https://llvm.org/bugs/show_bug.cgi?id=31399
Bug ID: 31399
Summary: [CodeGen] suboptimal code for ffs()
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: unassignedb...@nondot.org
Reporter: dav...@freebsd.org
CC: fil...@gmail.com, llvm-bugs@lists.llvm.org,
llvm-...@redking.me.uk, simon.f.whitta...@gmail.com
Classification: Unclassified
Given
#include <strings.h>
int f(int x) {
return ffs(x) + 1;
}
compiled with -O3, clang 3.9.0 generates
f(int): # @f(int)
bsfl %edi, %ecx
addl $2, %ecx
testl %edi, %edi
movl $1, %eax
cmovnel %ecx, %eax
retq
while gcc 6.2 generates:
f(int):
bsf eax, edi
mov edx, -1
cmove eax, edx
add eax, 2
ret
We could save a test instruction. I don't think it matters a lot in practice,
but maybe a fun exercise if somebody wants to take a look (cc:ing Simon W. who
was looking for small codegen bugs to fix).
(Note that we lower ffs() to (x != 0) ? llvm.cttz(x) + 1 : 0 in
SimplifyLibCalls.cpp)
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