Allow -1 to assign max value to unsigned bitfields.

Silence the -Wbitfield-constant-conversion warning for when -1 or other
negative values are assigned to unsigned bitfields, provided that the bitfield
is wider than the minimum number of bits needed to encode the negative value.

llvm-svn: 277796

clang/lib/Sema/SemaChecking.cpp

@@ -7827,6 +7827,12 @@ bool AnalyzeBitFieldAssignment(Sema &S, FieldDecl *Bitfield, Expr *Init,
   unsigned OriginalWidth = Value.getBitWidth();
   unsigned FieldWidth = Bitfield->getBitWidthValue(S.Context);
 
+  if (Value.isSigned() && Value.isNegative())
+    if (UnaryOperator *UO = dyn_cast<UnaryOperator>(OriginalInit))
+      if (UO->getOpcode() == UO_Minus)
+        if (isa<IntegerLiteral>(UO->getSubExpr()))
+          OriginalWidth = Value.getMinSignedBits();
+
   if (OriginalWidth <= FieldWidth)
     return false;
 

clang/test/Sema/bitfield.c

@@ -64,7 +64,7 @@ typedef signed Signed;
 
 struct Test5 { unsigned n : 2; } t5;
 // Bitfield is unsigned
-struct Test5 sometest5 = {-1}; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -1 to 3}}
+struct Test5 sometest5 = {-1};
 typedef __typeof__(+t5.n) Signed;  // ... but promotes to signed.
 
 typedef __typeof__(t5.n + 0) Signed; // Arithmetic promotes.

clang/test/Sema/constant-conversion.c

@@ -113,3 +113,15 @@ void test9() {
 
   char array_init[] = { 255, 127, 128, 129, 0 };
 }
+
+void test10() {
+  struct S {
+    unsigned a : 4;
+  } s;
+  s.a = -1;
+  s.a = 15;
+  s.a = -8;
+
+  s.a = -9;  // expected-warning{{implicit truncation from 'int' to bitfield changes value from -9 to 7}}
+  s.a = 16;  // expected-warning{{implicit truncation from 'int' to bitfield changes value from 16 to 0}}
+}