[PATCH] Fix PR 8639 (3.2.1 material)

[Christian Ehrhardt]

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

Hi,

I bootstrapped the 3.2.1 prerelease with the patch below and
it works for C and C++ without regressions. It fixes the PR 8639
which is about (80-4*x)/20 beeing evaluated to 4 instead of 3 if (x==1).

I really think this is 3.2.1 release material because all current
compilers silently generate bad code. Here's the patch as posted earlier
today:


--- 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)));
 

-- 
THAT'S ALL FOLKS!