https://bugs.llvm.org/show_bug.cgi?id=34642
Bug ID: 34642
Summary: [x86] missed optimizations for shift-counts: (32-n)
can be (-n)
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: pe...@cordes.ca
CC: llvm-bugs@lists.llvm.org
unsigned shift(unsigned a, unsigned n){
n = (32-n);
// n &= 31;
return a << n;
}
// https://godbolt.org/g/YFKxQw
// clang version 6.0.0 (trunk 313348) -march=skylake -O3
mov eax, 32 # -mno-bmi2 version is essentially the same
sub eax, esi
shlx eax, edi, eax
ret
But with n &= 31; we get the obviously-better
neg esi
shlx eax, edi, esi
ret
(shlx masks the shift count identically to how shl r,cl does, with &31 for
32-bit operand size. -mno-bmi2 code-gen is equivalent.)
So clang / llvm doesn't take full advantage of the masking behaviour of x86
shift counts for shlx, shl r,cl, or presumably bt / bts (which do also wrap
around at the destination register size with a register destination.)
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