[Bug c/36124] New: conditional loop becomes infinite loop in -O2 (gcc 4.2 -> 4.3 regression)

["cyprien+gccbug at cypou dot net" <gcc-bugzilla at gcc dot gnu dot org>]

Hi,

It seems to have some regression in gcc 4.3, visible on arm targets as well as
x86_64.
I originally already reported it to Debian bugtracking system
[http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=472867]

I tested it against the latest gcc snapshot:
    gcc (GCC) 4.3.1 20080501 (prerelease)

gcc has been built with no option, only srcdir/configure && make.

preprocessed content:
# 1 "c.c"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "c.c"


extern void func(void*);

void test()
{
    register long *foo = (long*) 1024;
    register int index;
    for(index=0;index<1024;index++)
        func(foo--);
}


This is a simple loop indexed on an integer. It should be finite, but when
compiling with -O2 (and -O3) the compiler removes the end condition.

gcc -S -O2 extract:
.LCFI0: 
        movl    $1024, %edi
        .p2align 4,,10
        .p2align 3
.L2:
        leaq    -8(%rdi), %rbx
        call    func
        movq    %rbx, %rdi
        jmp     .L2


Note that when using char* or non-pointer type for foo variable, it compiles
successfully.


-- 
           Summary: conditional loop becomes infinite loop in -O2 (gcc 4.2 -
                    > 4.3 regression)
           Product: gcc
           Version: 4.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: cyprien+gccbug at cypou dot net
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36124