Re: optimization/6258: floor() may return incorrect value when optimized

Re: optimization/6258: floor() may return incorrect value when optimized

[rth]

Synopsis: floor() may return incorrect value when optimized

State-Changed-From-To: open->closed
State-Changed-By: rth
State-Changed-When: Thu Apr 11 14:35:14 2002
State-Changed-Why:
    Not a bug.  -ffast-math allows the compiler to replace
    a/90 with a*(1/90).  Except that 1/90 can't be represented
    exactly, so the result of the multiplication is 
    -3.0000000000000001144917494144692682, not -3.  Since the
    value is indeed less than -3, floor rightly returns -4.
    
    If you don't want this to happen, don't use -ffast-math.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=6258

optimization/6258: floor() may return incorrect value when optimized

[Karl Burgess]

>Number:         6258
>Category:       optimization
>Synopsis:       floor() may return incorrect value when optimized
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Thu Apr 11 09:26:00 PDT 2002
>Closed-Date:
>Last-Modified:
>Originator:     Karl Burgess
>Release:        3.0.4 (Debian testing/unstable)
>Organization:
Tarantella, Inc.
>Environment:
System: Linux barrie.tarantella.com 2.4.12 #8 Thu Feb 21 11:48:06 UTC 2002
i686 unknown
Architecture: i686

host: i386-pc-linux-gnu
build: i386-pc-linux-gnu
target: i386-pc-linux-gnu
configured with:
../src/configure -v --enable-languages=c,c++,java,f77,proto,objc --prefix=/u
sr --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as -
-with-gnu-ld --with-system-zlib --enable-long-long --enable-nls --without-in
cluded-gettext --disable-checking --enable-threads=posix --enable-java-gc=bo
ehm --with-cpp-install-dir=bin --enable-objc-gc i386-linux
>Description:

Under some circumstances the GCC optimizations triggered by -ffast-math and
-O cause the maths function floor() to return incorrect values.
The supplied test program calls the function floor() in two places with
the same input values, but gets different values returned.


The gcc output is as follows:

egcc -v -save-temps -Wall -MMD -O -ffast-math -ofloor_opt floor.c -lm
Reading specs from /usr/lib/gcc-lib/i386-linux/3.0.4/specs
Configured with:
../src/configure -v --enable-languages=c,c++,java,f77,proto,objc --prefix=/u
sr --infodir=/share/info --mandir=/share/man --enable-shared --with-gnu-as -
-with-gnu-ld --with-system-zlib --enable-long-long --enable-nls --without-in
cluded-gettext --disable-checking --enable-threads=posix --enable-java-gc=bo
ehm --with-cpp-install-dir=bin --enable-objc-gc i386-linux
Thread model: posix
gcc version 3.0.4
 /usr/lib/gcc-lib/i386-linux/3.0.4/cpp0 -lang-c -v -MM -MF
floor.dfloor_opt -MQ
floor_opt -D__GNUC__=3 -D__GNUC_MINOR__=0 -D__GNUC_PATCHLEVEL__=4 -D__ELF__ 
-Dunix -Dlinux -D__ELF__ -D__unix__ -D__linux__ -D__unix -D__linux -Asystem=
posix -D__OPTIMIZE__ -D__FAST_MATH__ -D__STDC_HOSTED__=1 -Wall -Acpu=i386 -A
machine=i386 -Di386 -D__i386 -D__i386__ -D__tune_i386__ floor.c floor.i
GNU CPP version 3.0.4 (cpplib) (i386 Linux/ELF)
ignoring nonexistent directory "/usr/i386-linux/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/gcc-lib/i386-linux/3.0.4/include
 /usr/include
End of search list.
 /usr/lib/gcc-lib/i386-linux/3.0.4/cc1 -fpreprocessed
floor.i -quiet -dumpbase floor.c -O -Wall -version -ffast-math -o floor.s
GNU CPP version 3.0.4 (cpplib) (i386 Linux/ELF)
GNU C version 3.0.4 (i386-linux)
 compiled by GNU C version 3.0.4.
 as -V -Qy -o floor.o floor.s
GNU assembler version 2.12.90.0.1 (i386-linux) using BFD version 2.12.90.0.1
20020307 Debian/GNU Linux
 /usr/lib/gcc-lib/i386-linux/3.0.4/collect2 -m elf_i386 -dynamic-linker
/lib/ld-linux.so.2 -ofloor_opt
/usr/lib/gcc-lib/i386-linux/3.0.4/../../../crt1.o
/usr/lib/gcc-lib/i386-linux/3.0.4/../../../crti.o
/usr/lib/gcc-lib/i386-linux/3.0.4/crtbegin.o -L/usr/lib/gcc-lib/i386-linux/3
.0.4 -L/usr/lib/gcc-lib/i386-linux/3.0.4/../../..
floor.o -lm -lgcc -lc -lgcc /usr/lib/gcc-lib/i386-linux/3.0.4/crtend.o
/usr/lib/gcc-lib/i386-linux/3.0.4/../../../crtn.o


When the test program is run (by typing ./floor_opt), it prints out the
following:

    Test failed! floor(-270.000000/90) == -4.000000
    Test worked! floor(-270.000000/90) == -3.000000

The first calculation involving floor() has returned -4.0 instead of
-3.0, which is incorrect. The second occurrence of floor() is correct.


>How-To-Repeat:

Here is the .i file for the test, compressed with gzip -9 and uuencode:

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`
end

>Fix:

I have a choice of workarounds:
If you don't compile with -ffast-math, then the test program works.
If you compile with -ffast-math, but not -O, then the program also works.
If you compile with -O -ffast-math -fno-inline, then the program also works.
If you compile with gcc 2.7.2.1, then the program works.
If you redefine floor() as a call to one of your own functions (eg.
myfloor),
which then calls the real floor(), then the program also works.

I have also noticed that the problem does not occur with gcc 3.0.3 on
Solaris 2.6


>Release-Note:
>Audit-Trail:
>Unformatted:
 To: gcc-gnats@gcc.gnu.org,debian-gcc@lists.debian.org
 Subject: floor() may return incorrect value when optimized
 From: karl
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 Cc:
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