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From: "colin.pitrat+gcc at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/64442] New: -O1 modify output of a simple computation with rounding
Date: Tue, 30 Dec 2014 11:53:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64442

            Bug ID: 64442
           Summary: -O1 modify output of a simple computation with
                    rounding
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: colin.pitrat+gcc at gmail dot com

When building the following program with -O1, it outputs 85 whereas without -O1
it outputs 84:

#include <stdio.h>
#include <stdint.h>

int main()
{
    double max = 8.03;
    double a = 6.01;
    double b = 5;
    double c = (double)0xFF / (max - b);

    int16_t value = (a - b) * c;
    printf("Result = %d\n", value);
    return 0;
}

The compilation lines I use are really minimal:
- gcc main.c
vs
- gcc -O1 main.c

I tried to find which optimization flag was producing this issue but couldn't
find any !

- I don't have the issue when I don't use -O1 but provide explicitely all (or
any of) -O1 optimization flags (obtained with gcc -Q -O1 --help=optimizers |
grep "\-f" | grep enabled | awk '{ print $1 }' | xargs)
- Among all optimizer flags, I have the same issue only when providing either
-fsingle-precision-constant, -ffloat-store or -fshort-double but those flags
are supposed to be deactivated in -O1 

Note that I didn't test -fpack-struct because it didn't build

Regards,
Colin