Re: [patch] Fix PR tree-optimization/80426

[Eric Botcazou <ebotcazou at adacore dot com>]

> I know this attempts to be a copy of what is used elsewhere, but
> at least there it is a result of wi::sub etc.
> Wouldn't it be simpler to
>   if (sgn == SIGNED && wi::neg_p (min_op1) && wi::neg_p (wmin))
>     min_ovf = 1;
>   else if (sgn == UNSIGNED && wi::ne_p (min_op1, 0))
>     min_ovf = -1;
> 
> I mean, for SIGNED if min_op1 is 0, then wmin is 0 to and we want
> min_ovf = 0;
> If it is positive, wmin will be surely negative and again we want
> min_ovf = 0.  Only if it is negative and its negation is negative
> too we want min_ovf = 1 (i.e. wi::cmps (0, most_negative) result).
> For UNSIGNED, if min_op1 is 0, again all 3 wi::cmp will yield
> 0 and min_ovf = 0.  If it is non-zero, it is > 0, therefore it
> the first wi::cmp will return -1, the second wi::cmp returns
> 1 and the third one -1.

Fine with me.

> Is that what we want (e.g. the UNSIGNED case to overflow pretty much always
> except for 0 which should be optimized away anyway)?

I think so, you'd better be very cautious with overflow and symbolic ranges.

> Or, shouldn't we just set if (!min_op0 && min_op1 && minus_p) min_op0 =
> build_int_cst (expr_type, 0); before the if (min_op0 && min_op1) case
> and don't duplicate that?

This isn't better than my version IMO.

Tested on x86_64-suse-linux, OK for mainline and 6 branch?


2017-04-19  Eric Botcazou  <ebotcazou@adacore.com>
            Jakub Jelinek  <jakub@redhat.com>

        PR tree-optimization/80426
        * tree-vrp.c (extract_range_from_binary_expr_1): For an additive
        operation on symbolic operands, also compute the overflow for the
        invariant part when the operation degenerates into a negation.

-- 
Eric Botcazou

Index: tree-vrp.c
===================================================================
--- tree-vrp.c	(revision 246960)
+++ tree-vrp.c	(working copy)
@@ -2461,7 +2461,20 @@ extract_range_from_binary_expr_1 (value_
 	  else if (min_op0)
 	    wmin = min_op0;
 	  else if (min_op1)
-	    wmin = minus_p ? wi::neg (min_op1) : min_op1;
+	    {
+	      if (minus_p)
+		{
+		  wmin = wi::neg (min_op1);
+
+		  /* Check for overflow.  */
+		  if (sgn == SIGNED && wi::neg_p (min_op1) && wi::neg_p (wmin))
+		    min_ovf = 1;
+		  else if (sgn == UNSIGNED && wi::ne_p (min_op1, 0))
+		    min_ovf = -1;
+		}
+	      else
+		wmin = min_op1;
+	    }
 	  else
 	    wmin = wi::shwi (0, prec);
 
@@ -2489,7 +2502,20 @@ extract_range_from_binary_expr_1 (value_
 	  else if (max_op0)
 	    wmax = max_op0;
 	  else if (max_op1)
-	    wmax = minus_p ? wi::neg (max_op1) : max_op1;
+	    {
+	      if (minus_p)
+		{
+		  wmax = wi::neg (max_op1);
+
+		  /* Check for overflow.  */
+		  if (sgn == SIGNED && wi::neg_p (max_op1) && wi::neg_p (wmax))
+		    max_ovf = 1;
+		  else if (sgn == UNSIGNED && wi::ne_p (max_op1, 0))
+		    max_ovf = -1;
+		}
+	      else
+		wmax = max_op1;
+	    }
 	  else
 	    wmax = wi::shwi (0, prec);