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From: "jan at epgmod dot phys.tue.nl" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libfortran/66458] New: Loading libgfortran.so changes the FPU exception flags
Date: Mon, 08 Jun 2015 09:54:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66458

            Bug ID: 66458
           Summary: Loading libgfortran.so changes the FPU exception flags
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libfortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jan at epgmod dot phys.tue.nl
  Target Milestone: ---

It appears that the mere act of loading libgfortran.so changes the FPU
exception mask. It looks as if the library contains initialization code that I
would have expected to appear in the program's start-up code. (Indeed it
appears to do so in line 266 of libgfortran/runtime/main.c.)

The problem can be reproduced by compiling the following C program:

// fpuflags
#include <fenv.h>
#include <dlfcn.h>

int main(int argc, char* argv[])
{
        volatile double d=0;
        feenableexcept(FE_ALL_EXCEPT);
        if (argc==2)
        {
                dlopen(argv[1],RTLD_NOW);
        }
        // I expect the expression 1/d to generate an FPU exception:
        return 1/d;
}

> gcc fpuflags.c -ldl -lm
> ./a.out 
Floating point exception
> ./a.out /usr/lib64/libstdc++.so.6
Floating point exception
> ./a.out /usr/lib64/gcc/x86_64-suse-linux/4.8/libgfortran.so
> #nothing happens

After merely loading libgfortan.so, no FPU exceptions are generated anymore.
Loading libstdc++.so does not result in such problem.

This is a problem in my numerical application where libgfortran.so is loaded as
a depency of a plugin library that is loaded at runtime.

At first sight, this problem may be similar to that in PR20788.