Summary

Recently I encountered a weird issue regarding LTO and -ffast-math where I got inconsistent result for my "pow" ( in cmath ) calls depending on whether -flto is used.

Environment:

$ g   --version
g   (GCC) 8.3.0
Copyright (C) 2018 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ ll /lib64/libc.so.6
lrwxrwxrwx 1 root root 12 Sep  3  2019 /lib64/libc.so.6 -> libc-2.17.so

$ ll /lib64/libm.so.6
lrwxrwxrwx 1 root root 12 Sep  3  2019 /lib64/libm.so.6 -> libm-2.17.so

$ cat /etc/redhat-release 
CentOS Linux release 7.5.1804 (Core)

Minimal Example

Code

fixed.hxx

#include <cstdint>
double Power10f(const int16_t power);

fixed.cxx

#include "fixed.hxx"
#include <cmath>

double Power10f(const int16_t power)
{
    return pow(10.0, (double) power);
}

test.cxx

#include <iostream>
#include <cmath>
#include <iomanip>
#include <cstdint>
#include "fixed.hxx"

int main(int argc, char** argv)
{
    if (argc >= 3) {
        int64_t value = (int64_t)atoi(argv[1]);
        int16_t power = (int16_t)atoi(argv[2]);
        double x = Power10f(power);
        std::cout.precision(17);
        std::cout << std::scientific << x << std::endl;
        std::cout << std::scientific << (double)value * x << std::endl;
        return 0;   
    }
    return 1;
}

Compile & Run

Compile it with -ffast-math and with/without -flto gives different results

With -flto will eventually call the __pow_finite version and gives the an "accurate" result:

$ g   -O3 -DNDEBUG -ffast-math -std=c  17 -flto  -o fixed.cxx.o -c fixed.cxx
$ g   -O3 -DNDEBUG   -o fdtest fixed.cxx.o test.cxx
$ ./fdtest 81 20
1.00000000000000000e 20
8.10000000000000000e 21
$ objdump -DC fdtest > fdtest.dump
$ cat fdtest.dump
...
0000000000400930 <Power10f(short)>:
  400930:       0f bf ff                movswl %di,