re PR c/7284 (incorrectly simplifies leftshift followed by signed power-of-2 division)

	PR c/7284
	* fold-const.c (extract_muldiv_1): Do not treat signed left shift
	as multiplication.

testsuite:
	* gcc.c-torture/execute/pr7284-1.c: New test.

From-SVN: r85059

gcc/ChangeLog

 2004-07-22  Joseph S. Myers  <jsm@polyomino.org.uk>
 
+	PR c/7284
+	* fold-const.c (extract_muldiv_1): Do not treat signed left shift
+	as multiplication.
+
+2004-07-22  Joseph S. Myers  <jsm@polyomino.org.uk>
+
 	* doc/implement-c.texi: New file.
 	* doc/extend.texi (C Implementation): Move to there.
 	* doc/gcc.texi: Include implement-c.texi.

gcc/fold-const.c

@@ -5139,8 +5139,12 @@ extract_muldiv_1 (tree t, tree c, enum tree_code code, tree wide_type)
     case LSHIFT_EXPR:  case RSHIFT_EXPR:
       /* If the second operand is constant, this is a multiplication
 	 or floor division, by a power of two, so we can treat it that
-	 way unless the multiplier or divisor overflows.  */
+	 way unless the multiplier or divisor overflows.  Signed
+	 left-shift overflow is implementation-defined rather than
+	 undefined in C90, so do not convert signed left shift into
+	 multiplication.  */
       if (TREE_CODE (op1) == INTEGER_CST
+	  && (tcode == RSHIFT_EXPR || TYPE_UNSIGNED (TREE_TYPE (op0)))
 	  /* const_binop may not detect overflow correctly,
 	     so check for it explicitly here.  */
 	  && TYPE_PRECISION (TREE_TYPE (size_one_node)) > TREE_INT_CST_LOW (op1)

gcc/testsuite/ChangeLog

+2004-07-22  Joseph S. Myers  <jsm@polyomino.org.uk>
+
+	PR c/7284
+	* gcc.c-torture/execute/pr7284-1.c: New test.
+
 2004-07-22  Brian Booth  <bbooth@redhat.com>
 
 	* gcc.dg/tree-ssa/20040721-1.c: New test.

gcc/testsuite/gcc.c-torture/execute/pr7284-1.c

+/* Signed left-shift is implementation-defined in C89 (and see
+   DR#081), not undefined.  Bug 7284 from Al Grant (AlGrant at
+   myrealbox.com).  */
+
+/* { dg-options "-std=c89" } */
+
+extern void abort (void);
+extern void exit (int);
+
+int
+f (int n)
+{
+  return (n << 24) / (1 << 23);
+}
+
+volatile int x = 128;
+
+int
+main (void)
+{
+  if (f(x) != -256)
+    abort ();
+  exit (0);
+}