https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100322
Bug ID: 100322
Summary: Switching from std=c++17 to std=c++20 causes
performance regression in relationals
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: aaron at aarongraham dot com
Target Milestone: ---
Experiment here: https://godbolt.org/z/PT73cn5e5
#include <chrono>
using clk = std::chrono::steady_clock;
bool compare_count(clk::duration a, clk::duration b) {
return a.count() > b.count();
}
bool compare(clk::duration a, clk::duration b) {
return a > b;
}
Compiling with -std=c++17 I get:
_Z13compare_countNSt6chrono8durationIxSt5ratioILx1ELx1000000000EEEES3_:
cmp r2, r0
sbcs r3, r3, r1
ite lt
movlt r0, #1
movge r0, #0
bx lr
_Z7compareNSt6chrono8durationIxSt5ratioILx1ELx1000000000EEEES3_:
cmp r2, r0
sbcs r3, r3, r1
ite lt
movlt r0, #1
movge r0, #0
bx lr
Compiling with -std=c++20 I get:
_Z13compare_countNSt6chrono8durationIxSt5ratioILx1ELx1000000000EEEES3_:
cmp r2, r0
sbcs r3, r3, r1
ite lt
movlt r0, #1
movge r0, #0
bx lr
_Z7compareNSt6chrono8durationIxSt5ratioILx1ELx1000000000EEEES3_:
cmp r1, r3
it eq
cmpeq r0, r2
beq .L4
cmp r0, r2
sbcs r3, r1, r3
bge .L5
mov r0, #-1
.L3:
cmp r0, #0
ite le
movle r0, #0
movgt r0, #1
bx lr
.L4:
movs r0, #0
b .L3
.L5:
movs r0, #1
b .L3
(Note that clang doesn't have this problem)
