From:	Eric Rudd <rudd AT cyberoptics DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Possible gcc bug
Date:	Tue, 09 Jun 1998 10:40:59 -0500
Organization:	CyberOptics
Lines:	29
Message-ID:	<357D578B.1088A0F9@cyberoptics.com>
Reply-To:	rudd AT cyberoptics DOT com
NNTP-Posting-Host:	rudd.cyberoptics.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

I have isolated what I think is a register-allocation bug in gcc
v2.7.2.1 having to do with incorrect handling of extended asm. I would
appreciate feedback from others as to what is wrong here. The little
subroutine does nothing useful in its condensed form besides
illustrating the problem. I compiled with

gcc -O2 -S bad.c -o bad.lst

Attached to this message is the contents of "bad.lst". Compiling without
-O2 produces different code, but also incorrect.

void alias(int arg1, int arg2, int *arg3, int *arg4) {
   /* The problem with this code is that gcc aliases
      arg3 to arg4, so arg3 never gets set. */
   asm ("
      xorl  %0, %0
      xorl  %1, %1
      xorl  %%ecx, %%ecx
   "
   :  "=a"  (*arg3),       /* Results */
      "=d"  (*arg4)
   :  "0"   (arg1),        /* Input args */
      "b"   (arg2)
   :  "%ecx"               /* Clobbered registers */
   );
}

-Eric Rudd
rudd AT cyberoptics DOT com

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