[llvm] [DAGCombiner] Add some very basic folds for ADDRSPACECAST (PR #127733)

[Kai Nacke via llvm-commits]

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@@ -16054,6 +16056,25 @@ SDValue DAGCombiner::visitBITCAST(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitADDRSPACECAST(SDNode *N) {
+  auto *ASCN1 = cast<AddrSpaceCastSDNode>(N);
+
+  if (auto *ASCN2 = dyn_cast<AddrSpaceCastSDNode>(ASCN1->getOperand(0))) {
+    assert(ASCN2->getDestAddressSpace() == ASCN1->getSrcAddressSpace());
+
+    // Fold asc[B -> A](asc[A -> B](x)) -> x
+    if (ASCN1->getDestAddressSpace() == ASCN2->getSrcAddressSpace())
+      return ASCN2->getOperand(0);
+
+    // Fold asc[B -> C](asc[A -> B](x)) -> asc[A -> C](x)
----------------
redstar wrote:

> For integral address spaces, if the pointer is dereferenceable, the cast is assumed reversible (though we should have an instruction flag to indicate when we know the object is valid)

Looking at a different aspect: Shouldn't there a check if the address spaces are integral? Because this certainly makes the first fold invalid. And reading https://llvm.org/docs/LangRef.html#nointptrtype, it seems to me that this also means that the transitive fold is invalid.

https://github.com/llvm/llvm-project/pull/127733