[Attributor] Don't replace AddrSpaceCast with ConstantPointerNull

`ConstantPointerNull` represents a pointer with value 0, but it doesn’t
necessarily mean a nullptr. `ptr addrspace(x) null` is not the same as
`addrspacecast (ptr null to ptr addrspace(x))` if the nullptr in AS x is not
zero. Therefore, we can't simply replace it.

Fixes #115083.

Author: shiltian

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

@@ -10970,6 +10970,15 @@ struct AAPotentialValuesImpl : AAPotentialValues {
       Value *NewV = getSingleValue(A, *this, getIRPosition(), Values);
       if (!NewV || NewV == &OldV)
         continue;
+      // FIXME: ConstantPointerNull represents a pointer with value 0, but it
+      // doesn’t necessarily mean a nullptr. `ptr addrspace(x) null` is not the
+      // same as `addrspacecast (ptr null to ptr addrspace(x))` if the nullptr
+      // in AS x is not zero. Therefore, we can't simply replace it.
+      if (isa<ConstantPointerNull>(NewV)) {
+        if (auto *CE = dyn_cast<ConstantExpr>(&OldV);
+            CE->getOpcode() == Instruction::AddrSpaceCast)
+          continue;
+      }
       if (getCtxI() &&
           !AA::isValidAtPosition({*NewV, *getCtxI()}, A.getInfoCache()))
         continue;

llvm/test/Transforms/Attributor/do-not-replace-addrspacecast-with-constantpointernull.ll

@@ -0,0 +1,18 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S -passes=attributor %s -o - | FileCheck %s
+
+define i32 @bar(ptr %p0, ptr addrspace(5) %p5) {
+; CHECK-LABEL: define i32 @bar(
+; CHECK-SAME: ptr nofree readonly captures(none) [[P0:%.*]], ptr addrspace(5) nofree readonly [[P5:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq ptr addrspace(5) [[P5]], addrspacecast (ptr null to ptr addrspace(5))
+; CHECK-NEXT:    [[TMP1:%.*]] = select i1 [[TMP0]], ptr [[P0]], ptr null
+; CHECK-NEXT:    [[TMP2:%.*]] = load i32, ptr [[TMP1]], align 4
+; CHECK-NEXT:    ret i32 [[TMP2]]
+;
+entry:
+  %icmp = icmp eq ptr addrspace(5) %p5, addrspacecast (ptr null to ptr addrspace(5))
+  %select = select i1 %icmp, ptr %p0, ptr null
+  %load = load i32, ptr %select, align 4
+  ret i32 %load
+}