[SDAGBuilder] Handle multi-part arguments in argument copy elision (P…

…R63430)

When eliding an argument copy, we need to update the chain to ensure
the argument reads are performed before later writes. However, the
code doing this only handled this for the first part of the argument.
If the argument had multiple parts, the chains of the later parts were
dropped. Make sure we preserve all chains.

Fixes #63430.

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -10616,9 +10616,9 @@ static void tryToElideArgumentCopy(
     DenseMap<int, int> &ArgCopyElisionFrameIndexMap,
     SmallPtrSetImpl<const Instruction *> &ElidedArgCopyInstrs,
     ArgCopyElisionMapTy &ArgCopyElisionCandidates, const Argument &Arg,
-    SDValue ArgVal, bool &ArgHasUses) {
+    ArrayRef<SDValue> ArgVals, bool &ArgHasUses) {
   // Check if this is a load from a fixed stack object.
-  auto *LNode = dyn_cast<LoadSDNode>(ArgVal);
+  auto *LNode = dyn_cast<LoadSDNode>(ArgVals[0]);
   if (!LNode)
     return;
   auto *FINode = dyn_cast<FrameIndexSDNode>(LNode->getBasePtr().getNode());
@@ -10661,7 +10661,8 @@ static void tryToElideArgumentCopy(
   MFI.setIsImmutableObjectIndex(FixedIndex, false);
   AllocaIndex = FixedIndex;
   ArgCopyElisionFrameIndexMap.insert({OldIndex, FixedIndex});
-  Chains.push_back(ArgVal.getValue(1));
+  for (SDValue ArgVal : ArgVals)
+    Chains.push_back(ArgVal.getValue(1));
 
   // Avoid emitting code for the store implementing the copy.
   const StoreInst *SI = ArgCopyIter->second.second;
@@ -10922,9 +10923,14 @@ void SelectionDAGISel::LowerArguments(const Function &F) {
     // Elide the copying store if the target loaded this argument from a
     // suitable fixed stack object.
     if (Ins[i].Flags.isCopyElisionCandidate()) {
+      unsigned NumParts = 0;
+      for (EVT VT : ValueVTs)
+        NumParts += TLI->getNumRegistersForCallingConv(*CurDAG->getContext(),
+                                                       F.getCallingConv(), VT);
+
       tryToElideArgumentCopy(*FuncInfo, Chains, ArgCopyElisionFrameIndexMap,
                              ElidedArgCopyInstrs, ArgCopyElisionCandidates, Arg,
-                             InVals[i], ArgHasUses);
+                             ArrayRef(&InVals[i], NumParts), ArgHasUses);
     }
 
     // If this argument is unused then remember its value. It is used to generate

llvm/test/CodeGen/X86/pr63430.ll

@@ -1,14 +1,14 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --no_x86_scrub_sp --version 2
 ; RUN: llc -mtriple=x86_64-unknown-linux < %s | FileCheck %s
 
-; TODO: This is a miscompile.
+; Make sure the argument is read before the stack slot is over-written.
 define i1 @test(ptr %a0, ptr %a1, ptr %a2, ptr %a3, ptr %a4, ptr %a5, i128 %x) {
 ; CHECK-LABEL: test:
 ; CHECK:       # %bb.0:
 ; CHECK-NEXT:    movq 8(%rsp), %rax
 ; CHECK-NEXT:    xorps %xmm0, %xmm0
-; CHECK-NEXT:    movaps %xmm0, 8(%rsp)
 ; CHECK-NEXT:    andq 16(%rsp), %rax
+; CHECK-NEXT:    movaps %xmm0, 8(%rsp)
 ; CHECK-NEXT:    cmpq $-1, %rax
 ; CHECK-NEXT:    sete %al
 ; CHECK-NEXT:    retq