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Date: Tue, 13 Apr 2004 11:17:00 -0000
From: "gcc-bugzilla at gcc dot gnu dot org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Message-ID: <20040413101110.14938.bruno@clisp.org>
Reply-To: gcc-bugzilla@gcc.gnu.org
Subject: [Bug optimization/14938] New: bug with global register variables and function pointer calls
X-Bugzilla-Reason: CC
X-SW-Source: 2004-04/txt/msg01063.txt.bz2
List-Id: <gcc-bugs.sourceware.org>


In a function using a global register variable, that calls a function
through a function pointer, the -O optimization omits the last update
of the register variable before the call.

Environment:
System: Linux linuix 2.4.21-99-default #1 Wed Sep 24 13:30:51 UTC 2003 i686 athlon i386 GNU/Linux
Architecture: i686

	<machine, os, target, libraries (multiple lines)>
host: i586-suse-linux-gnu
build: i586-suse-linux-gnu
target: i586-suse-linux-gnu
configured with: ../configure --enable-threads=posix --prefix=/usr --with-local-prefix=/usr/local --infodir=/usr/share/info --mandir=/usr/share/man --libdir=/usr/lib --enable-languages=c,c++,f77,objc,java,ada --disable-checking --enable-libgcj --with-gxx-include-dir=/usr/include/g++ --with-slibdir=/lib --with-system-zlib --enable-shared --enable-__cxa_atexit i586-suse-linux

How-To-Repeat:

================================== bug.c ===================================
register void** STACK __asm__("%ebx");
extern void** saved_STACK;
extern unsigned int func (void);
extern unsigned int (*func_pointer) (void);
void lisp_completion_indirect ()
{
  STACK[0] = (void*)0;
  STACK++;
  func_pointer();
  saved_STACK = STACK;
  STACK--;
}
void lisp_completion_direct ()
{
  STACK[0] = (void*)0;
  STACK++;
  func();
  saved_STACK = STACK;
  STACK--;
}
============================================================================

$ gcc -Wall -O -S bug.c

In the resulting bug.s, you see that the increment of %ebx before the call
is omitted in the lisp_completion_indirect function.

================================== bug.s ===================================
	.file	"bug.c"
	.text
.globl lisp_completion_indirect
	.type	lisp_completion_indirect, @function
lisp_completion_indirect:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$8, %esp
	movl	$0, (%ebx)
	call	*func_pointer
	movl	%ebx, saved_STACK
	subl	$4, %ebx
	leave
	ret
	.size	lisp_completion_indirect, .-lisp_completion_indirect
.globl lisp_completion_direct
	.type	lisp_completion_direct, @function
lisp_completion_direct:
	pushl	%ebp
	movl	%esp, %ebp
	subl	$8, %esp
	movl	$0, (%ebx)
	addl	$4, %ebx
	call	func
	movl	%ebx, saved_STACK
	subl	$4, %ebx
	leave
	ret
	.size	lisp_completion_direct, .-lisp_completion_direct
	.ident	"GCC: (GNU) 3.3.1 (SuSE Linux)"
============================================================================
------- Additional Comments From bruno at clisp dot org  2004-04-13 10:11 -------
Fix:

The bug goes away if -O is not passed to gcc.

-- 
           Summary: bug with global register variables and function pointer
                    calls
           Product: gcc
           Version: 3.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: bruno at clisp dot org
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i586-suse-linux-gnu
  GCC host triplet: i586-suse-linux-gnu
GCC target triplet: i586-suse-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14938