tree-ssa-forwprop.c (associate_plusminus): For widening conversions check for undefined overflow in (T)(P + A) - (T)P -> (T)A.

2014-06-25  Bernd Edlinger  <bernd.edlinger@hotmail.de>

        * tree-ssa-forwprop.c (associate_plusminus): For widening conversions
        check for undefined overflow in (T)(P + A) - (T)P -> (T)A.
        Issue a strict overflow warning if appropriate.

testsuite:
2014-06-25  Bernd Edlinger  <bernd.edlinger@hotmail.de>

        * gcc.c-torture/execute/20140622-1.c: New test.

From-SVN: r211988

gcc/ChangeLog

@@ -1,3 +1,9 @@
+2014-06-25  Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	* tree-ssa-forwprop.c (associate_plusminus): For widening conversions
+	check for undefined overflow in (T)(P + A) - (T)P -> (T)A.
+	Issue a strict overflow warning if appropriate.
+
 2014-06-25  Martin Liska  <mliska@suse.cz>
 
 	IPA REF refactoring

gcc/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2014-06-25  Bernd Edlinger  <bernd.edlinger@hotmail.de>
+
+	* gcc.c-torture/execute/20140622-1.c: New test.
+
 2014-06-25  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	DR 178

gcc/testsuite/gcc.c-torture/execute/20140622-1.c

@@ -0,0 +1,27 @@
+unsigned p;
+
+long __attribute__((noinline, noclone))
+test (unsigned a)
+{
+  return (long)(p + a) - (long)p;
+}
+
+int
+main ()
+{
+  p = (unsigned) -2;
+  if (test (0) != 0)
+    __builtin_abort ();
+  if (test (1) != 1)
+    __builtin_abort ();
+  if (test (2) != -(long)(unsigned)-2)
+    __builtin_abort ();
+  p = (unsigned) -1;
+  if (test (0) != 0)
+    __builtin_abort ();
+  if (test (1) != -(long)(unsigned)-1)
+    __builtin_abort ();
+  if (test (2) != -(long)(unsigned)-2)
+    __builtin_abort ();
+  return 0;
+}

gcc/tree-ssa-forwprop.c

@@ -47,6 +47,7 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-pass.h"
 #include "langhooks.h"
 #include "flags.h"
+#include "diagnostic.h"
 #include "expr.h"
 #include "cfgloop.h"
 #include "optabs.h"
@@ -2666,15 +2667,35 @@ associate_plusminus (gimple_stmt_iterator *gsi)
 		    {
 		      /* And finally (T)(P + A) - (T)P.  */
 		      tree a = gimple_assign_rhs2 (def_stmt2);
-		      /* For pointer types, if the conversion of A to the final
+		      if (TYPE_PRECISION (TREE_TYPE (rhs1))
-			 type requires a sign- or zero-extension, then we have
+			  <= TYPE_PRECISION (TREE_TYPE (a))
-			 to punt - it is not defined which one is correct.  */
+			  /* For integer types, if A has a smaller type
-		      if (!POINTER_TYPE_P (TREE_TYPE (rhs1))
+			     than T the result depends on the possible
-			  || TYPE_PRECISION (TREE_TYPE (rhs1))
+			     overflow in P + A.
-			     <= TYPE_PRECISION (TREE_TYPE (a))
+			     E.g. T=size_t, A=(unsigned)429497295, P>0.
-			  || (TREE_CODE (a) == INTEGER_CST
+			     However, if an overflow in P + A would cause
+			     undefined behavior, we can assume that there
+			     is no overflow.  */
+			  || (INTEGRAL_TYPE_P (TREE_TYPE (p))
+			      && TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (p)))
+			  /* For pointer types, if the conversion of A to the
+			     final type requires a sign- or zero-extension,
+			     then we have to punt - it is not defined which
+			     one is correct.  */
+			  || (POINTER_TYPE_P (TREE_TYPE (p))
+			      && TREE_CODE (a) == INTEGER_CST
 			      && tree_int_cst_sign_bit (a) == 0))
 			{
+			  if (issue_strict_overflow_warning
+			      (WARN_STRICT_OVERFLOW_MISC)
+			      && TYPE_PRECISION (TREE_TYPE (rhs1))
+				 > TYPE_PRECISION (TREE_TYPE (a))
+			      && INTEGRAL_TYPE_P (TREE_TYPE (p)))
+			    warning_at (gimple_location (stmt),
+					OPT_Wstrict_overflow,
+					"assuming signed overflow does not "
+					"occur when assuming that "
+					"(T)(P + A) - (T)P is always (T)A");
 			  if (useless_type_conversion_p (TREE_TYPE (rhs1),
 							 TREE_TYPE (a)))
 			    code = TREE_CODE (a);