Re: optimization/7799: [3.2/3.3 regression] Loop bug with optimization flag -Os in gcc

["Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>]

The following reply was made to PR optimization/7799; it has been noted by GNATS.

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: nejataydin@superonline.com, gcc-gnats@gcc.gnu.org, gcc@gcc.gnu.org,
  gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org
Cc:  
Subject: Re: optimization/7799: [3.2/3.3 regression] Loop bug with optimization flag -Os in gcc
Date: Fri, 13 Dec 2002 14:36:26 +0100

 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=7799
 
 Hi,
 
 this PR is about the following code snippet that is miscompiled with -Os
 
 void fill (int* p, int* q[10])
 {
         int i;
         for (i = 0; i < 10; i++)
                 *q++ = &p[i];
 }
 
 The asm-Code is this:
 
         pushl   %ebp
         movl    %esp, %ebp
         movl    8(%ebp), %eax
         movl    12(%ebp), %edx
         leal    36(%eax), %ecx
 .L6:
         movl    %eax, (%edx)
         addl    $4, %eax
         addl    $4, %edx
         cmpl    %ecx, %eax
         jle     .L6
         popl    %ebp
         ret
 
 This code roughly corresponds to the following C-Code:
 
 void fill (int * p, int *q[10])
 {
 	int ecx = (int)p + 9;
 	do {
 		*q = p; p++; q++;
 	} while ((int)p <= ecx);
 }
 
 This transformation is IMHO illegal because there is no way to make the
 comparison in general equivialent to that in the original for loop.
 If p is initially 0x7ffffffc the comparison must be treated as unsigned,
 however, if p is initially 0xfffffffc the comparison must be treated as
 signed.
 
     regards   Christian
 
 -- 
 THAT'S ALL FOLKS!