Wbitfiled-constant-conversion : allow ~0 , ~(1<<A), etc

[danielmarjamaki]

Summary

To set all bits,~0 is often used.

x.y = ~0;

and to set all bits except (1<<A) this is often used:

x.y = ~(1 << A);

.. do not warn about these.

I think it might make sense to warn about the opposite:

x.y = ~0xffffffff;

But that is a possible false negative for now.

I believe this is somewhat related to r277796.

[danielmarjamaki]

ping

cfe/trunk/lib/Sema/SemaChecking.cpp

bool AnalyzeBitFieldAssignment(Sema &S, FieldDecl *Bitfield, Expr *Init,
 
   llvm::APSInt Value;
   if (!OriginalInit->EvaluateAsInt(Value, S.Context, Expr::SE_AllowSideEffects))
     return false;
 
   unsigned OriginalWidth = Value.getBitWidth();
   unsigned FieldWidth = Bitfield->getBitWidthValue(S.Context);
 
-  if (Value.isSigned() && Value.isNegative())
+  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 (UO->getOpcode() == UO_Minus || UO->getOpcode() == UO_Not)
 
   if (OriginalWidth <= FieldWidth)
     return false;
 
   // Compute the value which the bitfield will contain.
   llvm::APSInt TruncatedValue = Value.trunc(FieldWidth);
   TruncatedValue.setIsSigned(Bitfield->getType()->isSignedIntegerType());
 

cfe/trunk/test/Sema/constant-conversion.c

 
 void test7() {
   struct {
     unsigned int twoBits1:2;
     unsigned int twoBits2:2;
     unsigned int reserved:28;
   } f;
 
-  f.twoBits1 = ~1; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -2 to 2}}
+  f.twoBits1 = ~0; // no-warning
+  f.twoBits1 = ~1; // no-warning
   f.twoBits2 = ~2; // expected-warning {{implicit truncation from 'int' to bitfield changes value from -3 to 1}}
   f.twoBits1 &= ~1; // no-warning
   f.twoBits2 &= ~2; // no-warning
 }
 
 void test8() {
   enum E { A, B, C };
   struct { enum E x : 1; } f;

 #define CHAR_MACRO_HEX 0xff
   char macro_char_hex = CHAR_MACRO_HEX;
 #define CHAR_MACRO_DEC 255
   char macro_char_dec = CHAR_MACRO_DEC;  // expected-warning {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
 
   char array_init[] = { 255, 127, 128, 129, 0 };
 }
 
+#define A 1
+
 void test10() {
   struct S {
     unsigned a : 4;
   } s;
   s.a = -1;
   s.a = 15;
   s.a = -8;
+  s.a = ~0;
+  s.a = ~0U;
+  s.a = ~(1<<A);
 
-  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}}
 }