp), %eax
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leal (%eax,%eax,2), %eax
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The current instruction priority is based on pattern complexity. The former is
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more "complex" because it folds a load so the latter will not be emitted.
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Perhaps we should use AddedComplexity to give LEA32r a higher priority? We
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should always try to match LEA first since the LEA matching code does some
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estimate to determine whether the match is profitable.
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However, if we care more about code size, then imull is better. It's two bytes
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shorter than movl + leal.
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//===---------------------------------------------------------------------===//
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__builtin_ffs codegen is messy.
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int ffs_(unsigned X) { return __builtin_ffs(X); }
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llvm produces:
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ffs_:
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movl 4(%esp), %ecx
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bsfl %ecx, %eax
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movl $32, %edx
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cmove %edx, %eax
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incl %eax
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xorl %edx, %edx
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testl %ecx, %ecx
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cmove %edx, %eax
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ret
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vs gcc:
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_ffs_:
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movl $-1, %edx
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bsfl 4(%esp), %eax
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cmove %edx, %eax
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addl $1, %eax
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ret
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Another example of __builtin_ffs (use predsimplify to eliminate a select):
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int foo (unsigned long j) {
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if (j)
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return __builtin_ffs (j) - 1;
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else
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return 0;
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}
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//===---------------------------------------------------------------------===//
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It appears gcc place string data with linkonce linkage in
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.section __TEXT,__const_coal,coalesced instead of
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.section __DATA,__const_coal,coalesced.
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Take a look at darwin.h, there are other Darwin assembler directives that we
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do not make use of.
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//===---------------------------------------------------------------------===//
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define i32 @foo(i32* %a, i32 %t) {
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entry:
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br label %cond_true
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cond_true: ; preds = %cond_true, %entry
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%x.0.0 = phi i32 [ 0, %entry ], [ %tmp9, %cond_true ] ; <i32> [#uses=3]
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%t_addr.0.0 = phi i32 [ %t, %entry ], [ %tmp7, %cond_true ] ; <i32> [#uses=1]
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%tmp2 = getelementptr i32* %a, i32 %x.0.0 ; <i32*> [#uses=1]
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%tmp3 = load i32* %tmp2 ; <i32> [#uses=1]
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%tmp5 = add i32 %t_addr.0.0, %x.0.0 ; <i32> [#uses=1]
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%tmp7 = add i32 %tmp5, %tmp3 ; <i32> [#uses=2]
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%tmp9 = add i32 %x.0.0, 1 ; <i32> [#uses=2]
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%tmp = icmp sgt i32 %tmp9, 39 ; <i1> [#uses=1]
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br i1 %tmp, label %bb12, label %cond_true
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bb12: ; preds = %cond_true
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ret i32 %tmp7
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}
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is pessimized by -loop-reduce and -indvars
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//===---------------------------------------------------------------------===//
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u32 to float conversion improvement:
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float uint32_2_float( unsigned u ) {
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float fl = (int) (u & 0xffff);
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float fh = (int) (u >> 16);
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fh *= 0x1.0p16f;
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return fh + fl;
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}
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00000000 subl $0x04,%esp
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00000003 movl 0x08(%esp,1),%eax
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00000007 movl %eax,%ecx
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00000009 shrl $0x10,%ecx
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0000000c cvtsi2ss %ecx,%xmm0
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00000010 andl $0x0000ffff,%eax
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00000015 cvtsi2ss %eax,%xmm1
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00000019 mulss 0x00000078,%xmm0
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00000021 addss %xmm1,%xmm0
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00000025 movss %xmm0,(%esp,1)
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0000002a flds (%esp,1)
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0000002d addl $0x04,%esp
|
|
00000030 ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
When using fastcc abi, align stack slot of argument of type double on 8 byte
|
|
boundary to improve performance.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Codegen:
|
|
|
|
int f(int a, int b) {
|
|
if (a == 4 || a == 6)
|
|
b++;
|
|
return b;
|
|
}
|
|
|
|
|
|
as:
|
|
|
|
or eax, 2
|
|
cmp eax, 6
|
|
jz label
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
GCC's ix86_expand_int_movcc function (in i386.c) has a ton of interesting
|
|
simplifications for integer "x cmp y ? a : b". For example, instead of:
|
|
|
|
int G;
|
|
void f(int X, int Y) {
|
|
G = X < 0 ? 14 : 13;
|
|
}
|
|
|
|
compiling to:
|
|
|
|
_f:
|
|
movl $14, %eax
|
|
movl $13, %ecx
|
|
movl 4(%esp), %edx
|
|
testl %edx, %edx
|
|
cmovl %eax, %ecx
|
|
movl %ecx, _G
|
|
ret
|
|
|
|
it could be:
|
|
_f:
|
|
movl 4(%esp), %eax
|
|
sarl $31, %eax
|
|
notl %eax
|
|
addl $14, %eax
|
|
movl %eax, _G
|
|
ret
|
|
|
|
etc.
|
|
|
|
Another is:
|
|
int usesbb(unsigned int a, unsigned int b) {
|
|
return (a < b ? -1 : 0);
|
|
}
|
|
to:
|
|
_usesbb:
|
|
movl 8(%esp), %eax
|
|
cmpl %eax, 4(%esp)
|
|
sbbl %eax, %eax
|
|
ret
|
|
|
|
instead of:
|
|
_usesbb:
|
|
xorl %eax, %eax
|
|
movl 8(%esp), %ecx
|
|
cmpl %ecx, 4(%esp)
|
|
movl $4294967295, %ecx
|
|
cmovb %ecx, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Currently we don't have elimination of redundant stack manipulations. Consider
|
|
the code:
|
|
|
|
int %main() {
|
|
entry:
|
|
call fastcc void %test1( )
|
|
call fastcc void %test2( sbyte* cast (void ()* %test1 to sbyte*) )
|
|
ret int 0
|
|
}
|
|
|
|
declare fastcc void %test1()
|
|
|
|
declare fastcc void %test2(sbyte*)
|
|
|
|
|
|
This currently compiles to:
|
|
|
|
subl $16, %esp
|
|
call _test5
|
|
addl $12, %esp
|
|
subl $16, %esp
|
|
movl $_test5, (%esp)
|
|
call _test6
|
|
addl $12, %esp
|
|
|
|
The add\sub pair is really unneeded here.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Consider the expansion of:
|
|
|
|
define i32 @test3(i32 %X) {
|
|
%tmp1 = urem i32 %X, 255
|
|
ret i32 %tmp1
|
|
}
|
|
|
|
Currently it compiles to:
|
|
|
|
...
|
|
movl $2155905153, %ecx
|
|
movl 8(%esp), %esi
|
|
movl %esi, %eax
|
|
mull %ecx
|
|
...
|
|
|
|
This could be "reassociated" into:
|
|
|
|
movl $2155905153, %eax
|
|
movl 8(%esp), %ecx
|
|
mull %ecx
|
|
|
|
to avoid the copy. In fact, the existing two-address stuff would do this
|
|
except that mul isn't a commutative 2-addr instruction. I guess this has
|
|
to be done at isel time based on the #uses to mul?
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Make sure the instruction which starts a loop does not cross a cacheline
|
|
boundary. This requires knowning the exact length of each machine instruction.
|
|
That is somewhat complicated, but doable. Example 256.bzip2:
|
|
|
|
In the new trace, the hot loop has an instruction which crosses a cacheline
|
|
boundary. In addition to potential cache misses, this can't help decoding as I
|
|
imagine there has to be some kind of complicated decoder reset and realignment
|
|
to grab the bytes from the next cacheline.
|
|
|
|
532 532 0x3cfc movb (1809(%esp, %esi), %bl <<<--- spans 2 64 byte lines
|
|
942 942 0x3d03 movl %dh, (1809(%esp, %esi)
|
|
937 937 0x3d0a incl %esi
|
|
3 3 0x3d0b cmpb %bl, %dl
|
|
27 27 0x3d0d jnz 0x000062db <main+11707>
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
In c99 mode, the preprocessor doesn't like assembly comments like #TRUNCATE.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This could be a single 16-bit load.
|
|
|
|
int f(char *p) {
|
|
if ((p[0] == 1) & (p[1] == 2)) return 1;
|
|
return 0;
|
|
}
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We should inline lrintf and probably other libc functions.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Start using the flags more. For example, compile:
|
|
|
|
int add_zf(int *x, int y, int a, int b) {
|
|
if ((*x += y) == 0)
|
|
return a;
|
|
else
|
|
return b;
|
|
}
|
|
|
|
to:
|
|
addl %esi, (%rdi)
|
|
movl %edx, %eax
|
|
cmovne %ecx, %eax
|
|
ret
|
|
instead of:
|
|
|
|
_add_zf:
|
|
addl (%rdi), %esi
|
|
movl %esi, (%rdi)
|
|
testl %esi, %esi
|
|
cmove %edx, %ecx
|
|
movl %ecx, %eax
|
|
ret
|
|
|
|
and:
|
|
|
|
int add_zf(int *x, int y, int a, int b) {
|
|
if ((*x + y) < 0)
|
|
return a;
|
|
else
|
|
return b;
|
|
}
|
|
|
|
to:
|
|
|
|
add_zf:
|
|
addl (%rdi), %esi
|
|
movl %edx, %eax
|
|
cmovns %ecx, %eax
|
|
ret
|
|
|
|
instead of:
|
|
|
|
_add_zf:
|
|
addl (%rdi), %esi
|
|
testl %esi, %esi
|
|
cmovs %edx, %ecx
|
|
movl %ecx, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
These two functions have identical effects:
|
|
|
|
unsigned int f(unsigned int i, unsigned int n) {++i; if (i == n) ++i; return i;}
|
|
unsigned int f2(unsigned int i, unsigned int n) {++i; i += i == n; return i;}
|
|
|
|
We currently compile them to:
|
|
|
|
_f:
|
|
movl 4(%esp), %eax
|
|
movl %eax, %ecx
|
|
incl %ecx
|
|
movl 8(%esp), %edx
|
|
cmpl %edx, %ecx
|
|
jne LBB1_2 #UnifiedReturnBlock
|
|
LBB1_1: #cond_true
|
|
addl $2, %eax
|
|
ret
|
|
LBB1_2: #UnifiedReturnBlock
|
|
movl %ecx, %eax
|
|
ret
|
|
_f2:
|
|
movl 4(%esp), %eax
|
|
movl %eax, %ecx
|
|
incl %ecx
|
|
cmpl 8(%esp), %ecx
|
|
sete %cl
|
|
movzbl %cl, %ecx
|
|
leal 1(%ecx,%eax), %eax
|
|
ret
|
|
|
|
both of which are inferior to GCC's:
|
|
|
|
_f:
|
|
movl 4(%esp), %edx
|
|
leal 1(%edx), %eax
|
|
addl $2, %edx
|
|
cmpl 8(%esp), %eax
|
|
cmove %edx, %eax
|
|
ret
|
|
_f2:
|
|
movl 4(%esp), %eax
|
|
addl $1, %eax
|
|
xorl %edx, %edx
|
|
cmpl 8(%esp), %eax
|
|
sete %dl
|
|
addl %edx, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This code:
|
|
|
|
void test(int X) {
|
|
if (X) abort();
|
|
}
|
|
|
|
is currently compiled to:
|
|
|
|
_test:
|
|
subl $12, %esp
|
|
cmpl $0, 16(%esp)
|
|
jne LBB1_1
|
|
addl $12, %esp
|
|
ret
|
|
LBB1_1:
|
|
call L_abort$stub
|
|
|
|
It would be better to produce:
|
|
|
|
_test:
|
|
subl $12, %esp
|
|
cmpl $0, 16(%esp)
|
|
jne L_abort$stub
|
|
addl $12, %esp
|
|
ret
|
|
|
|
This can be applied to any no-return function call that takes no arguments etc.
|
|
Alternatively, the stack save/restore logic could be shrink-wrapped, producing
|
|
something like this:
|
|
|
|
_test:
|
|
cmpl $0, 4(%esp)
|
|
jne LBB1_1
|
|
ret
|
|
LBB1_1:
|
|
subl $12, %esp
|
|
call L_abort$stub
|
|
|
|
Both are useful in different situations. Finally, it could be shrink-wrapped
|
|
and tail called, like this:
|
|
|
|
_test:
|
|
cmpl $0, 4(%esp)
|
|
jne LBB1_1
|
|
ret
|
|
LBB1_1:
|
|
pop %eax # realign stack.
|
|
call L_abort$stub
|
|
|
|
Though this probably isn't worth it.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We need to teach the codegen to convert two-address INC instructions to LEA
|
|
when the flags are dead (likewise dec). For example, on X86-64, compile:
|
|
|
|
int foo(int A, int B) {
|
|
return A+1;
|
|
}
|
|
|
|
to:
|
|
|
|
_foo:
|
|
leal 1(%edi), %eax
|
|
ret
|
|
|
|
instead of:
|
|
|
|
_foo:
|
|
incl %edi
|
|
movl %edi, %eax
|
|
ret
|
|
|
|
Another example is:
|
|
|
|
;; X's live range extends beyond the shift, so the register allocator
|
|
;; cannot coalesce it with Y. Because of this, a copy needs to be
|
|
;; emitted before the shift to save the register value before it is
|
|
;; clobbered. However, this copy is not needed if the register
|
|
;; allocator turns the shift into an LEA. This also occurs for ADD.
|
|
|
|
; Check that the shift gets turned into an LEA.
|
|
; RUN: llvm-as < %s | llc -march=x86 -x86-asm-syntax=intel | \
|
|
; RUN: not grep {mov E.X, E.X}
|
|
|
|
@G = external global i32 ; <i32*> [#uses=3]
|
|
|
|
define i32 @test1(i32 %X, i32 %Y) {
|
|
%Z = add i32 %X, %Y ; <i32> [#uses=1]
|
|
volatile store i32 %Y, i32* @G
|
|
volatile store i32 %Z, i32* @G
|
|
ret i32 %X
|
|
}
|
|
|
|
define i32 @test2(i32 %X) {
|
|
%Z = add i32 %X, 1 ; <i32> [#uses=1]
|
|
volatile store i32 %Z, i32* @G
|
|
ret i32 %X
|
|
}
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Sometimes it is better to codegen subtractions from a constant (e.g. 7-x) with
|
|
a neg instead of a sub instruction. Consider:
|
|
|
|
int test(char X) { return 7-X; }
|
|
|
|
we currently produce:
|
|
_test:
|
|
movl $7, %eax
|
|
movsbl 4(%esp), %ecx
|
|
subl %ecx, %eax
|
|
ret
|
|
|
|
We would use one fewer register if codegen'd as:
|
|
|
|
movsbl 4(%esp), %eax
|
|
neg %eax
|
|
add $7, %eax
|
|
ret
|
|
|
|
Note that this isn't beneficial if the load can be folded into the sub. In
|
|
this case, we want a sub:
|
|
|
|
int test(int X) { return 7-X; }
|
|
_test:
|
|
movl $7, %eax
|
|
subl 4(%esp), %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This is a "commutable two-address" register coallescing deficiency:
|
|
|
|
define <4 x float> @test1(<4 x float> %V) {
|
|
entry:
|
|
%tmp8 = shufflevector <4 x float> %V, <4 x float> undef,
|
|
<4 x i32> < i32 3, i32 2, i32 1, i32 0 >
|
|
%add = add <4 x float> %tmp8, %V
|
|
ret <4 x float> %add
|
|
}
|
|
|
|
this codegens to:
|
|
|
|
_test1:
|
|
pshufd $27, %xmm0, %xmm1
|
|
addps %xmm0, %xmm1
|
|
movaps %xmm1, %xmm0
|
|
ret
|
|
|
|
instead of:
|
|
|
|
_test1:
|
|
pshufd $27, %xmm0, %xmm1
|
|
addps %xmm1, %xmm0
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Leaf functions that require one 4-byte spill slot have a prolog like this:
|
|
|
|
_foo:
|
|
pushl %esi
|
|
subl $4, %esp
|
|
...
|
|
and an epilog like this:
|
|
addl $4, %esp
|
|
popl %esi
|
|
ret
|
|
|
|
It would be smaller, and potentially faster, to push eax on entry and to
|
|
pop into a dummy register instead of using addl/subl of esp. Just don't pop
|
|
into any return registers :)
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
The X86 backend should fold (branch (or (setcc, setcc))) into multiple
|
|
branches. We generate really poor code for:
|
|
|
|
double testf(double a) {
|
|
return a == 0.0 ? 0.0 : (a > 0.0 ? 1.0 : -1.0);
|
|
}
|
|
|
|
For example, the entry BB is:
|
|
|
|
_testf:
|
|
subl $20, %esp
|
|
pxor %xmm0, %xmm0
|
|
movsd 24(%esp), %xmm1
|
|
ucomisd %xmm0, %xmm1
|
|
setnp %al
|
|
sete %cl
|
|
testb %cl, %al
|
|
jne LBB1_5 # UnifiedReturnBlock
|
|
LBB1_1: # cond_true
|
|
|
|
|
|
it would be better to replace the last four instructions with:
|
|
|
|
jp LBB1_1
|
|
je LBB1_5
|
|
LBB1_1:
|
|
|
|
We also codegen the inner ?: into a diamond:
|
|
|
|
cvtss2sd LCPI1_0(%rip), %xmm2
|
|
cvtss2sd LCPI1_1(%rip), %xmm3
|
|
ucomisd %xmm1, %xmm0
|
|
ja LBB1_3 # cond_true
|
|
LBB1_2: # cond_true
|
|
movapd %xmm3, %xmm2
|
|
LBB1_3: # cond_true
|
|
movapd %xmm2, %xmm0
|
|
ret
|
|
|
|
We should sink the load into xmm3 into the LBB1_2 block. This should
|
|
be pretty easy, and will nuke all the copies.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This:
|
|
#include <algorithm>
|
|
inline std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
|
|
{ return std::make_pair(a + b, a + b < a); }
|
|
bool no_overflow(unsigned a, unsigned b)
|
|
{ return !full_add(a, b).second; }
|
|
|
|
Should compile to:
|
|
|
|
|
|
_Z11no_overflowjj:
|
|
addl %edi, %esi
|
|
setae %al
|
|
ret
|
|
|
|
FIXME: That code looks wrong; bool return is normally defined as zext.
|
|
|
|
on x86-64, not:
|
|
|
|
__Z11no_overflowjj:
|
|
addl %edi, %esi
|
|
cmpl %edi, %esi
|
|
setae %al
|
|
movzbl %al, %eax
|
|
ret
|
|
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Re-materialize MOV32r0 etc. with xor instead of changing them to moves if the
|
|
condition register is dead. xor reg reg is shorter than mov reg, #0.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We aren't matching RMW instructions aggressively
|
|
enough. Here's a reduced testcase (more in PR1160):
|
|
|
|
define void @test(i32* %huge_ptr, i32* %target_ptr) {
|
|
%A = load i32* %huge_ptr ; <i32> [#uses=1]
|
|
%B = load i32* %target_ptr ; <i32> [#uses=1]
|
|
%C = or i32 %A, %B ; <i32> [#uses=1]
|
|
store i32 %C, i32* %target_ptr
|
|
ret void
|
|
}
|
|
|
|
$ llvm-as < t.ll | llc -march=x86-64
|
|
|
|
_test:
|
|
movl (%rdi), %eax
|
|
orl (%rsi), %eax
|
|
movl %eax, (%rsi)
|
|
ret
|
|
|
|
That should be something like:
|
|
|
|
_test:
|
|
movl (%rdi), %eax
|
|
orl %eax, (%rsi)
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
The following code:
|
|
|
|
bb114.preheader: ; preds = %cond_next94
|
|
%tmp231232 = sext i16 %tmp62 to i32 ; <i32> [#uses=1]
|
|
%tmp233 = sub i32 32, %tmp231232 ; <i32> [#uses=1]
|
|
%tmp245246 = sext i16 %tmp65 to i32 ; <i32> [#uses=1]
|
|
%tmp252253 = sext i16 %tmp68 to i32 ; <i32> [#uses=1]
|
|
%tmp254 = sub i32 32, %tmp252253 ; <i32> [#uses=1]
|
|
%tmp553554 = bitcast i16* %tmp37 to i8* ; <i8*> [#uses=2]
|
|
%tmp583584 = sext i16 %tmp98 to i32 ; <i32> [#uses=1]
|
|
%tmp585 = sub i32 32, %tmp583584 ; <i32> [#uses=1]
|
|
%tmp614615 = sext i16 %tmp101 to i32 ; <i32> [#uses=1]
|
|
%tmp621622 = sext i16 %tmp104 to i32 ; <i32> [#uses=1]
|
|
%tmp623 = sub i32 32, %tmp621622 ; <i32> [#uses=1]
|
|
br label %bb114
|
|
|
|
produces:
|
|
|
|
LBB3_5: # bb114.preheader
|
|
movswl -68(%ebp), %eax
|
|
movl $32, %ecx
|
|
movl %ecx, -80(%ebp)
|
|
subl %eax, -80(%ebp)
|
|
movswl -52(%ebp), %eax
|
|
movl %ecx, -84(%ebp)
|
|
subl %eax, -84(%ebp)
|
|
movswl -70(%ebp), %eax
|
|
movl %ecx, -88(%ebp)
|
|
subl %eax, -88(%ebp)
|
|
movswl -50(%ebp), %eax
|
|
subl %eax, %ecx
|
|
movl %ecx, -76(%ebp)
|
|
movswl -42(%ebp), %eax
|
|
movl %eax, -92(%ebp)
|
|
movswl -66(%ebp), %eax
|
|
movl %eax, -96(%ebp)
|
|
movw $0, -98(%ebp)
|
|
|
|
This appears to be bad because the RA is not folding the store to the stack
|
|
slot into the movl. The above instructions could be:
|
|
movl $32, -80(%ebp)
|
|
...
|
|
movl $32, -84(%ebp)
|
|
...
|
|
This seems like a cross between remat and spill folding.
|
|
|
|
This has redundant subtractions of %eax from a stack slot. However, %ecx doesn't
|
|
change, so we could simply subtract %eax from %ecx first and then use %ecx (or
|
|
vice-versa).
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
For this code:
|
|
|
|
cond_next603: ; preds = %bb493, %cond_true336, %cond_next599
|
|
%v.21050.1 = phi i32 [ %v.21050.0, %cond_next599 ], [ %tmp344, %cond_true336 ], [ %v.2, %bb493 ] ; <i32> [#uses=1]
|
|
%maxz.21051.1 = phi i32 [ %maxz.21051.0, %cond_next599 ], [ 0, %cond_true336 ], [ %maxz.2, %bb493 ] ; <i32> [#uses=2]
|
|
%cnt.01055.1 = phi i32 [ %cnt.01055.0, %cond_next599 ], [ 0, %cond_true336 ], [ %cnt.0, %bb493 ] ; <i32> [#uses=2]
|
|
%byteptr.9 = phi i8* [ %byteptr.12, %cond_next599 ], [ %byteptr.0, %cond_true336 ], [ %byteptr.10, %bb493 ] ; <i8*> [#uses=9]
|
|
%bitptr.6 = phi i32 [ %tmp5571104.1, %cond_next599 ], [ %tmp4921049, %cond_true336 ], [ %bitptr.7, %bb493 ] ; <i32> [#uses=4]
|
|
%source.5 = phi i32 [ %tmp602, %cond_next599 ], [ %source.0, %cond_true336 ], [ %source.6, %bb493 ] ; <i32> [#uses=7]
|
|
%tmp606 = getelementptr %struct.const_tables* @tables, i32 0, i32 0, i32 %cnt.01055.1 ; <i8*> [#uses=1]
|
|
%tmp607 = load i8* %tmp606, align 1 ; <i8> [#uses=1]
|
|
|
|
We produce this:
|
|
|
|
LBB4_70: # cond_next603
|
|
movl -20(%ebp), %esi
|
|
movl L_tables$non_lazy_ptr-"L4$pb"(%esi), %esi
|
|
|
|
However, ICC caches this information before the loop and produces this:
|
|
|
|
movl 88(%esp), %eax #481.12
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This code:
|
|
|
|
%tmp659 = icmp slt i16 %tmp654, 0 ; <i1> [#uses=1]
|
|
br i1 %tmp659, label %cond_true662, label %cond_next715
|
|
|
|
produces this:
|
|
|
|
testw %cx, %cx
|
|
movswl %cx, %esi
|
|
jns LBB4_109 # cond_next715
|
|
|
|
Shark tells us that using %cx in the testw instruction is sub-optimal. It
|
|
suggests using the 32-bit register (which is what ICC uses).
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We compile this:
|
|
|
|
void compare (long long foo) {
|
|
if (foo < 4294967297LL)
|
|
abort();
|
|
}
|
|
|
|
to:
|
|
|
|
compare:
|
|
subl $4, %esp
|
|
cmpl $0, 8(%esp)
|
|
setne %al
|
|
movzbw %al, %ax
|
|
cmpl $1, 12(%esp)
|
|
setg %cl
|
|
movzbw %cl, %cx
|
|
cmove %ax, %cx
|
|
testb $1, %cl
|
|
jne .LBB1_2 # UnifiedReturnBlock
|
|
.LBB1_1: # ifthen
|
|
call abort
|
|
.LBB1_2: # UnifiedReturnBlock
|
|
addl $4, %esp
|
|
ret
|
|
|
|
(also really horrible code on ppc). This is due to the expand code for 64-bit
|
|
compares. GCC produces multiple branches, which is much nicer:
|
|
|
|
compare:
|
|
subl $12, %esp
|
|
movl 20(%esp), %edx
|
|
movl 16(%esp), %eax
|
|
decl %edx
|
|
jle .L7
|
|
.L5:
|
|
addl $12, %esp
|
|
ret
|
|
.p2align 4,,7
|
|
.L7:
|
|
jl .L4
|
|
cmpl $0, %eax
|
|
.p2align 4,,8
|
|
ja .L5
|
|
.L4:
|
|
.p2align 4,,9
|
|
call abort
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Tail call optimization improvements: Tail call optimization currently
|
|
pushes all arguments on the top of the stack (their normal place for
|
|
non-tail call optimized calls) that source from the callers arguments
|
|
or that source from a virtual register (also possibly sourcing from
|
|
callers arguments).
|
|
This is done to prevent overwriting of parameters (see example
|
|
below) that might be used later.
|
|
|
|
example:
|
|
|
|
int callee(int32, int64);
|
|
int caller(int32 arg1, int32 arg2) {
|
|
int64 local = arg2 * 2;
|
|
return callee(arg2, (int64)local);
|
|
}
|
|
|
|
[arg1] [!arg2 no longer valid since we moved local onto it]
|
|
[arg2] -> [(int64)
|
|
[RETADDR] local ]
|
|
|
|
Moving arg1 onto the stack slot of callee function would overwrite
|
|
arg2 of the caller.
|
|
|
|
Possible optimizations:
|
|
|
|
|
|
- Analyse the actual parameters of the callee to see which would
|
|
overwrite a caller parameter which is used by the callee and only
|
|
push them onto the top of the stack.
|
|
|
|
int callee (int32 arg1, int32 arg2);
|
|
int caller (int32 arg1, int32 arg2) {
|
|
return callee(arg1,arg2);
|
|
}
|
|
|
|
Here we don't need to write any variables to the top of the stack
|
|
since they don't overwrite each other.
|
|
|
|
int callee (int32 arg1, int32 arg2);
|
|
int caller (int32 arg1, int32 arg2) {
|
|
return callee(arg2,arg1);
|
|
}
|
|
|
|
Here we need to push the arguments because they overwrite each
|
|
other.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
main ()
|
|
{
|
|
int i = 0;
|
|
unsigned long int z = 0;
|
|
|
|
do {
|
|
z -= 0x00004000;
|
|
i++;
|
|
if (i > 0x00040000)
|
|
abort ();
|
|
} while (z > 0);
|
|
exit (0);
|
|
}
|
|
|
|
gcc compiles this to:
|
|
|
|
_main:
|
|
subl $28, %esp
|
|
xorl %eax, %eax
|
|
jmp L2
|
|
L3:
|
|
cmpl $262144, %eax
|
|
je L10
|
|
L2:
|
|
addl $1, %eax
|
|
cmpl $262145, %eax
|
|
jne L3
|
|
call L_abort$stub
|
|
L10:
|
|
movl $0, (%esp)
|
|
call L_exit$stub
|
|
|
|
llvm:
|
|
|
|
_main:
|
|
subl $12, %esp
|
|
movl $1, %eax
|
|
movl $16384, %ecx
|
|
LBB1_1: # bb
|
|
cmpl $262145, %eax
|
|
jge LBB1_4 # cond_true
|
|
LBB1_2: # cond_next
|
|
incl %eax
|
|
addl $4294950912, %ecx
|
|
cmpl $16384, %ecx
|
|
jne LBB1_1 # bb
|
|
LBB1_3: # bb11
|
|
xorl %eax, %eax
|
|
addl $12, %esp
|
|
ret
|
|
LBB1_4: # cond_true
|
|
call L_abort$stub
|
|
|
|
1. LSR should rewrite the first cmp with induction variable %ecx.
|
|
2. DAG combiner should fold
|
|
leal 1(%eax), %edx
|
|
cmpl $262145, %edx
|
|
=>
|
|
cmpl $262144, %eax
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
define i64 @test(double %X) {
|
|
%Y = fptosi double %X to i64
|
|
ret i64 %Y
|
|
}
|
|
|
|
compiles to:
|
|
|
|
_test:
|
|
subl $20, %esp
|
|
movsd 24(%esp), %xmm0
|
|
movsd %xmm0, 8(%esp)
|
|
fldl 8(%esp)
|
|
fisttpll (%esp)
|
|
movl 4(%esp), %edx
|
|
movl (%esp), %eax
|
|
addl $20, %esp
|
|
#FP_REG_KILL
|
|
ret
|
|
|
|
This should just fldl directly from the input stack slot.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This code:
|
|
int foo (int x) { return (x & 65535) | 255; }
|
|
|
|
Should compile into:
|
|
|
|
_foo:
|
|
movzwl 4(%esp), %eax
|
|
orl $255, %eax
|
|
ret
|
|
|
|
instead of:
|
|
_foo:
|
|
movl $255, %eax
|
|
orl 4(%esp), %eax
|
|
andl $65535, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We're codegen'ing multiply of long longs inefficiently:
|
|
|
|
unsigned long long LLM(unsigned long long arg1, unsigned long long arg2) {
|
|
return arg1 * arg2;
|
|
}
|
|
|
|
We compile to (fomit-frame-pointer):
|
|
|
|
_LLM:
|
|
pushl %esi
|
|
movl 8(%esp), %ecx
|
|
movl 16(%esp), %esi
|
|
movl %esi, %eax
|
|
mull %ecx
|
|
imull 12(%esp), %esi
|
|
addl %edx, %esi
|
|
imull 20(%esp), %ecx
|
|
movl %esi, %edx
|
|
addl %ecx, %edx
|
|
popl %esi
|
|
ret
|
|
|
|
This looks like a scheduling deficiency and lack of remat of the load from
|
|
the argument area. ICC apparently produces:
|
|
|
|
movl 8(%esp), %ecx
|
|
imull 12(%esp), %ecx
|
|
movl 16(%esp), %eax
|
|
imull 4(%esp), %eax
|
|
addl %eax, %ecx
|
|
movl 4(%esp), %eax
|
|
mull 12(%esp)
|
|
addl %ecx, %edx
|
|
ret
|
|
|
|
Note that it remat'd loads from 4(esp) and 12(esp). See this GCC PR:
|
|
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=17236
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We can fold a store into "zeroing a reg". Instead of:
|
|
|
|
xorl %eax, %eax
|
|
movl %eax, 124(%esp)
|
|
|
|
we should get:
|
|
|
|
movl $0, 124(%esp)
|
|
|
|
if the flags of the xor are dead.
|
|
|
|
Likewise, we isel "x<<1" into "add reg,reg". If reg is spilled, this should
|
|
be folded into: shl [mem], 1
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
This testcase misses a read/modify/write opportunity (from PR1425):
|
|
|
|
void vertical_decompose97iH1(int *b0, int *b1, int *b2, int width){
|
|
int i;
|
|
for(i=0; i<width; i++)
|
|
b1[i] += (1*(b0[i] + b2[i])+0)>>0;
|
|
}
|
|
|
|
We compile it down to:
|
|
|
|
LBB1_2: # bb
|
|
movl (%esi,%edi,4), %ebx
|
|
addl (%ecx,%edi,4), %ebx
|
|
addl (%edx,%edi,4), %ebx
|
|
movl %ebx, (%ecx,%edi,4)
|
|
incl %edi
|
|
cmpl %eax, %edi
|
|
jne LBB1_2 # bb
|
|
|
|
the inner loop should add to the memory location (%ecx,%edi,4), saving
|
|
a mov. Something like:
|
|
|
|
movl (%esi,%edi,4), %ebx
|
|
addl (%edx,%edi,4), %ebx
|
|
addl %ebx, (%ecx,%edi,4)
|
|
|
|
Here is another interesting example:
|
|
|
|
void vertical_compose97iH1(int *b0, int *b1, int *b2, int width){
|
|
int i;
|
|
for(i=0; i<width; i++)
|
|
b1[i] -= (1*(b0[i] + b2[i])+0)>>0;
|
|
}
|
|
|
|
We miss the r/m/w opportunity here by using 2 subs instead of an add+sub[mem]:
|
|
|
|
LBB9_2: # bb
|
|
movl (%ecx,%edi,4), %ebx
|
|
subl (%esi,%edi,4), %ebx
|
|
subl (%edx,%edi,4), %ebx
|
|
movl %ebx, (%ecx,%edi,4)
|
|
incl %edi
|
|
cmpl %eax, %edi
|
|
jne LBB9_2 # bb
|
|
|
|
Additionally, LSR should rewrite the exit condition of these loops to use
|
|
a stride-4 IV, would would allow all the scales in the loop to go away.
|
|
This would result in smaller code and more efficient microops.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
In SSE mode, we turn abs and neg into a load from the constant pool plus a xor
|
|
or and instruction, for example:
|
|
|
|
xorpd LCPI1_0, %xmm2
|
|
|
|
However, if xmm2 gets spilled, we end up with really ugly code like this:
|
|
|
|
movsd (%esp), %xmm0
|
|
xorpd LCPI1_0, %xmm0
|
|
movsd %xmm0, (%esp)
|
|
|
|
Since we 'know' that this is a 'neg', we can actually "fold" the spill into
|
|
the neg/abs instruction, turning it into an *integer* operation, like this:
|
|
|
|
xorl 2147483648, [mem+4] ## 2147483648 = (1 << 31)
|
|
|
|
you could also use xorb, but xorl is less likely to lead to a partial register
|
|
stall. Here is a contrived testcase:
|
|
|
|
double a, b, c;
|
|
void test(double *P) {
|
|
double X = *P;
|
|
a = X;
|
|
bar();
|
|
X = -X;
|
|
b = X;
|
|
bar();
|
|
c = X;
|
|
}
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
handling llvm.memory.barrier on pre SSE2 cpus
|
|
|
|
should generate:
|
|
lock ; mov %esp, %esp
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
The generated code on x86 for checking for signed overflow on a multiply the
|
|
obvious way is much longer than it needs to be.
|
|
|
|
int x(int a, int b) {
|
|
long long prod = (long long)a*b;
|
|
return prod > 0x7FFFFFFF || prod < (-0x7FFFFFFF-1);
|
|
}
|
|
|
|
See PR2053 for more details.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We should investigate using cdq/ctld (effect: edx = sar eax, 31)
|
|
more aggressively; it should cost the same as a move+shift on any modern
|
|
processor, but it's a lot shorter. Downside is that it puts more
|
|
pressure on register allocation because it has fixed operands.
|
|
|
|
Example:
|
|
int abs(int x) {return x < 0 ? -x : x;}
|
|
|
|
gcc compiles this to the following when using march/mtune=pentium2/3/4/m/etc.:
|
|
abs:
|
|
movl 4(%esp), %eax
|
|
cltd
|
|
xorl %edx, %eax
|
|
subl %edx, %eax
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Consider:
|
|
int test(unsigned long a, unsigned long b) { return -(a < b); }
|
|
|
|
We currently compile this to:
|
|
|
|
define i32 @test(i32 %a, i32 %b) nounwind {
|
|
%tmp3 = icmp ult i32 %a, %b ; <i1> [#uses=1]
|
|
%tmp34 = zext i1 %tmp3 to i32 ; <i32> [#uses=1]
|
|
%tmp5 = sub i32 0, %tmp34 ; <i32> [#uses=1]
|
|
ret i32 %tmp5
|
|
}
|
|
|
|
and
|
|
|
|
_test:
|
|
movl 8(%esp), %eax
|
|
cmpl %eax, 4(%esp)
|
|
setb %al
|
|
movzbl %al, %eax
|
|
negl %eax
|
|
ret
|
|
|
|
Several deficiencies here. First, we should instcombine zext+neg into sext:
|
|
|
|
define i32 @test2(i32 %a, i32 %b) nounwind {
|
|
%tmp3 = icmp ult i32 %a, %b ; <i1> [#uses=1]
|
|
%tmp34 = sext i1 %tmp3 to i32 ; <i32> [#uses=1]
|
|
ret i32 %tmp34
|
|
}
|
|
|
|
However, before we can do that, we have to fix the bad codegen that we get for
|
|
sext from bool:
|
|
|
|
_test2:
|
|
movl 8(%esp), %eax
|
|
cmpl %eax, 4(%esp)
|
|
setb %al
|
|
movzbl %al, %eax
|
|
shll $31, %eax
|
|
sarl $31, %eax
|
|
ret
|
|
|
|
This code should be at least as good as the code above. Once this is fixed, we
|
|
can optimize this specific case even more to:
|
|
|
|
movl 8(%esp), %eax
|
|
xorl %ecx, %ecx
|
|
cmpl %eax, 4(%esp)
|
|
sbbl %ecx, %ecx
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Take the following code (from
|
|
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16541):
|
|
|
|
extern unsigned char first_one[65536];
|
|
int FirstOnet(unsigned long long arg1)
|
|
{
|
|
if (arg1 >> 48)
|
|
return (first_one[arg1 >> 48]);
|
|
return 0;
|
|
}
|
|
|
|
|
|
The following code is currently generated:
|
|
FirstOnet:
|
|
movl 8(%esp), %eax
|
|
cmpl $65536, %eax
|
|
movl 4(%esp), %ecx
|
|
jb .LBB1_2 # UnifiedReturnBlock
|
|
.LBB1_1: # ifthen
|
|
shrl $16, %eax
|
|
movzbl first_one(%eax), %eax
|
|
ret
|
|
.LBB1_2: # UnifiedReturnBlock
|
|
xorl %eax, %eax
|
|
ret
|
|
|
|
There are a few possible improvements here:
|
|
1. We should be able to eliminate the dead load into %ecx
|
|
2. We could change the "movl 8(%esp), %eax" into
|
|
"movzwl 10(%esp), %eax"; this lets us change the cmpl
|
|
into a testl, which is shorter, and eliminate the shift.
|
|
|
|
We could also in theory eliminate the branch by using a conditional
|
|
for the address of the load, but that seems unlikely to be worthwhile
|
|
in general.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Take the following code:
|
|
|
|
#include <xmmintrin.h>
|
|
__m128i doload64(short x) {return _mm_set_epi16(x,x,x,x,x,x,x,x);}
|
|
|
|
LLVM currently generates the following on x86:
|
|
doload64:
|
|
movzwl 4(%esp), %eax
|
|
movd %eax, %xmm0
|
|
punpcklwd %xmm0, %xmm0
|
|
pshufd $0, %xmm0, %xmm0
|
|
ret
|
|
|
|
gcc's generated code:
|
|
doload64:
|
|
movd 4(%esp), %xmm0
|
|
punpcklwd %xmm0, %xmm0
|
|
pshufd $0, %xmm0, %xmm0
|
|
ret
|
|
|
|
LLVM should be able to generate the same thing as gcc.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Take the following code:
|
|
#include <xmmintrin.h>
|
|
__m128i doload64(short x) {return _mm_set_epi16(0,0,0,0,0,0,0,1);}
|
|
|
|
On x86, LLVM generates the following:
|
|
doload64:
|
|
subl $28, %esp
|
|
movl $0, 4(%esp)
|
|
movl $1, (%esp)
|
|
movq (%esp), %xmm0
|
|
addl $28, %esp
|
|
ret
|
|
|
|
LLVM should instead generate something more like the following:
|
|
doload64:
|
|
movl $1, %eax
|
|
movd %eax, %xmm0
|
|
ret
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
We compile this function:
|
|
|
|
define i32 @foo(i32 %a, i32 %b, i32 %c, i8 zeroext %d) nounwind {
|
|
entry:
|
|
%tmp2 = icmp eq i8 %d, 0 ; <i1> [#uses=1]
|
|
br i1 %tmp2, label %bb7, label %bb
|
|
|
|
bb: ; preds = %entry
|
|
%tmp6 = add i32 %b, %a ; <i32> [#uses=1]
|
|
ret i32 %tmp6
|
|
|
|
bb7: ; preds = %entry
|
|
%tmp10 = sub i32 %a, %c ; <i32> [#uses=1]
|
|
ret i32 %tmp10
|
|
}
|
|
|
|
to:
|
|
|
|
_foo:
|
|
cmpb $0, 16(%esp)
|
|
movl 12(%esp), %ecx
|
|
movl 8(%esp), %eax
|
|
movl 4(%esp), %edx
|
|
je LBB1_2 # bb7
|
|
LBB1_1: # bb
|
|
addl %edx, %eax
|
|
ret
|
|
LBB1_2: # bb7
|
|
movl %edx, %eax
|
|
subl %ecx, %eax
|
|
ret
|
|
|
|
The coallescer could coallesce "edx" with "eax" to avoid the movl in LBB1_2
|
|
if it commuted the addl in LBB1_1.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|