[llvm] [SPIR-V] Simplify addrspacecast for null in composite constant preprocessing (PR #192030)

[Juan Manuel Martinez Caamaño via llvm-commits]

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@@ -1534,6 +1534,22 @@ void SPIRVEmitIntrinsics::preprocessUndefs(IRBuilder<> &B) {
 }
 
 void SPIRVEmitIntrinsics::preprocessCompositeConstants(IRBuilder<> &B) {
+  // Simplify addrspacecast(null) to ConstantPointerNull of the target type.
+  // Casting null always yields null, and this avoids SPIR-V lowering issues
+  // where the null gets typed as an integer instead of a pointer. This handles
+  // addrspacecast instructions (from expanded ConstantExprs); ConstantExpr
+  // sub-elements inside composites are simplified in the decomposition below.
+  SmallVector<Instruction *, 4> ToErase;
+  for (Instruction &I : instructions(CurrF))
+    if (auto *ASC = dyn_cast<AddrSpaceCastInst>(&I))
+      if (isa<ConstantPointerNull>(ASC->getPointerOperand())) {
+        ASC->replaceAllUsesWith(
+            ConstantPointerNull::get(cast<PointerType>(ASC->getType())));
+        ToErase.push_back(ASC);
+      }
+  for (Instruction *I : ToErase)
+    I->eraseFromParent();
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jmmartinez wrote:

What if we moved this to another function? It doesn't seem related to the rest of preprocess composite constants.

PS: if we `make_early_inc_range` we should be able to avoid `ToErase`.

https://github.com/llvm/llvm-project/pull/192030