https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122116
Bug ID: 122116
Summary: __builtin_expect_with_probability vs
__builtin_unpredictable
Product: gcc
Version: 16.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: rockeet at gmail dot com
Target Milestone: ---
We expect clang's __builtin_unpredictable can be emulated by
GCC's __builtin_expect_with_probability, but his is not true,
see: https://godbolt.org/z/zf33a95PM
#if !defined(__clang__)
#define __builtin_unpredictable(x) __builtin_expect_with_probability(x, 1,
0.5)
#endif
size_t binary_search_byte(const byte_t* data, size_t len, byte_t key) {
size_t lo = 0, hi = len;
while (lo < hi) {
size_t mid = (lo + hi) / 2;
if (__builtin_unpredictable(data[mid] < key))
lo = mid + 1;
else
hi = mid;
}
return lo;
}
__builtin_unpredictable direct clang generate cmov instead
of branch in the loop, we can not direct GCC to generate
branchless code with cmov.
