[llvm] x86: fix musttail sibcall miscompilation (PR #168956)

[Folkert de Vries via llvm-commits]

================
@@ -2404,21 +2615,40 @@ X86TargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
       FIN = DAG.getFrameIndex(FI, getPointerTy(DAG.getDataLayout()));
 
       if (Flags.isByVal()) {
-        // Copy relative to framepointer.
-        SDValue Source = DAG.getIntPtrConstant(VA.getLocMemOffset(), dl);
-        if (!StackPtr.getNode())
-          StackPtr = DAG.getCopyFromReg(Chain, dl, RegInfo->getStackRegister(),
-                                        getPointerTy(DAG.getDataLayout()));
-        Source = DAG.getNode(ISD::ADD, dl, getPointerTy(DAG.getDataLayout()),
-                             StackPtr, Source);
-
-        MemOpChains2.push_back(
-            CreateCopyOfByValArgument(Source, FIN, Chain, Flags, DAG, dl));
+        SDValue ByValSrc;
+        bool NeedsStackCopy;
+        if (auto It = ByValTemporaries.find(OutsIndex);
+            It != ByValTemporaries.end()) {
+          ByValSrc = It->second;
+          NeedsStackCopy = true;
+        } else {
+          ByValSrc = Arg;
+          NeedsStackCopy = !isTailCall;
+        }
+
+        if (NeedsStackCopy) {
+
+          auto PtrVT = getPointerTy(DAG.getDataLayout());
+          SDValue DstAddr = DAG.getFrameIndex(FI, PtrVT);
+
+          // Copy the struct contents from ByValSrc to DstAddr.
+          MemOpChains2.push_back(CreateCopyOfByValArgument(
+              ByValSrc, DstAddr, Chain, Flags, DAG, dl));
+        }
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folkertdev wrote:

This now works, and prevents needless copies

https://github.com/llvm/llvm-project/pull/168956