bug report

[Holger Maaß]

Hi egcs team,

I have a problem with egcs, version pgcc-2.91.66 19990314 (egcs-1.1.2
release).

The following code (in foo.c)

#define __fastcall __attribute__ ((regparm(3)))

unsigned reverse_bits(unsigned value, int len) __fastcall;

  :
  :
  :

unsigned reverse_bits(unsigned code, int len)
{
  unsigned res = 0;
  asm ("decl %1
rev1:
        rcrl $1,%2
        rcll $1,%0
        loop rev1" : "=r" (res) : "c" (len), "d" (code));
  return res;
}

compiled on a pentium as

          gcc -fomit-frame-pointer -O2 -S foo.c

gives me that (taken from foo.s):

         .align 16
.globl reverse_bits
         .type  reverse_bits,@function
reverse_bits:
         movl %edx,%ecx
         movl %eax,%edx
#APP
         decl %ecx
rev1:
         rcrl $1,%edx
         rcll $1,%eax
         loop rev1
#NO_APP
         ret

As you can see, the compiler ignores the `unsigned res = 0' statement.
That's not a good idea because the code above works properly only with a
`len' argument of 32. My work around is inserting an additional
statement `xorl %0,%0' just before the `rev1:' label. That's not nasty
so far, but I don't know, whether its my mistake or is there a mistake
in the compilers optimization routine.

MfG Holger (Holger.Maass@t-online.de)

P.S.: MfG means "Mit freundlichen Gruessen" and based on a german song
which caricatured the nuisance to abbreviate all and anything :-).