[HLSL] Implement array temporary support (#79382)

HLSL constant sized array function parameters do not decay to pointers.
Instead constant sized array types are preserved as unique types for
overload resolution, template instantiation and name mangling.

This implements the change by adding a new `ArrayParameterType` which
represents a non-decaying `ConstantArrayType`. The new type behaves the
same as `ConstantArrayType` except that it does not decay to a pointer.

Values of `ConstantArrayType` in HLSL decay during overload resolution
via a new `HLSLArrayRValue` cast to `ArrayParameterType`.

`ArrayParamterType` values are passed indirectly by-value to functions
in IR generation resulting in callee generated memcpy instructions.

The behavior of HLSL function calls is documented in the [draft language
specification](https://microsoft.github.io/hlsl-specs/specs/hlsl.pdf)
under the Expr.Post.Call heading.

Additionally the design of this implementation approach is documented in
[Clang's
documentation](https://clang.llvm.org/docs/HLSL/FunctionCalls.html)

Resolves #70123

1 files changed, 4 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGExprAgg.cpp b/clang/lib/CodeGen/CGExprAgg.cpp
index 143855a..1b9287e 100644
--- a/clang/lib/CodeGen/CGExprAgg.cpp
+++ b/clang/lib/CodeGen/CGExprAgg.cpp
@@ -883,6 +883,9 @@ void AggExprEmitter::VisitCastExpr(CastExpr *E) {
 
     [[fallthrough]];
 
+  case CK_HLSLArrayRValue:
+    Visit(E->getSubExpr());
+    break;
 
   case CK_NoOp:
   case CK_UserDefinedConversion:
@@ -1524,6 +1527,7 @@ static bool castPreservesZero(const CastExpr *CE) {
   case CK_LValueToRValue:
   case CK_LValueToRValueBitCast:
   case CK_UncheckedDerivedToBase:
+  case CK_HLSLArrayRValue:
     return false;
   }
   llvm_unreachable("Unhandled clang::CastKind enum");