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.

2 files changed, 39 insertions, 2 deletions

diff --git a/gcc/match.pd b/gcc/match.pd
index 41f9e6d..097ed2e 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 0000000..0e96dd7
--- /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" } */
+}
+