if statement incorrectly optimized to 0 with recent CVS

[Peter Osterlund <peter dot osterlund at mailbox dot swipnet dot se>]

Appearently there is some optimization in gcc trying to combine tests
of adjacent bytes into a single 16 bit test. This optimization
produces incorrect code for the following source. The compiler also
generates a bogus "comparison is always 0" warning during compilation.

I bootstrapped with CFLAGS="-O2 -Wall -march=pentiumpro -mpentiumpro"

gcc2.95 compiles this code correctly.


bash$ cat test.c
struct t {
    signed char b[2];
    int x;
};

enum { v = 1 };

int f(struct t* t)
{
    if (((t->b[0]) == 2) && ((t->b[1]) == -v))
	return 1;
    return 0;
}
bash$ PATH=/home/petero/gcc/bin:$PATH gcc -v -O -S test.c
Reading specs from /home/petero/gcc/lib/gcc-lib/i686-pc-linux-gnu/2.96/specs
gcc version 2.96 20000218 (experimental)
 /home/petero/gcc/lib/gcc-lib/i686-pc-linux-gnu/2.96/cpp -lang-c -v -D__GNUC__=2 -D__GNUC_MINOR__=96 -D__GNUC_PATCHLEVEL__=0 -D__ELF__ -Dunix -Dlinux -D__ELF__ -D__unix__ -D__linux__ -D__unix -D__linux -Asystem(posix) -D__OPTIMIZE__ -Acpu(i386) -Amachine(i386) -Di386 -D__i386 -D__i386__ -D__tune_pentiumpro__ test.c /tmp/ccxoGXRr.i
GNU CPP version 2.96 20000218 (experimental) (cpplib)
 (i386 Linux/ELF)
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /home/petero/gcc/i686-pc-linux-gnu/include
 /home/petero/gcc/lib/gcc-lib/i686-pc-linux-gnu/2.96/include
 /usr/include
End of search list.
 /home/petero/gcc/lib/gcc-lib/i686-pc-linux-gnu/2.96/cc1 /tmp/ccxoGXRr.i -quiet -dumpbase test.c -O -version -o test.s
GNU C version 2.96 20000218 (experimental) (i686-pc-linux-gnu) compiled by GNU C version 2.96 20000218 (experimental).
test.c: In function `f':
test.c:10: warning: comparison is always 0
bash$ cat test.s
	.file	"test.c"
	.version	"01.01"
gcc2_compiled.:
.text
	.align 16
.globl f
	.type	 f,@function
f:
	pushl	%ebp
	movl	%esp, %ebp
	movl	$0, %eax
	popl	%ebp
	ret
.Lfe1:
	.size	 f,.Lfe1-f
	.ident	"GCC: (GNU) 2.96 20000218 (experimental)"
bash$ 

-- 
Peter Österlund          Email:     peter.osterlund@mailbox.swipnet.se
Sköndalsvägen 35                    f90-pos@nada.kth.se
S-128 66 Sköndal         Home page: http://home1.swipnet.se/~w-15919
Sweden                   Phone:     +46 8 942647