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

[Richard Biener <richard dot guenther at gmail dot com>]

On Sat, Nov 11, 2017 at 11:03 PM, Marc Glisse <marc.glisse@inria.fr> 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.

Index: gcc/testsuite/gcc.c-torture/execute/20040409-1.c
===================================================================
--- gcc/testsuite/gcc.c-torture/execute/20040409-1.c    (revision 254629)
+++ gcc/testsuite/gcc.c-torture/execute/20040409-1.c    (working copy)
@@ -1,10 +1,12 @@
+/* { dg-options "-fwrapv" } */
+

I think you should use dg-additional-options (if that works).  As said in the PR
it would be safest to copy the tests, add -fwrapv and just remove the -fno-wrapv
cases that do not work.

I think a better fix would be in the caller of extract_range_from_binary_expr_1,
like simply always replacing VARYING with [min,max] if either of the two
ranges is not varying.  In vr_values::extract_range_from_binary_expr that is,
and doing an early out for varying & varying in _1.  Might simplify some
special case code for other opts as well.

Richard.

> 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.
>
> --
> Marc Glisse