llvm-6502/lib/Target/X86/README.txt
Chris Lattner 48d3c108b6 more notes
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@34204 91177308-0d34-0410-b5e6-96231b3b80d8
2007-02-12 21:20:26 +00:00

923 lines
24 KiB
Plaintext

//===---------------------------------------------------------------------===//
// Random ideas for the X86 backend.
//===---------------------------------------------------------------------===//
Add a MUL2U and MUL2S nodes to represent a multiply that returns both the
Hi and Lo parts (combination of MUL and MULH[SU] into one node). Add this to
X86, & make the dag combiner produce it when needed. This will eliminate one
imul from the code generated for:
long long test(long long X, long long Y) { return X*Y; }
by using the EAX result from the mul. We should add a similar node for
DIVREM.
another case is:
long long test(int X, int Y) { return (long long)X*Y; }
... which should only be one imul instruction.
This can be done with a custom expander, but it would be nice to move this to
generic code.
//===---------------------------------------------------------------------===//
This should be one DIV/IDIV instruction, not a libcall:
unsigned test(unsigned long long X, unsigned Y) {
return X/Y;
}
This can be done trivially with a custom legalizer. What about overflow
though? http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14224
//===---------------------------------------------------------------------===//
Improvements to the multiply -> shift/add algorithm:
http://gcc.gnu.org/ml/gcc-patches/2004-08/msg01590.html
//===---------------------------------------------------------------------===//
Improve code like this (occurs fairly frequently, e.g. in LLVM):
long long foo(int x) { return 1LL << x; }
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01109.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01128.html
http://gcc.gnu.org/ml/gcc-patches/2004-09/msg01136.html
Another useful one would be ~0ULL >> X and ~0ULL << X.
One better solution for 1LL << x is:
xorl %eax, %eax
xorl %edx, %edx
testb $32, %cl
sete %al
setne %dl
sall %cl, %eax
sall %cl, %edx
But that requires good 8-bit subreg support.
64-bit shifts (in general) expand to really bad code. Instead of using
cmovs, we should expand to a conditional branch like GCC produces.
//===---------------------------------------------------------------------===//
Compile this:
_Bool f(_Bool a) { return a!=1; }
into:
movzbl %dil, %eax
xorl $1, %eax
ret
//===---------------------------------------------------------------------===//
Some isel ideas:
1. Dynamic programming based approach when compile time if not an
issue.
2. Code duplication (addressing mode) during isel.
3. Other ideas from "Register-Sensitive Selection, Duplication, and
Sequencing of Instructions".
4. Scheduling for reduced register pressure. E.g. "Minimum Register
Instruction Sequence Problem: Revisiting Optimal Code Generation for DAGs"
and other related papers.
http://citeseer.ist.psu.edu/govindarajan01minimum.html
//===---------------------------------------------------------------------===//
Should we promote i16 to i32 to avoid partial register update stalls?
//===---------------------------------------------------------------------===//
Leave any_extend as pseudo instruction and hint to register
allocator. Delay codegen until post register allocation.
//===---------------------------------------------------------------------===//
Count leading zeros and count trailing zeros:
int clz(int X) { return __builtin_clz(X); }
int ctz(int X) { return __builtin_ctz(X); }
$ gcc t.c -S -o - -O3 -fomit-frame-pointer -masm=intel
clz:
bsr %eax, DWORD PTR [%esp+4]
xor %eax, 31
ret
ctz:
bsf %eax, DWORD PTR [%esp+4]
ret
however, check that these are defined for 0 and 32. Our intrinsics are, GCC's
aren't.
Another example (use predsimplify to eliminate a select):
int foo (unsigned long j) {
if (j)
return __builtin_ffs (j) - 1;
else
return 0;
}
//===---------------------------------------------------------------------===//
Use push/pop instructions in prolog/epilog sequences instead of stores off
ESP (certain code size win, perf win on some [which?] processors).
Also, it appears icc use push for parameter passing. Need to investigate.
//===---------------------------------------------------------------------===//
Only use inc/neg/not instructions on processors where they are faster than
add/sub/xor. They are slower on the P4 due to only updating some processor
flags.
//===---------------------------------------------------------------------===//
The instruction selector sometimes misses folding a load into a compare. The
pattern is written as (cmp reg, (load p)). Because the compare isn't
commutative, it is not matched with the load on both sides. The dag combiner
should be made smart enough to cannonicalize the load into the RHS of a compare
when it can invert the result of the compare for free.
//===---------------------------------------------------------------------===//
How about intrinsics? An example is:
*res = _mm_mulhi_epu16(*A, _mm_mul_epu32(*B, *C));
compiles to
pmuludq (%eax), %xmm0
movl 8(%esp), %eax
movdqa (%eax), %xmm1
pmulhuw %xmm0, %xmm1
The transformation probably requires a X86 specific pass or a DAG combiner
target specific hook.
//===---------------------------------------------------------------------===//
In many cases, LLVM generates code like this:
_test:
movl 8(%esp), %eax
cmpl %eax, 4(%esp)
setl %al
movzbl %al, %eax
ret
on some processors (which ones?), it is more efficient to do this:
_test:
movl 8(%esp), %ebx
xor %eax, %eax
cmpl %ebx, 4(%esp)
setl %al
ret
Doing this correctly is tricky though, as the xor clobbers the flags.
//===---------------------------------------------------------------------===//
We should generate bts/btr/etc instructions on targets where they are cheap or
when codesize is important. e.g., for:
void setbit(int *target, int bit) {
*target |= (1 << bit);
}
void clearbit(int *target, int bit) {
*target &= ~(1 << bit);
}
//===---------------------------------------------------------------------===//
Instead of the following for memset char*, 1, 10:
movl $16843009, 4(%edx)
movl $16843009, (%edx)
movw $257, 8(%edx)
It might be better to generate
movl $16843009, %eax
movl %eax, 4(%edx)
movl %eax, (%edx)
movw al, 8(%edx)
when we can spare a register. It reduces code size.
//===---------------------------------------------------------------------===//
Evaluate what the best way to codegen sdiv X, (2^C) is. For X/8, we currently
get this:
int %test1(int %X) {
%Y = div int %X, 8
ret int %Y
}
_test1:
movl 4(%esp), %eax
movl %eax, %ecx
sarl $31, %ecx
shrl $29, %ecx
addl %ecx, %eax
sarl $3, %eax
ret
GCC knows several different ways to codegen it, one of which is this:
_test1:
movl 4(%esp), %eax
cmpl $-1, %eax
leal 7(%eax), %ecx
cmovle %ecx, %eax
sarl $3, %eax
ret
which is probably slower, but it's interesting at least :)
//===---------------------------------------------------------------------===//
The first BB of this code:
declare bool %foo()
int %bar() {
%V = call bool %foo()
br bool %V, label %T, label %F
T:
ret int 1
F:
call bool %foo()
ret int 12
}
compiles to:
_bar:
subl $12, %esp
call L_foo$stub
xorb $1, %al
testb %al, %al
jne LBB_bar_2 # F
It would be better to emit "cmp %al, 1" than a xor and test.
//===---------------------------------------------------------------------===//
Enable X86InstrInfo::convertToThreeAddress().
//===---------------------------------------------------------------------===//
We are currently lowering large (1MB+) memmove/memcpy to rep/stosl and rep/movsl
We should leave these as libcalls for everything over a much lower threshold,
since libc is hand tuned for medium and large mem ops (avoiding RFO for large
stores, TLB preheating, etc)
//===---------------------------------------------------------------------===//
Optimize this into something reasonable:
x * copysign(1.0, y) * copysign(1.0, z)
//===---------------------------------------------------------------------===//
Optimize copysign(x, *y) to use an integer load from y.
//===---------------------------------------------------------------------===//
%X = weak global int 0
void %foo(int %N) {
%N = cast int %N to uint
%tmp.24 = setgt int %N, 0
br bool %tmp.24, label %no_exit, label %return
no_exit:
%indvar = phi uint [ 0, %entry ], [ %indvar.next, %no_exit ]
%i.0.0 = cast uint %indvar to int
volatile store int %i.0.0, int* %X
%indvar.next = add uint %indvar, 1
%exitcond = seteq uint %indvar.next, %N
br bool %exitcond, label %return, label %no_exit
return:
ret void
}
compiles into:
.text
.align 4
.globl _foo
_foo:
movl 4(%esp), %eax
cmpl $1, %eax
jl LBB_foo_4 # return
LBB_foo_1: # no_exit.preheader
xorl %ecx, %ecx
LBB_foo_2: # no_exit
movl L_X$non_lazy_ptr, %edx
movl %ecx, (%edx)
incl %ecx
cmpl %eax, %ecx
jne LBB_foo_2 # no_exit
LBB_foo_3: # return.loopexit
LBB_foo_4: # return
ret
We should hoist "movl L_X$non_lazy_ptr, %edx" out of the loop after
remateralization is implemented. This can be accomplished with 1) a target
dependent LICM pass or 2) makeing SelectDAG represent the whole function.
//===---------------------------------------------------------------------===//
The following tests perform worse with LSR:
lambda, siod, optimizer-eval, ackermann, hash2, nestedloop, strcat, and Treesor.
//===---------------------------------------------------------------------===//
Teach the coalescer to coalesce vregs of different register classes. e.g. FR32 /
FR64 to VR128.
//===---------------------------------------------------------------------===//
mov $reg, 48(%esp)
...
leal 48(%esp), %eax
mov %eax, (%esp)
call _foo
Obviously it would have been better for the first mov (or any op) to store
directly %esp[0] if there are no other uses.
//===---------------------------------------------------------------------===//
Adding to the list of cmp / test poor codegen issues:
int test(__m128 *A, __m128 *B) {
if (_mm_comige_ss(*A, *B))
return 3;
else
return 4;
}
_test:
movl 8(%esp), %eax
movaps (%eax), %xmm0
movl 4(%esp), %eax
movaps (%eax), %xmm1
comiss %xmm0, %xmm1
setae %al
movzbl %al, %ecx
movl $3, %eax
movl $4, %edx
cmpl $0, %ecx
cmove %edx, %eax
ret
Note the setae, movzbl, cmpl, cmove can be replaced with a single cmovae. There
are a number of issues. 1) We are introducing a setcc between the result of the
intrisic call and select. 2) The intrinsic is expected to produce a i32 value
so a any extend (which becomes a zero extend) is added.
We probably need some kind of target DAG combine hook to fix this.
//===---------------------------------------------------------------------===//
We generate significantly worse code for this than GCC:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21150
http://gcc.gnu.org/bugzilla/attachment.cgi?id=8701
There is also one case we do worse on PPC.
//===---------------------------------------------------------------------===//
If shorter, we should use things like:
movzwl %ax, %eax
instead of:
andl $65535, %EAX
The former can also be used when the two-addressy nature of the 'and' would
require a copy to be inserted (in X86InstrInfo::convertToThreeAddress).
//===---------------------------------------------------------------------===//
Bad codegen:
char foo(int x) { return x; }
_foo:
movl 4(%esp), %eax
shll $24, %eax
sarl $24, %eax
ret
SIGN_EXTEND_INREG can be implemented as (sext (trunc)) to take advantage of
sub-registers.
//===---------------------------------------------------------------------===//
Consider this:
typedef struct pair { float A, B; } pair;
void pairtest(pair P, float *FP) {
*FP = P.A+P.B;
}
We currently generate this code with llvmgcc4:
_pairtest:
movl 8(%esp), %eax
movl 4(%esp), %ecx
movd %eax, %xmm0
movd %ecx, %xmm1
addss %xmm0, %xmm1
movl 12(%esp), %eax
movss %xmm1, (%eax)
ret
we should be able to generate:
_pairtest:
movss 4(%esp), %xmm0
movl 12(%esp), %eax
addss 8(%esp), %xmm0
movss %xmm0, (%eax)
ret
The issue is that llvmgcc4 is forcing the struct to memory, then passing it as
integer chunks. It does this so that structs like {short,short} are passed in
a single 32-bit integer stack slot. We should handle the safe cases above much
nicer, while still handling the hard cases.
While true in general, in this specific case we could do better by promoting
load int + bitcast to float -> load fload. This basically needs alignment info,
the code is already implemented (but disabled) in dag combine).
//===---------------------------------------------------------------------===//
Another instruction selector deficiency:
void %bar() {
%tmp = load int (int)** %foo
%tmp = tail call int %tmp( int 3 )
ret void
}
_bar:
subl $12, %esp
movl L_foo$non_lazy_ptr, %eax
movl (%eax), %eax
call *%eax
addl $12, %esp
ret
The current isel scheme will not allow the load to be folded in the call since
the load's chain result is read by the callseq_start.
//===---------------------------------------------------------------------===//
Don't forget to find a way to squash noop truncates in the JIT environment.
//===---------------------------------------------------------------------===//
Implement anyext in the same manner as truncate that would allow them to be
eliminated.
//===---------------------------------------------------------------------===//
How about implementing truncate / anyext as a property of machine instruction
operand? i.e. Print as 32-bit super-class register / 16-bit sub-class register.
Do this for the cases where a truncate / anyext is guaranteed to be eliminated.
For IA32 that is truncate from 32 to 16 and anyext from 16 to 32.
//===---------------------------------------------------------------------===//
For this:
int test(int a)
{
return a * 3;
}
We currently emits
imull $3, 4(%esp), %eax
Perhaps this is what we really should generate is? Is imull three or four
cycles? Note: ICC generates this:
movl 4(%esp), %eax
leal (%eax,%eax,2), %eax
The current instruction priority is based on pattern complexity. The former is
more "complex" because it folds a load so the latter will not be emitted.
Perhaps we should use AddedComplexity to give LEA32r a higher priority? We
should always try to match LEA first since the LEA matching code does some
estimate to determine whether the match is profitable.
However, if we care more about code size, then imull is better. It's two bytes
shorter than movl + leal.
//===---------------------------------------------------------------------===//
Implement CTTZ, CTLZ with bsf and bsr.
//===---------------------------------------------------------------------===//
It appears gcc place string data with linkonce linkage in
.section __TEXT,__const_coal,coalesced instead of
.section __DATA,__const_coal,coalesced.
Take a look at darwin.h, there are other Darwin assembler directives that we
do not make use of.
//===---------------------------------------------------------------------===//
int %foo(int* %a, int %t) {
entry:
br label %cond_true
cond_true: ; preds = %cond_true, %entry
%x.0.0 = phi int [ 0, %entry ], [ %tmp9, %cond_true ]
%t_addr.0.0 = phi int [ %t, %entry ], [ %tmp7, %cond_true ]
%tmp2 = getelementptr int* %a, int %x.0.0
%tmp3 = load int* %tmp2 ; <int> [#uses=1]
%tmp5 = add int %t_addr.0.0, %x.0.0 ; <int> [#uses=1]
%tmp7 = add int %tmp5, %tmp3 ; <int> [#uses=2]
%tmp9 = add int %x.0.0, 1 ; <int> [#uses=2]
%tmp = setgt int %tmp9, 39 ; <bool> [#uses=1]
br bool %tmp, label %bb12, label %cond_true
bb12: ; preds = %cond_true
ret int %tmp7
}
is pessimized by -loop-reduce and -indvars
//===---------------------------------------------------------------------===//
u32 to float conversion improvement:
float uint32_2_float( unsigned u ) {
float fl = (int) (u & 0xffff);
float fh = (int) (u >> 16);
fh *= 0x1.0p16f;
return fh + fl;
}
00000000 subl $0x04,%esp
00000003 movl 0x08(%esp,1),%eax
00000007 movl %eax,%ecx
00000009 shrl $0x10,%ecx
0000000c cvtsi2ss %ecx,%xmm0
00000010 andl $0x0000ffff,%eax
00000015 cvtsi2ss %eax,%xmm1
00000019 mulss 0x00000078,%xmm0
00000021 addss %xmm1,%xmm0
00000025 movss %xmm0,(%esp,1)
0000002a flds (%esp,1)
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.
//===---------------------------------------------------------------------===//
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.
//===---------------------------------------------------------------------===//
We currently compile sign_extend_inreg into two shifts:
long foo(long X) {
return (long)(signed char)X;
}
becomes:
_foo:
movl 4(%esp), %eax
shll $24, %eax
sarl $24, %eax
ret
This could be:
_foo:
movsbl 4(%esp),%eax
ret
//===---------------------------------------------------------------------===//
Consider the expansion of:
uint %test3(uint %X) {
%tmp1 = rem uint %X, 255
ret uint %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
//===---------------------------------------------------------------------===//
This:
#include <math.h>
int foo(double X) { return isnan(X); }
compiles to (-m64):
_foo:
pxor %xmm1, %xmm1
ucomisd %xmm1, %xmm0
setp %al
movzbl %al, %eax
ret
the pxor is not needed, we could compare the value against itself.
//===---------------------------------------------------------------------===//
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.
//===---------------------------------------------------------------------===//