Date: Wed, 4 Apr 2012 12:29:23 +0300 From: Andrey Simonenko <simon@comsys.ntu-kpi.kiev.ua> To: Steve Kargl <sgk@troutmask.apl.washington.edu> Cc: freebsd-current@freebsd.org Subject: Re: -ffast-math in Ports and wrong generated code Message-ID: <20120404092923.GA1087@pm513-1.comsys.ntu-kpi.kiev.ua> In-Reply-To: <20120403134300.GA98102@troutmask.apl.washington.edu> References: <20120403112111.GA39616@pm513-1.comsys.ntu-kpi.kiev.ua> <20120403134300.GA98102@troutmask.apl.washington.edu>
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On Tue, Apr 03, 2012 at 06:43:00AM -0700, Steve Kargl wrote:
> On Tue, Apr 03, 2012 at 02:21:11PM +0300, Andrey Simonenko wrote:
> >
> > I use one port from the Ports Collection, that works with FP. Having
> > reinstalled it (its version was not changed) I noticed that it started
> > to work incorrectly. After debugging and disassembling its code I found
> > out that the -ffast-math option used for building was the result of
> > wrongly generated code (I did not specify this option in /etc/make.conf).
> >
> > At least finite() function call was eliminated from the result Assembler
> > code when -ffast-math option is used, tested on 9.0-STABLE and 10.0-CURRENT.
> >
> > Example test source code and generated code under 9.0-STABLE on amd64
> > by gcc from the base system:
> >
> > -----------------------------
> > #include <math.h>
> > #include <stdio.h>
> >
> > void
> > check_finite(double x)
> > {
> > printf("%d\n", finite(x));
> > }
> > -----------------------------
> >
> > % gcc -Wall -O2 -S finite.c
> > -----------------------------
> > check_finite:
> > .LFB3:
> > subq $8, %rsp
> > .LCFI0:
> > call finite <-- call to finite()
> > movl $.LC0, %edi
> > movl %eax, %esi
> > addq $8, %rsp
> > xorl %eax, %eax
> > jmp printf
> > .LFE3:
> > .size check_finite, .-check_finite
> > -----------------------------
> >
> > % gcc -Wall -O2 -ffast-math -S finite.c
> > -----------------------------
> > check_finite:
> > .LFB3:
> > xorl %esi, %esi <-- fake result from finite()
> > movl $.LC0, %edi
> > xorl %eax, %eax
> > jmp printf
> > .LFE3:
> > .size check_finite, .-check_finite
> > -----------------------------
> >
> > Can somebody comment this?
>
> Read the man page for gcc. With --fast-math,
> gcc assumes that the result of any FP operation
> is finite. So, the function call to finite()
> is eliminated as it is always true.
Looks like that I was misunderstood. I did not ask why finite() was
eliminated, I asked why fake result from finite() is wrong. Obviously
that -ffast-math can optimize FP arithmetics and as a result some functions
can be eliminated. The problem is not respecting IEEE specifications for
FP, the problem is wrongly generated code when -ffast-math is used.
Actually there is a bug in GCC used in the base system. There was made
a change to builtins.c from gcc in revision 1.12 [1] and as a result gcc
started to eliminate finite() function calls with -ffinite-math-only.
The true result from finite() is non-zero value, but GCC generated always
false value, so any program that uses finite() and has -ffinite-math-only
works incorrectly if it was built by this version of gcc.
Here is the correction for builtins.c:
--- builtins.c.orig 2012-01-06 14:50:41.000000000 +0200
+++ builtins.c 2012-04-04 10:27:23.000000000 +0300
@@ -8738,7 +8738,7 @@ fold_builtin_classify (tree fndecl, tree
case BUILT_IN_FINITE:
if (!HONOR_NANS (TYPE_MODE (TREE_TYPE (arg)))
&& !HONOR_INFINITIES (TYPE_MODE (TREE_TYPE (arg))))
- return omit_one_operand (type, integer_zero_node, arg);
+ return omit_one_operand (type, integer_one_node, arg);
if (TREE_CODE (arg) == REAL_CST)
{
After this change the corresponding Assembler code for my test file is:
% gcc -Wall -O2 -ffast-math -S finite.c
-----------------------------
check_finite:
.LFB3:
movl $1, %esi <-- fake result from finite()
movl $.LC0, %edi
xorl %eax, %eax
jmp printf
.LFE3:
.size check_finite, .-check_finite
-----------------------------
What do you think? If there is no objections, I'll create PR.
[1] http://www.freebsd.org/cgi/cvsweb.cgi/src/contrib/gcc/builtins.c.diff?r1=1.11;r2=1.12
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