Re: c/8639: [3.2/3.3 regression] simple integer arithmetic expression broken

Re: c/8639: [3.2/3.3 regression] simple integer arithmetic expression broken

[Christian Ehrhardt]

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

From: "Christian Ehrhardt" <ehrhardt@mathematik.uni-ulm.de>
To: gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org, steveJepsen@netscape.net,
  gcc-bugs@gcc.gnu.org, nobody@gcc.gnu.org
Cc:  
Subject: Re: c/8639: [3.2/3.3 regression] simple integer arithmetic expression broken
Date: Tue, 19 Nov 2002 19:09:59 +0100

 Hi,
 
 this patch will be bootstrapping here in a minute. An non bootstapped
 compile indicates that this fixes the bug. I'm not sure if we can be
 more clever here without doing a big rewrite of fold-const.c. I'll
 report back tonight/tomorrow (CET) when the testsuite has completed.
 
 Someone please consider to check this in. I don't have CVS access.
 Also adding a testcase would probably be a good idea.
 
     regards  Christian
 
 
 --- gcc-3.2.1-20021111/gcc/fold-const.c.orig	Tue Nov 19 18:47:57 2002
 +++ gcc-3.2.1-20021111/gcc/fold-const.c	Tue Nov 19 18:57:04 2002
 @@ -4617,10 +4617,12 @@
        t2 = extract_muldiv (op1, c, code, wide_type);
        if (t1 != 0 && t2 != 0
  	  && (code == MULT_EXPR
 -	      /* If not multiplication, we can only do this if either operand
 -		 is divisible by c.  */
 -	      || multiple_of_p (ctype, op0, c)
 -	      || multiple_of_p (ctype, op1, c)))
 +	      /* If not multiplication, we can only do this if this is 
 +	       * PLUS_EXPR and both operands are divisible by c. Note that
 +	       * (80 - 4*x)/20 or (80 + (-4)*x)/20 must not degenerate
 +	       * into (20 - x/5). */
 +	      || (multiple_of_p (ctype, op0, c) &&
 +	          multiple_of_p (ctype, op1, c))))
  	return fold (build (tcode, ctype, convert (ctype, t1),
  			    convert (ctype, t2)));
  
 
 http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8639
 
 -- 
 THAT'S ALL FOLKS!

Re: c/8639: [3.2/3.3 regression] simple integer arithmetic expression broken

[bangerth]

Old Synopsis: simple integer arithmetic expression broken
New Synopsis: [3.2/3.3 regression] simple integer arithmetic expression broken

State-Changed-From-To: open->analyzed
State-Changed-By: bangerth
State-Changed-When: Tue Nov 19 07:36:06 2002
State-Changed-Why:
    Unless I am very blind, I think this reduced testcase
    should indeed pass the assertion:
    -------------------------
    #include <assert.h>
    
    int foo (int i) {
      int r;
      r = (80 - 4 * i) / 20;
      assert (r == 3);
    }
    
    int main () {
      foo(1);
    }
    -----------------------------
    Unfortunately, it aborts, with all gccs since 3.0. It
    passes with 2.95, though. Since this seems like a regression
    to me, I raise the priority. Note that no optimization
    flags are necessary to trigger this behavior. The same
    happens with the C++ front end, by the way.

http://gcc.gnu.org/cgi-bin/gnatsweb.pl?cmd=view%20audit-trail&database=gcc&pr=8639