Create a new 'flag_enum' attribute.
This attribute serves as a hint to improve warnings about the ranges of
enumerators used as flag types. It currently has no working C++ implementation
due to different semantics for enums in C++. For more explanation, see the docs
and testcases.
Reviewed by Aaron Ballman.
llvm-svn: 222906
diff --git a/clang/lib/Sema/SemaDecl.cpp b/clang/lib/Sema/SemaDecl.cpp
index d299ec1..f75a2ce 100644
--- a/clang/lib/Sema/SemaDecl.cpp
+++ b/clang/lib/Sema/SemaDecl.cpp
@@ -13501,6 +13501,49 @@
}
}
+bool
+Sema::IsValueInFlagEnum(const EnumDecl *ED, const llvm::APInt &Val,
+ bool AllowMask) const {
+ FlagEnumAttr *FEAttr = ED->getAttr<FlagEnumAttr>();
+ assert(FEAttr && "looking for value in non-flag enum");
+
+ llvm::APInt FlagMask = ~FEAttr->getFlagBits();
+ unsigned Width = FlagMask.getBitWidth();
+
+ // We will try a zero-extended value for the regular check first.
+ llvm::APInt ExtVal = Val.zextOrSelf(Width);
+
+ // A value is in a flag enum if either its bits are a subset of the enum's
+ // flag bits (the first condition) or we are allowing masks and the same is
+ // true of its complement (the second condition). When masks are allowed, we
+ // allow the common idiom of ~(enum1 | enum2) to be a valid enum value.
+ //
+ // While it's true that any value could be used as a mask, the assumption is
+ // that a mask will have all of the insignificant bits set. Anything else is
+ // likely a logic error.
+ if (!(FlagMask & ExtVal))
+ return true;
+
+ if (AllowMask) {
+ // Try a one-extended value instead. This can happen if the enum is wider
+ // than the constant used, in C with extensions to allow for wider enums.
+ // The mask will still have the correct behaviour, so we give the user the
+ // benefit of the doubt.
+ //
+ // FIXME: This heuristic can cause weird results if the enum was extended
+ // to a larger type and is signed, because then bit-masks of smaller types
+ // that get extended will fall out of range (e.g. ~0x1u). We currently don't
+ // detect that case and will get a false positive for it. In most cases,
+ // though, it can be fixed by making it a signed type (e.g. ~0x1), so it may
+ // be fine just to accept this as a warning.
+ ExtVal |= llvm::APInt::getHighBitsSet(Width, Width - Val.getBitWidth());
+ if (!(FlagMask & ~ExtVal))
+ return true;
+ }
+
+ return false;
+}
+
void Sema::ActOnEnumBody(SourceLocation EnumLoc, SourceLocation LBraceLoc,
SourceLocation RBraceLoc, Decl *EnumDeclX,
ArrayRef<Decl *> Elements,
@@ -13586,10 +13629,8 @@
BestPromotionType = Context.getPromotedIntegerType(BestType);
else
BestPromotionType = BestType;
- // We don't need to set BestWidth, because BestType is going to be the type
- // of the enumerators, but we do anyway because otherwise some compilers
- // warn that it might be used uninitialized.
- BestWidth = CharWidth;
+
+ BestWidth = Context.getIntWidth(BestType);
}
else if (NumNegativeBits) {
// If there is a negative value, figure out the smallest integer type (of
@@ -13654,10 +13695,15 @@
}
}
+ FlagEnumAttr *FEAttr = Enum->getAttr<FlagEnumAttr>();
+ if (FEAttr)
+ FEAttr->getFlagBits() = llvm::APInt(BestWidth, 0);
+
// Loop over all of the enumerator constants, changing their types to match
- // the type of the enum if needed.
- for (unsigned i = 0, e = Elements.size(); i != e; ++i) {
- EnumConstantDecl *ECD = cast_or_null<EnumConstantDecl>(Elements[i]);
+ // the type of the enum if needed. If we have a flag type, we also prepare the
+ // FlagBits cache.
+ for (auto *D : Elements) {
+ auto *ECD = cast_or_null<EnumConstantDecl>(D);
if (!ECD) continue; // Already issued a diagnostic.
// Standard C says the enumerators have int type, but we allow, as an
@@ -13687,7 +13733,7 @@
// enum-specifier, each enumerator has the type of its
// enumeration.
ECD->setType(EnumType);
- continue;
+ goto flagbits;
} else {
NewTy = BestType;
NewWidth = BestWidth;
@@ -13714,8 +13760,32 @@
ECD->setType(EnumType);
else
ECD->setType(NewTy);
+
+flagbits:
+ // Check to see if we have a constant with exactly one bit set. Note that x
+ // & (x - 1) will be nonzero if and only if x has more than one bit set.
+ if (FEAttr) {
+ llvm::APInt ExtVal = InitVal.zextOrSelf(BestWidth);
+ if (ExtVal != 0 && !(ExtVal & (ExtVal - 1))) {
+ FEAttr->getFlagBits() |= ExtVal;
+ }
+ }
}
+ if (FEAttr) {
+ for (Decl *D : Elements) {
+ EnumConstantDecl *ECD = cast_or_null<EnumConstantDecl>(D);
+ if (!ECD) continue; // Already issued a diagnostic.
+
+ llvm::APSInt InitVal = ECD->getInitVal();
+ if (InitVal != 0 && !IsValueInFlagEnum(Enum, InitVal, true))
+ Diag(ECD->getLocation(), diag::warn_flag_enum_constant_out_of_range)
+ << ECD << Enum;
+ }
+ }
+
+
+
Enum->completeDefinition(BestType, BestPromotionType,
NumPositiveBits, NumNegativeBits);