Tweak to bitfield-overflow warning: don't warn about storing

a positive value into a signed bitfield of the exact width of
the value.

llvm-svn: 118657

2 files changed, 13 insertions, 1 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index caaa08c0bc2..e69ebe2250d 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -2612,7 +2612,14 @@ void AnalyzeAssignment(Sema &S, BinaryOperator *E) {
       if (OriginalWidth > FieldWidth) {
         llvm::APSInt TruncatedValue = Value;
         TruncatedValue.trunc(FieldWidth);
-        TruncatedValue.extend(OriginalWidth);
+
+        // It's fairly common to write values into signed bitfields
+        // that, if sign-extended, would end up becoming a different
+        // value.  We don't want to warn about that.
+        if (Value.isSigned() && Value.isNegative())
+          TruncatedValue.sext(OriginalWidth);
+        else
+          TruncatedValue.zext(OriginalWidth);
 
         if (Value != TruncatedValue) {
           std::string PrettyValue = Value.toString(10);
diff --git a/clang/test/Sema/constant-conversion.c b/clang/test/Sema/constant-conversion.c
index 5f734f3649d..958621266b7 100644
--- a/clang/test/Sema/constant-conversion.c
+++ b/clang/test/Sema/constant-conversion.c
@@ -13,3 +13,8 @@ void test_7809123(void) {
 
   a.i5 = 36; // expected-warning {{implicit truncation from 'int' to bitfield changes value from 36 to 4}}
 }
+
+void test() {
+  struct { int bit : 1; } a;
+  a.bit = 1; // shouldn't warn
+}