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After much back and forth, I decided to deviate from ARM design and split LDR into 4 instructions (r + imm12, r + imm8, r + r << imm12, constantpool). The advantage of this is 1) it follows the latest ARM technical manual, and 2) makes it easier to reduce the width of the instruction later. The down side is this creates more inconsistency between the two sub-targets. We should split ARM LDR instruction in a similar fashion later. I've added a README entry for this. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@74420 91177308-0d34-0410-b5e6-96231b3b80d8
540 lines
16 KiB
Plaintext
540 lines
16 KiB
Plaintext
//===---------------------------------------------------------------------===//
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// Random ideas for the ARM backend.
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//===---------------------------------------------------------------------===//
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Reimplement 'select' in terms of 'SEL'.
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* We would really like to support UXTAB16, but we need to prove that the
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add doesn't need to overflow between the two 16-bit chunks.
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* Implement pre/post increment support. (e.g. PR935)
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* Coalesce stack slots!
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* Implement smarter constant generation for binops with large immediates.
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* Consider materializing FP constants like 0.0f and 1.0f using integer
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immediate instructions then copy to FPU. Slower than load into FPU?
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//===---------------------------------------------------------------------===//
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Crazy idea: Consider code that uses lots of 8-bit or 16-bit values. By the
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time regalloc happens, these values are now in a 32-bit register, usually with
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the top-bits known to be sign or zero extended. If spilled, we should be able
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to spill these to a 8-bit or 16-bit stack slot, zero or sign extending as part
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of the reload.
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Doing this reduces the size of the stack frame (important for thumb etc), and
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also increases the likelihood that we will be able to reload multiple values
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from the stack with a single load.
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//===---------------------------------------------------------------------===//
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The constant island pass is in good shape. Some cleanups might be desirable,
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but there is unlikely to be much improvement in the generated code.
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1. There may be some advantage to trying to be smarter about the initial
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placement, rather than putting everything at the end.
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2. There might be some compile-time efficiency to be had by representing
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consecutive islands as a single block rather than multiple blocks.
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3. Use a priority queue to sort constant pool users in inverse order of
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position so we always process the one closed to the end of functions
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first. This may simply CreateNewWater.
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//===---------------------------------------------------------------------===//
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Eliminate copysign custom expansion. We are still generating crappy code with
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default expansion + if-conversion.
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//===---------------------------------------------------------------------===//
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Eliminate one instruction from:
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define i32 @_Z6slow4bii(i32 %x, i32 %y) {
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%tmp = icmp sgt i32 %x, %y
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%retval = select i1 %tmp, i32 %x, i32 %y
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ret i32 %retval
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}
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__Z6slow4bii:
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cmp r0, r1
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movgt r1, r0
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mov r0, r1
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bx lr
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=>
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__Z6slow4bii:
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cmp r0, r1
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movle r0, r1
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bx lr
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//===---------------------------------------------------------------------===//
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Implement long long "X-3" with instructions that fold the immediate in. These
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were disabled due to badness with the ARM carry flag on subtracts.
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//===---------------------------------------------------------------------===//
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We currently compile abs:
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int foo(int p) { return p < 0 ? -p : p; }
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into:
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_foo:
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rsb r1, r0, #0
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cmn r0, #1
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movgt r1, r0
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mov r0, r1
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bx lr
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This is very, uh, literal. This could be a 3 operation sequence:
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t = (p sra 31);
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res = (p xor t)-t
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Which would be better. This occurs in png decode.
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//===---------------------------------------------------------------------===//
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More load / store optimizations:
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1) Better representation for block transfer? This is from Olden/power:
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fldd d0, [r4]
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fstd d0, [r4, #+32]
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fldd d0, [r4, #+8]
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fstd d0, [r4, #+40]
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fldd d0, [r4, #+16]
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fstd d0, [r4, #+48]
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fldd d0, [r4, #+24]
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fstd d0, [r4, #+56]
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If we can spare the registers, it would be better to use fldm and fstm here.
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Need major register allocator enhancement though.
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2) Can we recognize the relative position of constantpool entries? i.e. Treat
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ldr r0, LCPI17_3
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ldr r1, LCPI17_4
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ldr r2, LCPI17_5
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as
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ldr r0, LCPI17
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ldr r1, LCPI17+4
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ldr r2, LCPI17+8
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Then the ldr's can be combined into a single ldm. See Olden/power.
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Note for ARM v4 gcc uses ldmia to load a pair of 32-bit values to represent a
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double 64-bit FP constant:
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adr r0, L6
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ldmia r0, {r0-r1}
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.align 2
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L6:
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.long -858993459
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.long 1074318540
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3) struct copies appear to be done field by field
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instead of by words, at least sometimes:
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struct foo { int x; short s; char c1; char c2; };
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void cpy(struct foo*a, struct foo*b) { *a = *b; }
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llvm code (-O2)
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ldrb r3, [r1, #+6]
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ldr r2, [r1]
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ldrb r12, [r1, #+7]
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ldrh r1, [r1, #+4]
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str r2, [r0]
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strh r1, [r0, #+4]
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strb r3, [r0, #+6]
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strb r12, [r0, #+7]
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gcc code (-O2)
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ldmia r1, {r1-r2}
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stmia r0, {r1-r2}
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In this benchmark poor handling of aggregate copies has shown up as
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having a large effect on size, and possibly speed as well (we don't have
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a good way to measure on ARM).
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//===---------------------------------------------------------------------===//
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* Consider this silly example:
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double bar(double x) {
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double r = foo(3.1);
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return x+r;
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}
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_bar:
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stmfd sp!, {r4, r5, r7, lr}
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add r7, sp, #8
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mov r4, r0
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mov r5, r1
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fldd d0, LCPI1_0
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fmrrd r0, r1, d0
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bl _foo
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fmdrr d0, r4, r5
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fmsr s2, r0
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fsitod d1, s2
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faddd d0, d1, d0
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fmrrd r0, r1, d0
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ldmfd sp!, {r4, r5, r7, pc}
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Ignore the prologue and epilogue stuff for a second. Note
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mov r4, r0
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mov r5, r1
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the copys to callee-save registers and the fact they are only being used by the
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fmdrr instruction. It would have been better had the fmdrr been scheduled
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before the call and place the result in a callee-save DPR register. The two
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mov ops would not have been necessary.
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//===---------------------------------------------------------------------===//
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Calling convention related stuff:
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* gcc's parameter passing implementation is terrible and we suffer as a result:
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e.g.
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struct s {
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double d1;
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int s1;
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};
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void foo(struct s S) {
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printf("%g, %d\n", S.d1, S.s1);
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}
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'S' is passed via registers r0, r1, r2. But gcc stores them to the stack, and
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then reload them to r1, r2, and r3 before issuing the call (r0 contains the
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address of the format string):
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stmfd sp!, {r7, lr}
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add r7, sp, #0
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sub sp, sp, #12
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stmia sp, {r0, r1, r2}
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ldmia sp, {r1-r2}
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ldr r0, L5
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ldr r3, [sp, #8]
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L2:
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add r0, pc, r0
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bl L_printf$stub
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Instead of a stmia, ldmia, and a ldr, wouldn't it be better to do three moves?
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* Return an aggregate type is even worse:
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e.g.
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struct s foo(void) {
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struct s S = {1.1, 2};
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return S;
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}
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mov ip, r0
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ldr r0, L5
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sub sp, sp, #12
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L2:
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add r0, pc, r0
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@ lr needed for prologue
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ldmia r0, {r0, r1, r2}
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stmia sp, {r0, r1, r2}
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stmia ip, {r0, r1, r2}
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mov r0, ip
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add sp, sp, #12
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bx lr
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r0 (and later ip) is the hidden parameter from caller to store the value in. The
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first ldmia loads the constants into r0, r1, r2. The last stmia stores r0, r1,
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r2 into the address passed in. However, there is one additional stmia that
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stores r0, r1, and r2 to some stack location. The store is dead.
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The llvm-gcc generated code looks like this:
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csretcc void %foo(%struct.s* %agg.result) {
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entry:
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%S = alloca %struct.s, align 4 ; <%struct.s*> [#uses=1]
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%memtmp = alloca %struct.s ; <%struct.s*> [#uses=1]
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cast %struct.s* %S to sbyte* ; <sbyte*>:0 [#uses=2]
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call void %llvm.memcpy.i32( sbyte* %0, sbyte* cast ({ double, int }* %C.0.904 to sbyte*), uint 12, uint 4 )
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cast %struct.s* %agg.result to sbyte* ; <sbyte*>:1 [#uses=2]
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call void %llvm.memcpy.i32( sbyte* %1, sbyte* %0, uint 12, uint 0 )
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cast %struct.s* %memtmp to sbyte* ; <sbyte*>:2 [#uses=1]
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call void %llvm.memcpy.i32( sbyte* %2, sbyte* %1, uint 12, uint 0 )
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ret void
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}
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llc ends up issuing two memcpy's (the first memcpy becomes 3 loads from
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constantpool). Perhaps we should 1) fix llvm-gcc so the memcpy is translated
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into a number of load and stores, or 2) custom lower memcpy (of small size) to
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be ldmia / stmia. I think option 2 is better but the current register
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allocator cannot allocate a chunk of registers at a time.
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A feasible temporary solution is to use specific physical registers at the
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lowering time for small (<= 4 words?) transfer size.
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* ARM CSRet calling convention requires the hidden argument to be returned by
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the callee.
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//===---------------------------------------------------------------------===//
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We can definitely do a better job on BB placements to eliminate some branches.
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It's very common to see llvm generated assembly code that looks like this:
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LBB3:
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...
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LBB4:
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...
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beq LBB3
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b LBB2
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If BB4 is the only predecessor of BB3, then we can emit BB3 after BB4. We can
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then eliminate beq and and turn the unconditional branch to LBB2 to a bne.
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See McCat/18-imp/ComputeBoundingBoxes for an example.
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//===---------------------------------------------------------------------===//
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Pre-/post- indexed load / stores:
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1) We should not make the pre/post- indexed load/store transform if the base ptr
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is guaranteed to be live beyond the load/store. This can happen if the base
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ptr is live out of the block we are performing the optimization. e.g.
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mov r1, r2
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ldr r3, [r1], #4
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...
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vs.
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ldr r3, [r2]
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add r1, r2, #4
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...
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In most cases, this is just a wasted optimization. However, sometimes it can
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negatively impact the performance because two-address code is more restrictive
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when it comes to scheduling.
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Unfortunately, liveout information is currently unavailable during DAG combine
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time.
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2) Consider spliting a indexed load / store into a pair of add/sub + load/store
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to solve #1 (in TwoAddressInstructionPass.cpp).
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3) Enhance LSR to generate more opportunities for indexed ops.
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4) Once we added support for multiple result patterns, write indexed loads
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patterns instead of C++ instruction selection code.
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5) Use FLDM / FSTM to emulate indexed FP load / store.
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//===---------------------------------------------------------------------===//
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Implement support for some more tricky ways to materialize immediates. For
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example, to get 0xffff8000, we can use:
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mov r9, #&3f8000
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sub r9, r9, #&400000
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//===---------------------------------------------------------------------===//
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We sometimes generate multiple add / sub instructions to update sp in prologue
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and epilogue if the inc / dec value is too large to fit in a single immediate
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operand. In some cases, perhaps it might be better to load the value from a
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constantpool instead.
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//===---------------------------------------------------------------------===//
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GCC generates significantly better code for this function.
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int foo(int StackPtr, unsigned char *Line, unsigned char *Stack, int LineLen) {
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int i = 0;
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if (StackPtr != 0) {
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while (StackPtr != 0 && i < (((LineLen) < (32768))? (LineLen) : (32768)))
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Line[i++] = Stack[--StackPtr];
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if (LineLen > 32768)
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{
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while (StackPtr != 0 && i < LineLen)
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{
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i++;
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--StackPtr;
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}
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}
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}
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return StackPtr;
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}
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//===---------------------------------------------------------------------===//
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This should compile to the mlas instruction:
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int mlas(int x, int y, int z) { return ((x * y + z) < 0) ? 7 : 13; }
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//===---------------------------------------------------------------------===//
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At some point, we should triage these to see if they still apply to us:
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19598
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18560
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27016
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11831
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11826
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11824
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11823
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11820
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10982
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10242
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9831
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9760
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9759
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9703
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9702
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9663
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http://www.inf.u-szeged.hu/gcc-arm/
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http://citeseer.ist.psu.edu/debus04linktime.html
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//===---------------------------------------------------------------------===//
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gcc generates smaller code for this function at -O2 or -Os:
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void foo(signed char* p) {
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if (*p == 3)
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bar();
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else if (*p == 4)
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baz();
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else if (*p == 5)
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quux();
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}
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llvm decides it's a good idea to turn the repeated if...else into a
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binary tree, as if it were a switch; the resulting code requires -1
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compare-and-branches when *p<=2 or *p==5, the same number if *p==4
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or *p>6, and +1 if *p==3. So it should be a speed win
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(on balance). However, the revised code is larger, with 4 conditional
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branches instead of 3.
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More seriously, there is a byte->word extend before
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each comparison, where there should be only one, and the condition codes
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are not remembered when the same two values are compared twice.
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//===---------------------------------------------------------------------===//
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More register scavenging work:
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1. Use the register scavenger to track frame index materialized into registers
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(those that do not fit in addressing modes) to allow reuse in the same BB.
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2. Finish scavenging for Thumb.
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//===---------------------------------------------------------------------===//
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More LSR enhancements possible:
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1. Teach LSR about pre- and post- indexed ops to allow iv increment be merged
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in a load / store.
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2. Allow iv reuse even when a type conversion is required. For example, i8
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and i32 load / store addressing modes are identical.
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//===---------------------------------------------------------------------===//
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This:
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int foo(int a, int b, int c, int d) {
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long long acc = (long long)a * (long long)b;
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acc += (long long)c * (long long)d;
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return (int)(acc >> 32);
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}
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Should compile to use SMLAL (Signed Multiply Accumulate Long) which multiplies
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two signed 32-bit values to produce a 64-bit value, and accumulates this with
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a 64-bit value.
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We currently get this with both v4 and v6:
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_foo:
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smull r1, r0, r1, r0
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smull r3, r2, r3, r2
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adds r3, r3, r1
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adc r0, r2, r0
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bx lr
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//===---------------------------------------------------------------------===//
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This:
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#include <algorithm>
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std::pair<unsigned, bool> full_add(unsigned a, unsigned b)
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{ return std::make_pair(a + b, a + b < a); }
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bool no_overflow(unsigned a, unsigned b)
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{ return !full_add(a, b).second; }
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Should compile to:
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_Z8full_addjj:
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adds r2, r1, r2
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movcc r1, #0
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movcs r1, #1
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str r2, [r0, #0]
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strb r1, [r0, #4]
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mov pc, lr
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_Z11no_overflowjj:
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cmn r0, r1
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movcs r0, #0
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movcc r0, #1
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mov pc, lr
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not:
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__Z8full_addjj:
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add r3, r2, r1
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str r3, [r0]
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mov r2, #1
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mov r12, #0
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cmp r3, r1
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movlo r12, r2
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str r12, [r0, #+4]
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bx lr
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__Z11no_overflowjj:
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add r3, r1, r0
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mov r2, #1
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mov r1, #0
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|
cmp r3, r0
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|
movhs r1, r2
|
|
mov r0, r1
|
|
bx lr
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Some of the NEON intrinsics may be appropriate for more general use, either
|
|
as target-independent intrinsics or perhaps elsewhere in the ARM backend.
|
|
Some of them may also be lowered to target-independent SDNodes, and perhaps
|
|
some new SDNodes could be added.
|
|
|
|
For example, maximum, minimum, and absolute value operations are well-defined
|
|
and standard operations, both for vector and scalar types.
|
|
|
|
The current NEON-specific intrinsics for count leading zeros and count one
|
|
bits could perhaps be replaced by the target-independent ctlz and ctpop
|
|
intrinsics. It may also make sense to add a target-independent "ctls"
|
|
intrinsic for "count leading sign bits". Likewise, the backend could use
|
|
the target-independent SDNodes for these operations.
|
|
|
|
ARMv6 has scalar saturating and halving adds and subtracts. The same
|
|
intrinsics could possibly be used for both NEON's vector implementations of
|
|
those operations and the ARMv6 scalar versions.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
ARM::MOVCCr is commutable (by flipping the condition). But we need to implement
|
|
ARMInstrInfo::commuteInstruction() to support it.
|
|
|
|
//===---------------------------------------------------------------------===//
|
|
|
|
Split out LDR (literal) from normal ARM LDR instruction. Also consider spliting
|
|
LDR into imm12 and so_reg forms. This allows us to clean up some code. e.g.
|
|
ARMLoadStoreOptimizer does not need to look at LDR (literal) and LDR (so_reg)
|
|
while ARMConstantIslandPass only need to worry about LDR (literal).
|