[C++2a] Add __builtin_bit_cast, used to implement std::bit_cast
This commit adds a new builtin, __builtin_bit_cast(T, v), which performs a
bit_cast from a value v to a type T. This expression can be evaluated at
compile time under specific circumstances.
The compile time evaluation currently doesn't support bit-fields, but I'm
planning on fixing this in a follow up (some of the logic for figuring this out
is in CodeGen). I'm also planning follow-ups for supporting some more esoteric
types that the constexpr evaluator supports, as well as extending
__builtin_memcpy constexpr evaluation to use the same infrastructure.
rdar://44987528
Differential revision: https://reviews.llvm.org/D62825
llvm-svn: 364954
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index fe31f83..695facd 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -711,6 +711,25 @@
break;
}
+ case CK_LValueToRValueBitCast: {
+ if (Dest.isIgnored()) {
+ CGF.EmitAnyExpr(E->getSubExpr(), AggValueSlot::ignored(),
+ /*ignoreResult=*/true);
+ break;
+ }
+
+ LValue SourceLV = CGF.EmitLValue(E->getSubExpr());
+ Address SourceAddress =
+ Builder.CreateElementBitCast(SourceLV.getAddress(), CGF.Int8Ty);
+ Address DestAddress =
+ Builder.CreateElementBitCast(Dest.getAddress(), CGF.Int8Ty);
+ llvm::Value *SizeVal = llvm::ConstantInt::get(
+ CGF.SizeTy,
+ CGF.getContext().getTypeSizeInChars(E->getType()).getQuantity());
+ Builder.CreateMemCpy(DestAddress, SourceAddress, SizeVal);
+ break;
+ }
+
case CK_DerivedToBase:
case CK_BaseToDerived:
case CK_UncheckedDerivedToBase: {