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From: "timo.kreuzer at reactos dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug c/66282] New: Missing x86 floating point conversion on explicit casts violates C standard
Date: Mon, 25 May 2015 21:27:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66282

            Bug ID: 66282
           Summary: Missing x86 floating point conversion on explicit
                    casts violates C standard
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: timo.kreuzer at reactos dot org
  Target Milestone: ---

When compiling floating point code, the compiler omits to convert an
intermediate computation result, which is cast to double, to 64 bit precision
and instead keeps the internal 80 bit precision.

Example code:

void bugbug()
{
    double x = 4.4;
    double y;

    y = (1 / x);
    if ( y != ((double)(1 / x)) )
    {
        printf("Standard violation!\n");
    }
}

compiled with "gcc.exe -Wall -g -c fpu.c -o fpu.o"

The expression on the right side inside the if() is not converted to double
precision, which is in violation of the C standard.

Quote:
"Implementations employing wide registers have to take care to honor
appropriate
semantics. Values are independent of whether they are represented in a register
or in memory. For example, an implicit spilling of a register is not permitted
to alter the value. Also, an explicit store and load is required to round to
the precision of the storage type. In particular, casts and assignments are
required to perform their specified conversion."

Emphasis is on the last sentence.

This was tested on a mingw version of the compiler, but I do not think this is
mingw specific.