https://bugs.llvm.org/show_bug.cgi?id=50187
Bug ID: 50187
Summary: LLVM prefer lzcnt too strongly - extra computation
required
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Backend: X86
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected],
[email protected]
long long clz64(long long num) {
return 63LL - __builtin_clzll(num | 1);
}
int clz32(unsigned num) {
return 31 - __builtin_clz(num | 1);
}
int clz32_a(unsigned num) {
return 31 - __builtin_clz(num);
}
LLVM -march=haswell -O3
clz64(long long): # @clz64(long long)
or rdi, 1
lzcnt rax, rdi
xor rax, 63
ret
clz32(unsigned int): # @clz32(unsigned int)
or edi, 1
lzcnt eax, edi
xor eax, 31
ret
clz32_a(unsigned int): # @clz32_a(unsigned int)
lzcnt eax, edi
xor eax, 31
ret
GCC -march=haswell -O3
clz64(long long):
or rdi, 1
bsr rax, rdi
ret
clz32(unsigned int):
or edi, 1
bsr eax, edi
ret
clz32_a(unsigned int):
bsr eax, edi
ret
https://gcc.godbolt.org/z/r4bYfocKG
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