Re: [PATCH] tree-optimization/94899: Remove "+ 0x80000000" in int comparisons

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Feb 1, 2022 at 5:54 AM Arjun Shankar via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Expressions of the form "X + CST < Y + CST" where X and Y are of int
> type and CST is of unsigned type with only the MSB on can be simplified
> to "X < Y" because "X + 0x80000000" increases monotonically with X.

+/* As a special case, X + C < Y + C is the same as X < Y even with wrapping
+   overflow if X and Y are signed integers of the same size, and C is an
+   unsigned constant with all bits except MSB set to 0 and size >= that of
+   X/Y.  */
+(for op (lt le ge gt)
+ (simplify
+  (op (plus:c (convert@0 @1) @4) (plus:c (convert@2 @3) @4))
+  (if (CONSTANT_CLASS_P (@4)
+       && TYPE_UNSIGNED (TREE_TYPE (@4))

why include (convert ..) here?  It looks like you could do without,
merging the special case with the preceding pattern and let a followup
pattern simplify (lt (convert @1) (convert @2)) instead?

> gcc/
>         * match.pd (X + C < Y + C -> X < Y, if C is 0x80000000): New
>         simplification.
> gcc/testsuite/
>         * gcc.dg/pr94899.c: New test.
> ---
>  gcc/match.pd                   | 18 ++++++++++++++++++
>  gcc/testsuite/gcc.dg/pr94899.c | 28 ++++++++++++++++++++++++++++
>  2 files changed, 46 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/pr94899.c