https://gcc.gnu.org/bugzilla/show_bug.cgi?id=121720
Bug ID: 121720
Summary: [13.4 / 14.3 / 15 / 16 Regression] std::min_element is
not optimized at -O2
Product: gcc
Version: 14.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: moncef.mechri at gmail dot com
Target Milestone: ---
Consider the following code:
#include <algorithm>
#include <cstdint>
#include <vector>
size_t min1(const std::vector<uint64_t>& v)
{
return *std::min_element(v.begin(), v.end());
}
size_t min2(const std::vector<uint64_t>& v)
{
auto min = v[0];
for (size_t i = 1; i < v.size(); ++i)
if (v[i] < min)
min = v[i];
return min;
}
On Linux x86_64, with g++ 13.3 or 14.2 at -O2, the codegen for the main loop is
identical between the two versions:
mov rcx, QWORD PTR [rdx]
cmp rax, rcx
cmova rax, rcx
mov rcx, QWORD PTR [rdx+8]
cmp rax, rcx
cmova rax, rcx
add rdx, 16
cmp rsi, rdx
jne .L4
Indicating that the iterator in min1() has been optimized away successfully.
However, starting with GCC 13.4 and 14.3 (as well as 15 and 16 trunk), the
iterator isn't optimized away anymore at -O2:
.L4:
mov rsi, QWORD PTR [rdx]
cmp QWORD PTR [rax], rsi
cmovb rdx, rax
add rax, 8
cmp rcx, rax
jne .L4
With the impacted compiler versions, the iterator does get optimized away at
-O3.
Demo: https://godbolt.org/z/85rdj4s36
Note: There are existing issues related to missed optimizations of
std::min_element (for example [1] and [2]), but I felt like this one warranted
its own issue since it's a regression introduced in versions released just a
few months ago.
[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89121
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102684
