[clang] [HLSL] Implement array temporary support (PR #79382)

[Eli Friedman via cfe-commits]

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@@ -10524,6 +10524,11 @@ Sema::PerformCopyInitialization(const InitializedEntity &Entity,
   Expr *InitE = Init.get();
   assert(InitE && "No initialization expression?");
 
+  if (LangOpts.HLSL)
+    if (auto AdjTy = dyn_cast<DecayedType>(Entity.getType()))
+      if (AdjTy->getOriginalType()->isConstantArrayType())
+        InitE = HLSLArrayTemporaryExpr::Create(getASTContext(), InitE);
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efriedma-quic wrote:

This implementation is surprising.  If arrays are supposed to be passed by value, decay shouldn't be happening in the first place.

Allowing decay to happen, then trying to fix it up later, has some weird implications: for example, `void f(int[2])` and `void f(int[4])` have the same type.  You introduce pointer types in places where they shouldn't really exist, and casts which aren't really supposed to happen.  And it's not clear to me the non-canonical type wrapper will be preserved in all the cases you need (preserving non-canonical types is best-effort; sometimes we're forced to canonicalize).

As an alternative approach, maybe we can introduce a new type to represent a "function argument of array type".  When we construct the function type, instead of decaying to a pointer, it would "decay" to "function argument of array type". The result is sort of similar, but instead of a non-canonical DecayedType, you have a canonical type which would be reliably preserved across the compiler.  For argument passing, initialization would work similarly to what you're doing now.

https://github.com/llvm/llvm-project/pull/79382