https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115267
Bug ID: 115267
Summary: Undue warning about undefined behavior in
Product: gcc
Version: 14.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tobi at gcc dot gnu.org
Target Milestone: ---
This seems to be a simpler case of PR103724. I originally observed it with 4.9
but was able to reproduce it with gcc 14.
Godbolt: https://godbolt.org/z/6h8bEYsvP
=== Testcase ===================
#include <iostream>
#include <sstream>
double arr[4];
void read(std::istream& is) {
for (unsigned int i = 0; i < 2; ++i) {
for (unsigned int j = i; j < 2; ++j) {
is >> arr[2*i+j];
if (i != j) {
arr[2*j+i] = arr[2*i+j];
}
}
}
}
int main()
{
std::stringstream ss("1 2 3");
read(ss);
return int(arr[0] + arr[1]);
}
===
Gives with -O3 -Wall:
=========================
<source>:11:39: warning: iteration 4 invokes undefined behavior
[-Waggressive-loop-optimizations]
11 | arr[2*j+i] = arr[2*i+j];
| ~~~~~~~~~^
<source>:8:36: note: within this loop
8 | for (unsigned int j = i; j < 2; ++j) {
|
==========================
Try as I might, I don't see the undefined behavior, the matrix is also
correctly filled in, no infinite loop obtains. Signedness of the loop index
doesn't play a role. The original code used a Eigen::Matrix2d instead of
manually indexing the 2x2 matirx, and then the warning is about iteration 2.
The stringstream seems to be necessary to confuse the compiler enough, but
maybe there's a way of getting the same behavior with a less complex thing
inside the loop.
