optimization/10232: else clause is executed gratuitously

[p dot van-hoof at qub dot ac dot uk]

>Number:         10232
>Category:       optimization
>Synopsis:       else clause is executed gratuitously
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Wed Mar 26 21:56:01 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Peter van Hoof
>Release:        gcc 3.2.2
>Organization:
>Environment:
System: Linux tpol.pst.qub.ac.uk 2.4.18-27.7.x #1 Fri Mar 14 06:44:53 EST 2003 i686 unknown

host: i686-pc-linux-gnu
build: i686-pc-linux-gnu
target: i686-pc-linux-gnu
Configured with: ../gcc-3.2.2/configure --prefix=/usr/local/gcc322
>Description:
In the program below, the else clause should never be executed. However, the (long)pp is executed nevertheless and triggers an FP exception because pp is too large to fit in a 32-bit integer. This bug is to some extent target specific; it is triggered for the following targets: i686, pentiumpro, pentium2, pentium3, athlon, athlon-tbird, athlon-4, athlon-xp, athlon-mp.

In order to trigger the bug with gcc 3.3, the flag -ffast-math needs to be added.
>How-To-Repeat:
tpol.pst.qub.ac.uk> gcc -O2 h.c
tpol.pst.qub.ac.uk> a.out 
survived 2.15e+09 3

tpol.pst.qub.ac.uk> gcc -v -march=athlon -O2 h.c
Reading specs from /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/specs
Configured with: ../gcc-3.2.2/configure --prefix=/usr/local/gcc322
Thread model: posix
gcc version 3.2.2
 /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/cc1 -lang-c -v -iprefix /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/ -D__GNUC__=3 -D__GNUC_MINOR__=2 -D__GNUC_PATCHLEVEL__=2 -D__GXX_ABI_VERSION=102 -D__ELF__ -Dunix -D__gnu_linux__ -Dlinux -D__ELF__ -D__unix__ -D__gnu_linux__ -D__linux__ -D__unix -D__linux -Asystem=posix -D__OPTIMIZE__ -D__STDC_HOSTED__=1 -Acpu=i386 -Amachine=i386 -Di386 -D__i386 -D__i386__ -D__athlon -D__athlon__ -D__tune_athlon__ -D__MMX__ -D__3dNOW__ -D__3dNOW_A__ h.c -quiet -dumpbase h.c -march=athlon -O2 -version -o /tmp/ccS2PZ2B.s
GNU CPP version 3.2.2 (cpplib) (i386 Linux/ELF)
GNU C version 3.2.2 (i686-pc-linux-gnu)
	compiled by GNU C version 3.2.2.
ignoring nonexistent directory "/usr/local/gcc3/i686-pc-linux-gnu/include"
ignoring nonexistent directory "/usr/local/gcc322/i686-pc-linux-gnu/include"
ignoring duplicate directory "/usr/local/gcc322/lib/gcc-lib/i686-pc-linux-gnu/3.2.2/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/gcc3/lib/gcc-lib/i686-pc-linux-gnu/3.2.2/include
 /usr/local/include
 /usr/local/gcc322/include
 /usr/include
End of search list.
 as -V -Qy -o /tmp/ccSiJcG4.o /tmp/ccS2PZ2B.s
GNU assembler version 2.11.93.0.2 (i386-redhat-linux) using BFD version 2.11.93.0.2 20020207
 /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/collect2 -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 /usr/lib/crt1.o /usr/lib/crti.o /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/crtbegin.o -L/usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2 -L/usr/local/gcc3/bin/../lib/gcc-lib -L/usr/local/gcc322/lib/gcc-lib/i686-pc-linux-gnu/3.2.2 -L/usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/../../.. -L/usr/local/gcc322/lib/gcc-lib/i686-pc-linux-gnu/3.2.2/../../.. /tmp/ccSiJcG4.o -lgcc -lgcc_eh -lc -lgcc -lgcc_eh /usr/local/gcc3/bin/../lib/gcc-lib/i686-pc-linux-gnu/3.2.2/crtend.o /usr/lib/crtn.o
tpol.pst.qub.ac.uk> a.out 
Floating exception

tpol.pst.qub.ac.uk> cat h.c
/* INT_MAX+1 */
double pp = 2147483647. + 1.;

int main()
{
	int nd;

	/* make sure the program crashes on FP exception */
	unsigned short int Mask;
	__asm__ ("fnstcw %0" : "=m" (*&Mask));
	Mask &= 0xFFF2;
	__asm__ ("fldcw %0" : : "m" (*&Mask));

	if( pp > 3. )
		nd = 3;
	else
		nd = (long)pp;
	printf( "survived %.2e %d\n", pp, nd );
	return 0;
}
>Fix:
Workaround: optimize for a different target, or use -O1, or omit -ffast-math in the case of gcc 3.3
>Release-Note:
>Audit-Trail:
>Unformatted: