[llvm] [DSE] Handle provenance when eliminating tautological assignments (PR #184311)

[Nikita Popov via llvm-commits]

================
@@ -1179,3 +1181,56 @@ if.else:
 end:
   ret void
 }
+
+; Should not optimize as `*x` and `y` ptrs may have different provenance.
+define void @remove_tautological_store_of_ptr(ptr %x, ptr %y) {
+; CHECK-LABEL: @remove_tautological_store_of_ptr(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[VAL:%.*]] = load ptr, ptr [[X:%.*]], align 8
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq ptr [[VAL]], [[Y:%.*]]
+; CHECK-NEXT:    br i1 [[CMP]], label [[THEN:%.*]], label [[END:%.*]]
+; CHECK:       then:
+; CHECK-NEXT:    store ptr [[Y]], ptr [[X]], align 8
+; CHECK-NEXT:    br label [[END]]
+; CHECK:       end:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %val = load ptr, ptr %x, align 8
+  %cmp = icmp eq ptr %val, %y
+  br i1 %cmp, label %then, label %end
+
+then:
+  store ptr %y, ptr %x, align 8
+  br label %end
+
+end:
+  ret void
+}
+
+; Dominating equality `*x == null` holds, the store would introduce nullary
+; provenance. Going from non-nullary provenance to nullary provenance does
+; maintain provenance monotonicity.
----------------
nikic wrote:

```suggestion
; provenance. Thanks to provenance monotonicity, we are allowed to replace a
; pointer with nullary provenance with one with potentially non-nullary provenance.
```

https://github.com/llvm/llvm-project/pull/184311