https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113896
Bug ID: 113896
Summary: Assigning array elements in the wrong order after
floating point optimization
Product: gcc
Version: 12.3.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: noobie-iv at mail dot ru
Target Milestone: ---
Created attachment 57404
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=57404&action=edit
Test program and build_and_run script
/* file: f.cpp */
extern double a1; // 1.0
extern double a2; // 1.0
void f(double K[2], bool b)
{
double A[] = {
b ? a1 : a2,
0,
0,
0
};
double sum{};
for(double a : A) sum += a;
for(double& a : A) a /= sum;
if (b) {
K[0] = A[0]; // 1.0
K[1] = A[1]; // 0.0
} else {
K[0] = A[0] + A[1];
}
}
/* file: main.cpp */
#include <iostream>
double a1 = 1.0;
double a2 = 1.0;
void f(double K[2], bool b);
int main()
{
double K[2]{};
f(K, true);
std::cout << K[0] << "\t" << K[1] << "\n";
}
Bug: Returns different results when compiled with different optimization
settings:
g++ -O2 -ffast-math f.cpp main.cpp -o good
g++ -O3 -ffast-math f.cpp main.cpp -o bad
./good outputs "1 0"
./bad outputs "0 1"
The bug is reproduced in systems:
* Fedora-37-1.7 x64 with latest gcc autoupdate (12.3.1 20230508)
* Debian-12.2.0 x64 with gcc-12.3.1 build from sources
In the gcc-12 branch:
* Last good commit: d127348d7711e148e5ddd205a8c3409b37fae64c (12.2.1 20221017)
* First bad commit: fe7d74313736b8e1c30812bc49419f419bdf1c53 (12.2.1 20221017)
* Last tested bad commit: 4ced4ca95001f1583623c801c9c3642224a2c4f0 (12.3.1
20240210)
In the gcc-13 branch - There is no bug:
* Last tested good commit: c891d8dc23e1a46ad9f3e757d09e57b500d40044 (13.2.0)
In the gcc-14 branch - There is no bug:
* Last tested good commit: cff174fabd6c980c09aee95db1d9d5c22421761f (14.0.1
20240210)
Note: Function f() is a simplified version of the
XSpline::linearCombinationFor() function from the scantailor-experimental
project:
https://github.com/ImageProcessing-ElectronicPublications/scantailor-experimental/blob/main/src/math/XSpline.cpp
Now the bug has been temporarily resolved by declaring array A volatile.
