3 files changed, 52 insertions, 2 deletions
diff --git a/gcc/match.pd b/gcc/match.pd
index 0f9c34f..35e3d82 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -2395,12 +2395,14 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
   (mult (convert @0) @1)))
 
 /* Narrow integer multiplication by a zero_one_valued_p operand.
-   Multiplication by [0,1] is guaranteed not to overflow.  */
+   Multiplication by [0,1] is guaranteed not to overflow except for
+   1bit signed types.  */
 (simplify
  (convert (mult@0 zero_one_valued_p@1 INTEGER_CST@2))
  (if (INTEGRAL_TYPE_P (type)
       && INTEGRAL_TYPE_P (TREE_TYPE (@0))
-      && TYPE_PRECISION (type) < TYPE_PRECISION (TREE_TYPE (@0)))
+      && TYPE_PRECISION (type) < TYPE_PRECISION (TREE_TYPE (@0))
+      && (TYPE_UNSIGNED (type) || TYPE_PRECISION (type) > 1))
   (mult (convert @1) (convert @2))))
 
 /* (X << C) != 0 can be simplified to X, when C is zero_one_valued_p.
diff --git a/gcc/testsuite/c-c++-common/ubsan/signed1bitfield-1.c b/gcc/testsuite/c-c++-common/ubsan/signed1bitfield-1.c
new file mode 100644
index 0000000..2ba8cf4
--- /dev/null
+++ b/gcc/testsuite/c-c++-common/ubsan/signed1bitfield-1.c
@@ -0,0 +1,25 @@
+/* { dg-do run } */
+/* { dg-options "-O2 -fsanitize=undefined" } */
+
+/* PR tree-optimization/115154 */
+/* This was being miscompiled with -fsanitize=undefined due to
+   `(signed:1)(t*5)` being transformed into `-((signed:1)t)` which
+   is undefined. */
+
+struct s {
+  signed b : 1;
+} f;
+int i = 55;
+__attribute__((noinline))
+void check(int a)
+{
+        if (!a)
+        __builtin_abort();
+}
+int main() {
+    int t = i != 5;
+    t = t*5;
+    f.b = t;
+    int tt = f.b;
+    check(f.b);
+}
diff --git a/gcc/testsuite/gcc.c-torture/execute/signed1bitfield-1.c b/gcc/testsuite/gcc.c-torture/execute/signed1bitfield-1.c
new file mode 100644
index 0000000..ab888ca
--- /dev/null
+++ b/gcc/testsuite/gcc.c-torture/execute/signed1bitfield-1.c
@@ -0,0 +1,23 @@
+/* PR tree-optimization/115154 */
+/* This was being miscompiled to `(signed:1)(t*5)`
+   being transformed into `-((signed:1)t)` which is undefined.
+   Note there is a pattern which removes the negative in some cases
+   which works around the issue.  */
+
+struct {
+  signed b : 1;
+} f;
+int i = 55;
+__attribute__((noinline))
+void check(int a)
+{
+        if (!a)
+        __builtin_abort();
+}
+int main() {
+    int t = i != 5;
+    t = t*5;
+    f.b = t;
+    int tt = f.b;
+    check(f.b);
+}