FPE in zrotg with g++-13 -O3

[shardest]

I have a simple test program that worked on Ubuntu 22.04 and now fails after upgrading to 24.04. I filed an Ubuntu bug report and then realized that I can reproduce the issue without using Ubuntu's OpenBLAS package but rather building from source. For reference, here's the bug report:

https://bugs.launchpad.net/ubuntu/+source/openblas/+bug/2083157

This is the test program (g++ -o test_blas test_blas.cpp -lopenblas):

#include
#include

#include <fenv.h>

typedef std::complex dcomp;

extern "C" {
void zrotg_(dcomp*, dcomp*, double*, dcomp*);
}

int main(void) {
feenableexcept(FE_INVALID |
FE_DIVBYZERO |
FE_OVERFLOW |
FE_UNDERFLOW);

dcomp ain(0.28048120941867205058,0.028460086243503417841), bin(0.018279187257355982571,0);
double c(0.0);
dcomp s(0.0);

dcomp a=ain, b=bin;
zrotg_(&a, &b, &c, &s);
std::cout << a << " " << b << " " << c << " " << s << std::endl;

return 0;

}

Expected output (get this with -O0):

(0.28107,0.101469) (0.0182792,0) 0.997905 (0.0643715,0.0065317)

Output with -O1, -O2, -O3:
Floating point exception (core dumped)

Output with -O3 but FPE turned off:
(0.28107,inf) (0.0182792,0) 0.997905 (0.0643715,0.0065317)

Additional notes:

• Also fails with the Intel compiler and OpenBLAS
• Works with the Intel compiler and MKL BLAS

Here are some results for different OpenBLAS versions:

• 0.3.20 (version in Ubuntu 22.04, where my program used to work): works
• 0.3.23: works
• 0.3.24: fails
• 0.3.26 (version in Ubuntu 24.04): fails

It seems this may be related to the implementation of "safe scaling" that went in for 0.3.24, possibly from #4130.

[martin-frbg]

What architecture/cpu do you see this with ?

[shardest]

My system is a dual Xeon e5-2630v4.

[martin-frbg]

So a fairly common Haswell-generation x86_64... I cannot reproduce this with the current 0.3.28 and gcc 13.2 or 14.2 here

[shardest]

I can try with 0.3.28 but I won't have time to get to it until this evening. Did you try with 0.3.26?

[martin-frbg]

This (a typo leading to uninitialized data being used as denominator) was fixed in the context of #4499 (released as 0.3.27)

[shardest]

Thanks - I confirmed that it works on my system with 0.3.27. I will mention it on the Ubuntu bug report.