Re: optimization/8613: [3.2/3.3/3.4 regression] -O2 optimization generates wrong code

[Glen Nakamura <glen@imodulo.com>]

The following reply was made to PR optimization/8613; it has been noted by GNATS.

From: Glen Nakamura <glen@imodulo.com>
To: gcc-gnats@gcc.gnu.org, gcc-bugs@gcc.gnu.org,
	mark.odonohue@cytopia.com.au
Cc:  
Subject: Re: optimization/8613: [3.2/3.3/3.4 regression] -O2 optimization generates wrong code
Date: Tue, 18 Feb 2003 23:10:09 -1000

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8613
 
 I've tracked this bug down to a problem with postincrements and the strlen
 builtin function.  What happens is that the postincrement code isn't emitted
 before the builtin strlen function is expanded and gets emitted within the
 loop created by ix86_expand_strlensi_unroll_1.  This causes the postincrement
 code to be executed multiple times when calculating the string length.
 The fix is to call emit_queue before expanding the strlen builtin so the
 postincrement code is emitted outside of the loop.
 
 I'm not exactly sure where the emit_queue call should be placed.  Although
 adding emit_queue to expand_builtin_strlen would work, I added it to
 expand_builtin instead, because other builtins may have the same problem.
 It passed regression testing on i686-pc-linux-gnu for the 3.3 branch,
 but a more experienced developer should decide where the best place is.
 
 Here is a reduced testcase:
 
 extern void abort (void);
 
 int
 main ()
 {
   char buf[16] = "1234567890";
   char *p = buf;
   *p++ = (char) __builtin_strlen (buf);
   if ((buf[0] != 10) || (p - buf != 1))
     abort ();
 }
 
 And here is the patch:
 
 2003-02-19  Glen Nakamura  <glen@imodulo.com>
 
 	* builtins.c (expand_builtin): Emit postincrements before expanding
 	builtin functions.
 
 diff -Nru3p gcc-3.3.orig/gcc/builtins.c gcc-3.3/gcc/builtins.c
 --- gcc-3.3.orig/gcc/builtins.c	2002-12-01 17:51:43.000000000 +0000
 +++ gcc-3.3/gcc/builtins.c	2002-12-01 17:51:43.000000000 +0000
 @@ -3691,6 +3691,9 @@ expand_builtin (exp, target, subtarget, 
    tree arglist = TREE_OPERAND (exp, 1);
    enum built_in_function fcode = DECL_FUNCTION_CODE (fndecl);
  
 +  /* Perform postincrements before expanding builtin functions.  */
 +  emit_queue ();
 +
    if (DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_MD)
      return (*targetm.expand_builtin) (exp, target, subtarget, mode, ignore);