Re: [PATCH] Fix (-A) - B -> (-B) - A optimization in fold_binary_loc (PR tree-optimization/83269)

[Jakub Jelinek <jakub at redhat dot com>]

On Fri, Dec 15, 2017 at 09:38:52AM +0100, Richard Biener wrote:
> On Thu, 14 Dec 2017, Jakub Jelinek wrote:
> 
> > Hi!
> > 
> > As the following testcase shows, the (-A) - B -> (-B) - A optimization can't
> > be done the way it is if the negation of A is performed in type with
> > wrapping behavior while the subtraction is done in signed type (with the
> > same precision), as if A is (unsigned) INT_MIN, then (int) -(unsigned) INT_MIN
> > is INT_MIN and INT_MIN - B is different from (-B) - INT_MIN.
> > The reason we can see this is because we check that arg0 is NEGATE_EXPR, but
> > arg0 is STRIP_NOPS from op0.  If the NEGATE_EXPR is already done in signed
> > type, then it would be already UB if A was INT_MIN and so we can safely do
> > it.
> > 
> > Whether we perform the subtraction in the unsigned type or just don't
> > optimize I think doesn't matter that much, at least the only spot during
> > x86_64-linux and i686-linux bootstraps/regtests this new condition triggered
> > was the new testcase, nothing else.  So if you instead prefer to punt, I can
> > tweak the patch, move the negated condition to the if above it.
> > 
> > Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
> 
> I think a better fix would be to just check TREE_CODE (op0) == NEGATE_EXPR
> and use op0, like we do for op1 (probably fixed that earlier).  I'd rather
> not complicate the fold-const.c code more at this point.

That would regress the case when type is unsigned.  If you don't want to
complicate fold-const.c, my preference would be to add the extra && !, it
isn't that much.

Of course, a question is why this optimization hasn't been moved to match.pd
when others had been.

2017-12-15  Jakub Jelinek  <jakub@redhat.com>

	PR tree-optimization/83269
	* fold-const.c (fold_binary_loc): Perform (-A) - B -> (-B) - A
	subtraction in arg0's type if type is signed and arg0 is unsigned.
	Formatting fix.

	* gcc.c-torture/execute/pr83269.c: New test.

--- gcc/fold-const.c.jj	2017-12-08 00:50:27.000000000 +0100
+++ gcc/fold-const.c	2017-12-14 17:42:31.221398170 +0100
@@ -9098,8 +9098,8 @@ expr_not_equal_to (tree t, const wide_in
    return NULL_TREE.  */
 
 tree
-fold_binary_loc (location_t loc,
-	     enum tree_code code, tree type, tree op0, tree op1)
+fold_binary_loc (location_t loc, enum tree_code code, tree type,
+		 tree op0, tree op1)
 {
   enum tree_code_class kind = TREE_CODE_CLASS (code);
   tree arg0, arg1, tem;
@@ -9769,11 +9769,18 @@ fold_binary_loc (location_t loc,
 
       /* (-A) - B -> (-B) - A  where B is easily negated and we can swap.  */
       if (TREE_CODE (arg0) == NEGATE_EXPR
-	  && negate_expr_p (op1))
-	return fold_build2_loc (loc, MINUS_EXPR, type,
-				negate_expr (op1),
-				fold_convert_loc (loc, type,
-						  TREE_OPERAND (arg0, 0)));
+	  && negate_expr_p (op1)
+	  /* If arg0 is e.g. unsigned int and type is int, then this could
+	     introduce UB, because if A is INT_MIN at runtime, the original
+	     expression can be well defined while the latter is not.
+	     See PR83269.  */
+	  && !(ANY_INTEGRAL_TYPE_P (type)
+	       && TYPE_OVERFLOW_UNDEFINED (type)
+	       && ANY_INTEGRAL_TYPE_P (TREE_TYPE (arg0))
+	       && !TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0))))
+	return fold_build2_loc (loc, MINUS_EXPR, type, negate_expr (op1),
+			        fold_convert_loc (loc, type,
+						  TREE_OPERAND (arg0, 0)));
 
       /* Fold __complex__ ( x, 0 ) - __complex__ ( 0, y ) to
 	 __complex__ ( x, -y ).  This is not the same for SNaNs or if
--- gcc/testsuite/gcc.c-torture/execute/pr83269.c.jj	2017-12-14 17:43:24.534710997 +0100
+++ gcc/testsuite/gcc.c-torture/execute/pr83269.c	2017-12-14 17:43:10.000000000 +0100
@@ -0,0 +1,14 @@
+/* PR tree-optimization/83269 */
+
+int
+main ()
+{
+#if __SIZEOF_INT__ == 4 && __SIZEOF_LONG_LONG__ > 4 && __CHAR_BIT__ == 8
+  volatile unsigned char a = 1;
+  long long b = 0x80000000L;
+  int c = -((int)(-b) - (-0x7fffffff * a));
+  if (c != 1)
+    __builtin_abort ();
+#endif
+  return 0;
+}


	Jakub