[llvm] [CodeGen] Handling Oversized Alloca Types under 32 bit Mode to Avoid Code Generator Crash (PR #71472)

[Qiongsi Wu via llvm-commits]

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@@ -4138,9 +4138,10 @@ void SelectionDAGBuilder::visitAlloca(const AllocaInst &I) {
                                           APInt(IntPtr.getScalarSizeInBits(),
                                                 TySize.getKnownMinValue())));
   else
-    AllocSize =
-        DAG.getNode(ISD::MUL, dl, IntPtr, AllocSize,
-                    DAG.getConstant(TySize.getFixedValue(), dl, IntPtr));
+    AllocSize = DAG.getNode(ISD::MUL, dl, IntPtr, AllocSize,
+                            DAG.getConstant(APInt(IntPtr.getScalarSizeInBits(),
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qiongsiwu wrote:

It seems to me that `getZExtOrTrunc` takes an `SDValue` as its input. `TySize` here is not a `Value *`, but a constant, so I am not sure how to create an `SDValue` out of it, other than using `DAG.getConstant` (which is crashing because `getConstant` does not like oversized constants). That is why an `APInt` is used, similar to the `isScalable` case above. A few lines above (https://github.com/llvm/llvm-project/pull/71472/files#diff-34ee9626bb04949852b8ca80ba74ba7e9a43d7875410dcf993d36720f3ea6058R4132), the code performs the check we want on `AllocaSize`. I could add an additional check there for `TySize` as well, and I was limited by the fact that `TySize` is not a `Value *`. 

I must be missing some APIs to do such a thing. Could you kindly point out what I am missing here? Thanks so much! 

https://github.com/llvm/llvm-project/pull/71472