From 6483f05989ef31ebf2583deeb4753211ca2b7e17 Mon Sep 17 00:00:00 2001
From: Eugene Rozenfeld <erozen@microsoft.com>
Date: Thu, 5 Nov 2020 20:35:45 -0700
Subject: [PATCH] Simplify x >> x to 0. This fixes PR96701.

gcc/
	* match.pd (x >> x): New pattern.

gcc/testsuite

	* gcc.dg/self-right-shift.c: New test.
---
 gcc/match.pd                            |  5 +++++
 gcc/testsuite/gcc.dg/self-right-shift.c | 12 ++++++++++++
 2 files changed, 17 insertions(+)
 create mode 100644 gcc/testsuite/gcc.dg/self-right-shift.c

diff --git a/gcc/match.pd b/gcc/match.pd
index 17ba04100c7..d82a62052fc 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2925,6 +2925,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
       && (wi::ltu_p (wi::to_wide (@1), element_precision (type))))
   (bit_and @0 (rshift { build_minus_one_cst (type); } @1))))
 
+/* Optimize x >> x into 0 */
+(simplify
+ (rshift @0 @0)
+  { build_zero_cst (type); })
+
 (for shiftrotate (lrotate rrotate lshift rshift)
  (simplify
   (shiftrotate @0 integer_zerop)
diff --git a/gcc/testsuite/gcc.dg/self-right-shift.c b/gcc/testsuite/gcc.dg/self-right-shift.c
new file mode 100644
index 00000000000..c457ee53ec4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/self-right-shift.c
@@ -0,0 +1,12 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+/* Self right-shift should be optimized to 0. */
+
+int
+foo (int i)
+{
+  return i >> i;
+}
+
+/* { dg-final { scan-tree-dump "return 0;" "optimized" } } */
-- 
2.30.2