Re: c/8639: simple integer arithmetic expression broken

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

The following reply was made to PR c/8639; it has been noted by GNATS.

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: steveJepsen@netscape.net
Cc: gcc-bugs@gcc.gnu.org, gcc-gnats@gcc.gnu.org
Subject: Re: c/8639: simple integer arithmetic expression broken
Date: Tue, 19 Nov 2002 17:10:33 +0100

 On Tue, Nov 19, 2002 at 02:42:14PM -0000, steveJepsen@netscape.net wrote:
 > >Number:         8639
 > >Category:       c
 > >Synopsis:       simple integer arithmetic expression broken
 
 This is:
 a) real
 b) present in 3.2, 3.2.1 and 3.3
 c) present at least in C and C++
 d) A regression from 2.95.2
 d) arch independant (!)
 e) A show stopper (IMHO)
 
 Someone please mark this as confirmed, priority high. It should probably
 also be reclassified as middle end.
 
 > >Confidential:   no
 > >Severity:       critical
 > >Priority:       medium
 > >Responsible:    unassigned
 > >State:          open
 > >Class:          wrong-code
 > >Submitter-Id:   net
 > >Arrival-Date:   Tue Nov 19 06:46:02 PST 2002
 > >Closed-Date:
 > >Last-Modified:
 > >Originator:     Steve Jepsen
 > >Release:        unknown-1.0
 > >Organization:
 > >Environment:
 > gcc version 3.2 20020903 (Red Hat Linux 8.0 3.2-7)
 > >Description:
 > the expression:
 > 
 >    bug_result = (80 - 4 * intvar) / 20;
 > 
 > with intvar == 1 returns 4 (should return 3)
 
 I did a few experiments with expression of the form (A-B*intvar)/C with
 A, B and C beeing constant. In these expressions the Bug only seems to
 happen if C divides A and B divides C. According to the assembler output
 this is rewritten as A/C - intvar/(C/B) which is NOT the same.
 A/C - (intvar-1+C/B)/(C/B) would be the same as far as I can see.
 
 A minimal example is this:
 
 int main ()
 {
 	int x = 1;
 	int res = (80 - 4*x)/20;
 	return res;
 }
 
 Compiled with gcc t.c this gives:
 elbereth$ gcc -Wall t.c
 elbereth$ a.out
 elbereth$ echo $? 
 4
 elbereth$ 
 
 Here's the asm output on sparc which shows that the expression
 is rewritten as (20 - x/5).
 
 main:
         !#PROLOGUE# 0
         save    %sp, -120, %sp
         !#PROLOGUE# 1
         mov     1, %o0
         st      %o0, [%fp-20]
         ld      [%fp-20], %o0
         sll     %o0, 2, %o1
         mov     81, %o0
         sub     %o0, %o1, %o0
         mov     20, %o1
         call    .div, 0
          nop
         st      %o0, [%fp-24]
         ld      [%fp-24], %o0
         mov     %o0, %i0
         nop
         ret
         restore
 
 
    regards    Christian