From 2578a3870ef849dc77e98796600181b64ae4fd61 Mon Sep 17 00:00:00 2001
From: Roger Sayle <roger@nextmovesoftware.com>
Date: Fri, 17 Sep 2021 15:57:34 +0100
Subject: [PATCH] PR c/102245: Disable sign-changing optimization for shifts by
 zero.

Respecting Jakub's suggestion that it may be better to warn-on-valid for
"if (x << 0)" as the author might have intended "if (x < 0)" [which will
also warn when x is _Bool], the simplest way to resolve this regression
is to disable the recently added fold transformation for shifts by zero;
these will be optimized later (elsewhere).  Guarding against integer_zerop
is the simplest of three alternatives; the second being to only apply
this transformation to GIMPLE and not GENERIC, and the third (potentially)
being to explicitly handle shifts by zero here, with an (if cond then else),
optimizing the expression to a convert, but awkwardly duplicating a
more general transformation earlier in match.pd's shift simplifications.

2021-09-17  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	PR c/102245
	* match.pd (shift optimizations): Disable recent sign-changing
	optimization for shifts by zero, these will be folded later.

gcc/testsuite/ChangeLog
	PR c/102245
	* gcc.dg/Wint-in-bool-context-4.c: New test case.
---
 gcc/match.pd                                  |  6 ++--
 gcc/testsuite/gcc.dg/Wint-in-bool-context-4.c | 35 +++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 gcc/testsuite/gcc.dg/Wint-in-bool-context-4.c

diff --git a/gcc/match.pd b/gcc/match.pd
index 41f9e6d97f0f..097ed2e5dffd 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3401,13 +3401,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
      (cmp @0 @2)))))
 
 /* Both signed and unsigned lshift produce the same result, so use
-   the form that minimizes the number of conversions.  */
+   the form that minimizes the number of conversions.  Postpone this
+   transformation until after shifts by zero have been folded.  */
 (simplify
  (convert (lshift:s@0 (convert:s@1 @2) INTEGER_CST@3))
  (if (INTEGRAL_TYPE_P (type)
       && tree_nop_conversion_p (type, TREE_TYPE (@0))
       && INTEGRAL_TYPE_P (TREE_TYPE (@2))
-      && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type))
+      && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type)
+      && !integer_zerop (@3))
   (lshift (convert @2) @3)))
 
 /* Simplifications of conversions.  */
diff --git a/gcc/testsuite/gcc.dg/Wint-in-bool-context-4.c b/gcc/testsuite/gcc.dg/Wint-in-bool-context-4.c
new file mode 100644
index 000000000000..0e96dd7bd7c8
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Wint-in-bool-context-4.c
@@ -0,0 +1,35 @@
+/* PR c/102245 */
+/* { dg-options "-Wint-in-bool-context" } */
+/* { dg-do compile } */
+
+_Bool test1(_Bool x)
+{
+  return !(x << 0);  /* { dg-warning "boolean context" } */
+}
+
+_Bool test2(_Bool x)
+{
+  return !(x << 1);  /* { dg-warning "boolean context" } */
+}
+
+_Bool test3(_Bool x, int y)
+{
+  return !(x << y);  /* { dg-warning "boolean context" } */
+}
+
+_Bool test4(int x, int y)
+{
+  return !(x << y);  /* { dg-warning "boolean context" } */
+}
+
+_Bool test5(int x, int y)
+{
+  return !((x << y) << 0);  /* { dg-warning "boolean context" } */
+}
+
+int test6(_Bool x)
+{
+  int v = 0;
+  return (v & ~1L) | (1L & (x << 0));  /* { dg-bogus "boolean context" } */
+}
+
-- 
2.47.2