https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110170
Bug ID: 110170
Summary: Sub-optimal conditional jumps in conditional-swap with
floating point
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: antoshkka at gmail dot com
Target Milestone: ---
Some of the C++ algorithms are written in attempt to avoid conditional jumps in
tight loops. For example, code close the following could be seen in libc++:
void __cond_swap(double* __x, double* __y) {
bool __r = (*__x < *__y);
auto __tmp = __r ? *__x : *__y;
*__y = __r ? *__y : *__x;
*__x = __tmp;
}
GCC-14 with -O2 and -march=x86-64 options generates the following code:
__cond_swap(double*, double*):
movsd xmm1, QWORD PTR [rdi]
movsd xmm0, QWORD PTR [rsi]
comisd xmm0, xmm1
jbe .L2
movq rax, xmm1
movapd xmm1, xmm0
movq xmm0, rax
.L2:
movsd QWORD PTR [rsi], xmm1
movsd QWORD PTR [rdi], xmm0
ret
A conditional jump could be probably avoided in the following way:
__cond_swap(double*, double*):
movsd xmm0, qword ptr [rdi]
movsd xmm1, qword ptr [rsi]
movapd xmm2, xmm0
minsd xmm2, xmm1
maxsd xmm1, xmm0
movsd qword ptr [rsi], xmm1
movsd qword ptr [rdi], xmm2
ret
Playground: https://godbolt.org/z/v3jW67x91
