[clang] [clang] put indirect parameters in the target-requested AS, if appplicable (PR #181256)

[Matt Arsenault via cfe-commits]

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@@ -283,30 +283,47 @@ void AggExprEmitter::withReturnValueSlot(
 
   // If it makes no observable difference, save a memcpy + temporary.
   //
-  // We need to always provide our own temporary if destruction is required.
+  // We need to always provide an address if destruction is required.
   // Otherwise, EmitCall will emit its own, notice that it's "unused", and end
   // its lifetime before we have the chance to emit a proper destructor call.
-  //
-  // We also need a temporary if the destination is in a different address space
-  // from the sret AS. Use the target hook to get the actual sret AS for this
-  // return type.
   const CXXRecordDecl *RD = RetTy->getAsCXXRecordDecl();
-  LangAS SRetLangAS = CGF.CGM.getTargetCodeGenInfo().getSRetAddrSpace(RD);
-  unsigned SRetAS = CGF.getContext().getTargetAddressSpace(SRetLangAS);
-  bool CanAggregateCopy =
-      RD ? (RD->hasTrivialCopyConstructor() ||
-            RD->hasTrivialMoveConstructor() || RD->hasTrivialCopyAssignment() ||
-            RD->hasTrivialMoveAssignment() || RD->hasAttr<TrivialABIAttr>() ||
-            RD->isUnion())
-         : RetTy.isTriviallyCopyableType(CGF.getContext());
-  bool DestASMismatch = !Dest.isIgnored() && CanAggregateCopy &&
-                        Dest.getAddress()
-                                .getBasePointer()
-                                ->stripPointerCasts()
-                                ->getType()
-                                ->getPointerAddressSpace() != SRetAS;
-  bool UseTemp = Dest.isPotentiallyAliased() || Dest.requiresGCollection() ||
-                 (RequiresDestruction && Dest.isIgnored()) || DestASMismatch;
+  // Initially assume we will need to pass through the destination memory.
+  unsigned SRetAS =
+      CGF.getContext().getTargetAddressSpace(RetTy.getAddressSpace());
+  bool DestASMismatch = false;
+  // However, ABIInfo might be permitted to make this return indirect. If it
+  // does, and RetTy doesn't specify an address space, it is also permitted (but
+  // not required) to change the ABI addrspace to alloca. We don't yet know what
+  // it will decide, so make a copy here for the optimizer to remove if it can
+  // prove it unnecessary later.
+  if (!RD || RD->canPassInRegisters()) {
+    if (!RetTy.hasAddressSpace()) {
+      SRetAS = CGF.CGM.getDataLayout().getAllocaAddrSpace();
+      // This will now require a copy unless we can find an underlying object
+      // with the correct addrspace to use for dest.
+      if (!Dest.isIgnored()) {
+        llvm::Value *DestV = Dest.getAddress().getBasePointer();
+        if (DestV->getType()->getPointerAddressSpace() != SRetAS) {
+          DestV = DestV->stripPointerCasts();
+          if (DestV->getType()->getPointerAddressSpace() != SRetAS)
+            DestASMismatch = true;
+        }
+      }
+    }
+  }
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arsenm wrote:

Would be nicer to write this as a helper function / initialize DestASMismatch to an expression 

https://github.com/llvm/llvm-project/pull/181256