re PR middle-end/28651 (signed compare incorrectly false for (int)(U+4)<(int)U where U is unsigned INT_MAX (for optimized x86))

2006-08-11  Richard Guenther  <rguenther@suse.de>

	PR middle-end/28651
	* simplify-rtx.c (simplify_const_relational_operation):
	Simplify A CMP B to A - B CMP 0 only for EQ and NE comparison
	codes.

	* gcc.c-torture/execute/pr28651.c: New testcase.

From-SVN: r116079

4 files changed, 44 insertions, 9 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index d42b0aa..92fd1fe 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2006-08-11  Richard Guenther  <rguenther@suse.de>
+
+	PR middle-end/28651
+	* simplify-rtx.c (simplify_const_relational_operation):
+	Simplify A CMP B to A - B CMP 0 only for EQ and NE comparison
+	codes.
+
 2006-08-10  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* tree.c (build1_stat): Also propagate the TREE_CONSTANT and
diff --git a/gcc/simplify-rtx.c b/gcc/simplify-rtx.c
index 3f627e5..8c437e4 100644
--- a/gcc/simplify-rtx.c
+++ b/gcc/simplify-rtx.c
@@ -3733,19 +3733,18 @@ simplify_const_relational_operation (enum rtx_code code,
      a register or a CONST_INT, this can't help; testing for these cases will
      prevent infinite recursion here and speed things up.
 
-     If CODE is an unsigned comparison, then we can never do this optimization,
-     because it gives an incorrect result if the subtraction wraps around zero.
-     ANSI C defines unsigned operations such that they never overflow, and
-     thus such cases can not be ignored; but we cannot do it even for
-     signed comparisons for languages such as Java, so test flag_wrapv.  */
+     We can only do this for EQ and NE comparisons as otherwise we may
+     lose or introduce overflow which we cannot disregard as undefined as
+     we do not know the signedness of the operation on either the left or
+     the right hand side of the comparison.  */
 
-  if (!flag_wrapv && INTEGRAL_MODE_P (mode) && trueop1 != const0_rtx
+  if (INTEGRAL_MODE_P (mode) && trueop1 != const0_rtx
+      && (code == EQ || code == NE)
       && ! ((REG_P (op0) || GET_CODE (trueop0) == CONST_INT)
 	    && (REG_P (op1) || GET_CODE (trueop1) == CONST_INT))
       && 0 != (tem = simplify_binary_operation (MINUS, mode, op0, op1))
-      /* We cannot do this for == or != if tem is a nonzero address.  */
-      && ((code != EQ && code != NE) || ! nonzero_address_p (tem))
-      && code != GTU && code != GEU && code != LTU && code != LEU)
+      /* We cannot do this if tem is a nonzero address.  */
+      && ! nonzero_address_p (tem))
     return simplify_const_relational_operation (signed_condition (code),
 						mode, tem, const0_rtx);
 
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 44a4894..3bc320f 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2006-08-11  Richard Guenther  <rguenther@suse.de>
+
+	PR middle-end/28651
+	* gcc.c-torture/execute/pr28651.c: New testcase.
+
 2006-08-10  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gnat.dg/specs/static_initializer.ads: New test.
diff --git a/gcc/testsuite/gcc.c-torture/execute/pr28651.c b/gcc/testsuite/gcc.c-torture/execute/pr28651.c
new file mode 100644
index 0000000..1262f9f
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/pr28651.c
@@ -0,0 +1,24 @@
+extern void abort (void);
+int
+foo (unsigned int u)
+{
+  return (int)(u + 4) < (int)u;
+}
+
+int
+main (int argc, char *argv[])
+{
+  unsigned int u;
+
+  /* Run with no arguments so u will be MAX_INT and the optimizers
+     won't know its value.  */
+  if (argc > 1)
+    u = 1;
+  else
+    u = 0x7fffffff;
+
+  if (foo (u) == 0)
+    abort();
+  return 0;
+}
+