[Bug target/52451] gcc w/i387 float generates fucom rather than fcom for floating point comparsons

[nszabolcs at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=52451

Szabolcs Nagy <nszabolcs at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |nszabolcs at gmail dot com

--- Comment #4 from Szabolcs Nagy <nszabolcs at gmail dot com> ---
this bug is still present in gcc-4.9 on i386 and x86_64 targets
and possibly others (clang is broken too but that's another story)

the ieee-754 and thus iso c99/c11 + annex f,  requires that

 ==, != are quiet (never raise any exception)

 <, >, <=, >= are signaling (raise invalid if an operand is nan)

eg. with

int f(float x, float y)
{
    return x < y;
}

int g(float x, float y)
{
    return x == y;
}

on x87 (i386):
fcom* should be used for f (signaling)
fucom* should be used for g (quiet)

with sse2 (x86_64):
comis* should be used for f (signaling)
ucomis* should be used for g (quiet)

(on arm it is vcmpe.f32 vs vcmp.f32)

it is easy to check that gcc always emits quiet comparisions
and with -mno-ieee-fp it always emits signaling ones
(both of them are wrong)

-O2 -std=c99 on x86_64:

    http://goo.gl/awmRGP

-O2 -std=c99 -mno-ieee-fp on x86_64:

    http://goo.gl/9CHNjK