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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
This commit is contained in:
Richard Trieu 2016-08-05 02:39:30 +00:00
parent a425623dd9
commit 7561ed01cb
3 changed files with 19 additions and 1 deletions
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6
clang/lib/Sema/SemaChecking.cpp
View File

@ -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;

2
clang/test/Sema/bitfield.c
View File

@ -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.

12
clang/test/Sema/constant-conversion.c
View File

@ -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}}
}