Re: VRP: x+1 and -x cannot be INT_MIN

[Martin Sebor <msebor@gmail.com>]

On 11/11/2017 03:03 PM, Marc Glisse wrote:
> Hello,
>
> with undefined overflow, just because we know nothing about one of the
> arguments of an addition doesn't mean we can't say something about the
> result. We could constrain more the cases where we replace VR_VARYING
> with a full VR_RANGE, but I didn't want to duplicate too much logic.
>
> The 20040409 testcases were introduced to test an RTL transformation, so
> I don't feel too bad adding -fwrapv to work around the undefined
> overflows they exhibit.
>
> Bootstrap+regtest on powerpc64le-unknown-linux-gnu.
>
> 2017-11-13  Marc Glisse  <marc.glisse@inria.fr>
>
> gcc/
>     * tree-vrp.c (extract_range_from_binary_expr_1) [PLUS_EXPR,
>     MINUS_EXPR]: Use a full range for VR_VARYING.
>
> gcc/testsuite/
>     PR testsuite/82951
>     * gcc.c-torture/execute/20040409-1.c: Use -fwrapv.
>     * gcc.c-torture/execute/20040409-2.c: Likewise.
>     * gcc.c-torture/execute/20040409-3.c: Likewise.
>     * gcc.dg/tree-ssa/vrp118.c: New file.
>

I'm curious about the 4 in the added test case (copied below).
Is there some significance to it or is actually meant to be
(or could it be) a 2?

FWIW, if there's some significance to the 4 then it would be
nice to have a comment there explaining it.  If there isn't
any then may I suggest to either change it to 2 or, perhaps
even better, change the second if condition to
(x < -__INT_MAX__ + 3) to make the effect on the range of
x clear and (presumably) also obviate questions like this
one.

Martin

+++ gcc/testsuite/gcc.dg/tree-ssa/vrp118.c	(working copy)
@@ -0,0 +1,13 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+void eliminate_me();
+void f(int x,int y){
+    if (y < 4)
+      __builtin_unreachable();
+    x += y;
+    if (x == -__INT_MAX__)
+      eliminate_me ();
+}
+
+/* { dg-final { scan-tree-dump-not "eliminate_me" "optimized" } } */