[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

5 files changed, 39 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index abe10d383dc..4e97fba2621 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -4209,6 +4209,7 @@ LValue CodeGenFunction::EmitCastLValue(const CastExpr *E) {
   switch (E->getCastKind()) {
   case CK_ToVoid:
   case CK_BitCast:
+  case CK_LValueToRValueBitCast:
   case CK_ArrayToPointerDecay:
   case CK_FunctionToPointerDecay:
   case CK_NullToMemberPointer:
diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index fe31f836286..695facd50b6 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -711,6 +711,25 @@ void AggExprEmitter::VisitCastExpr(CastExpr *E) {
     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: {
diff --git a/clang/lib/CodeGen/CGExprComplex.cpp b/clang/lib/CodeGen/CGExprComplex.cpp
index 4906b632925..6a5fb45ba25 100644
--- a/clang/lib/CodeGen/CGExprComplex.cpp
+++ b/clang/lib/CodeGen/CGExprComplex.cpp
@@ -464,6 +464,15 @@ ComplexPairTy ComplexExprEmitter::EmitCast(CastKind CK, Expr *Op,
     return EmitLoadOfLValue(CGF.MakeAddrLValue(V, DestTy), Op->getExprLoc());
   }
 
+  case CK_LValueToRValueBitCast: {
+    LValue SourceLVal = CGF.EmitLValue(Op);
+    Address Addr = Builder.CreateElementBitCast(SourceLVal.getAddress(),
+                                                CGF.ConvertTypeForMem(DestTy));
+    LValue DestLV = CGF.MakeAddrLValue(Addr, DestTy);
+    DestLV.setTBAAInfo(TBAAAccessInfo::getMayAliasInfo());
+    return EmitLoadOfLValue(DestLV, Op->getExprLoc());
+  }
+
   case CK_BitCast:
   case CK_BaseToDerived:
   case CK_DerivedToBase:
diff --git a/clang/lib/CodeGen/CGExprConstant.cpp b/clang/lib/CodeGen/CGExprConstant.cpp
index 4a0494295d2..9f4b686ad90 100644
--- a/clang/lib/CodeGen/CGExprConstant.cpp
+++ b/clang/lib/CodeGen/CGExprConstant.cpp
@@ -1115,6 +1115,7 @@ public:
     case CK_ToVoid:
     case CK_Dynamic:
     case CK_LValueBitCast:
+    case CK_LValueToRValueBitCast:
     case CK_NullToMemberPointer:
     case CK_UserDefinedConversion:
     case CK_CPointerToObjCPointerCast:
diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 5e979c0de97..ef6b84737b0 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -2033,6 +2033,15 @@ Value *ScalarExprEmitter::VisitCastExpr(CastExpr *CE) {
     return EmitLoadOfLValue(LV, CE->getExprLoc());
   }
 
+  case CK_LValueToRValueBitCast: {
+    LValue SourceLVal = CGF.EmitLValue(E);
+    Address Addr = Builder.CreateElementBitCast(SourceLVal.getAddress(),
+                                                CGF.ConvertTypeForMem(DestTy));
+    LValue DestLV = CGF.MakeAddrLValue(Addr, DestTy);
+    DestLV.setTBAAInfo(TBAAAccessInfo::getMayAliasInfo());
+    return EmitLoadOfLValue(DestLV, CE->getExprLoc());
+  }
+
   case CK_CPointerToObjCPointerCast:
   case CK_BlockPointerToObjCPointerCast:
   case CK_AnyPointerToBlockPointerCast: