[llvm] [NVPTX] Check Before inserting AddrSpaceCastInst in NVPTXLoweringAlloca (PR #106127)

[weiwei chen via llvm-commits]

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@@ -72,12 +72,26 @@ bool NVPTXLowerAlloca::runOnFunction(Function &F) {
         Changed = true;
         auto ETy = allocaInst->getAllocatedType();
         auto LocalAddrTy = PointerType::get(ETy, ADDRESS_SPACE_LOCAL);
-        auto NewASCToLocal = new AddrSpaceCastInst(allocaInst, LocalAddrTy, "");
-        auto GenericAddrTy = PointerType::get(ETy, ADDRESS_SPACE_GENERIC);
-        auto NewASCToGeneric =
-            new AddrSpaceCastInst(NewASCToLocal, GenericAddrTy, "");
-        NewASCToLocal->insertAfter(allocaInst);
-        NewASCToGeneric->insertAfter(NewASCToLocal);
+        PointerType *AllocInstPtrTy =
+            cast<PointerType>(allocaInst->getType()->getScalarType());
+        Instruction *NewASCToGeneric = allocaInst;
+        unsigned AllocAddrSpace = AllocInstPtrTy->getAddressSpace();
+        assert((AllocAddrSpace == ADDRESS_SPACE_GENERIC ||
+                AllocAddrSpace == ADDRESS_SPACE_LOCAL) &&
+               "AllocaInst can only be in Generic or Local address space for "
+               "NVPTX.");
+        if (AllocAddrSpace != ADDRESS_SPACE_LOCAL) {
+          // Only insert a new AddrSpaceCastInst if
+          // allocaInst is not already in ADDRESS_SPACE_LOCAL.
+          auto NewASCToLocal =
+              new AddrSpaceCastInst(allocaInst, LocalAddrTy, "");
+          auto GenericAddrTy = PointerType::get(ETy, ADDRESS_SPACE_GENERIC);
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weiweichen wrote:

Oh, actually, I found out that `-infer-address-spaces` won't work properly if the extra ASCast to LOCAL is not inserted if `alloca` is already in Generic . So even `alloca` is already in generic, we still need to insert two ASCast to make go to `local` and back to `generic` to make `-infer-address-spaces` work properly (i.e. pass the test) 🤯 🤯 🤯 .

https://github.com/llvm/llvm-project/pull/106127