[flang-commits] [flang] [flang] AliasAnalysis: Fix pointer component logic (PR #94242)

[via flang-commits]

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@@ -159,21 +193,56 @@ AliasResult AliasAnalysis::alias(Value lhs, Value rhs) {
     src2->attributes.set(Attribute::Target);
   }
 
-  // Dummy TARGET/POINTER argument may alias with a global TARGET/POINTER.
+  // Two TARGET/POINTERs may alias.
   if (src1->isTargetOrPointer() && src2->isTargetOrPointer() &&
       src1->isData() == src2->isData()) {
     LLVM_DEBUG(llvm::dbgs() << "  aliasing because of target or pointer\n");
     return AliasResult::MayAlias;
   }
 
-  // Box for POINTER component inside an object of a derived type
-  // may alias box of a POINTER object, as well as boxes for POINTER
-  // components inside two objects of derived types may alias.
-  if ((src1->isRecordWithPointerComponent() && src2->isTargetOrPointer()) ||
-      (src2->isRecordWithPointerComponent() && src1->isTargetOrPointer()) ||
-      (src1->isRecordWithPointerComponent() &&
-       src2->isRecordWithPointerComponent())) {
-    LLVM_DEBUG(llvm::dbgs() << "  aliasing because of pointer components\n");
+  // A pointer dummy arg (but not a pointer component of a dummy arg) may alias
+  // a pointer component and thus the associated composite.  That composite
+  // might be a global or another dummy arg.  This is an example of the global
+  // composite case:
+  //
+  // module m
+  //   type t
+  //      real, pointer :: p
+  //   end type
+  //   type(t) :: a
+  //   type(t) :: b
+  // contains
+  //   subroutine test(p)
+  //     real, pointer :: p
+  //     p = 42
+  //     a = b
+  //     print *, p
+  //   end subroutine
+  // end module
+  // program
+  //   use m
+  //   real, target :: x1 = 1
+  //   real, target :: x2 = 2
+  //   a%p => x1
+  //   b%p => x2
+  //   call test(a%p)
+  // end
+  //
+  // The dummy argument p is an alias for a%p, even for the purposes of pointer
+  // association during the assignment a = b.  Thus, the program should print 2.
+  if ((isRecordWithPointerComponent(val1->getType()) &&
+       src1->kind != SourceKind::Allocate &&
+       src2->kind == SourceKind::Argument &&
----------------
jeanPerier wrote:

This misses the case where the pointer is HostAssoc, where aliasing is also possible.

```
module m 
  type t 
     real, pointer :: p 
  end type
  type(t) :: a
  type(t) :: b
contains
subroutine test(p)
  real, pointer :: p
  call internal()
contains
subroutine internal()
  p = 42
  a = b
  print *, p
end subroutine
end subroutine
end module

  use m
  real, target :: x1 = 1
  real, target :: x2 = 2
  a%p => x1
  b%p => x2
  call test(a%p)
end
```

I would rather reverse the logic, that is, precisely list the cases that are known to fall into the NoAlias, and fallback into MayAlias: Use `(src2->kind != SourceKind::Allocate || src2->kind != SourceKind::Global)`



https://github.com/llvm/llvm-project/pull/94242