[flang-commits] [flang] [flang] AliasAnalysis: More formally define and distinguish between data and non-data (PR #91020)

[Joel E. Denny via flang-commits]

================
@@ -46,6 +47,56 @@ struct AliasAnalysis {
   /// Attributes of the memory source object.
   ENUM_CLASS(Attribute, Target, Pointer, IntentIn);
 
+  // See
+  // https://discourse.llvm.org/t/rfc-distinguish-between-data-and-non-data-in-fir-alias-analysis/78759/1
+  //
+  // It is possible, while following the source of a memory reference through
+  // the use-def chain, to arrive at the same origin, even though the starting
+  // points were known to not alias.
+  //
+  // Example:
+  // clang-format off
+  //    fir.global @_QMtopEa : !fir.box<!fir.ptr<!fir.array<?xf32>>> 
+  //  
+  //  func.func @_QPtest() {
+  //    %c1 = arith.constant 1 : index
+  //    %cst = arith.constant 1.000000e+00 : f32
+  //    %0 = fir.address_of(@_QMtopEa) : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
+  //    %1 = fir.declare %0 {fortran_attrs = #fir.var_attrs<pointer>, uniq_name = "_QMtopEa"} : (!fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>) -> !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
+  //    %2 = fir.load %1 : !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>>
+  //    ...
+  //    %5 = fir.array_coor %2 %c1 : (!fir.box<!fir.ptr<!fir.array<?xf32>>>, !fir.shift<1>, index) -> !fir.ref<f32>
+  //    fir.store %cst to %5 : !fir.ref<f32>
+  //    return
+  //  }
+  //
+  // With high level operations, such as fir.array_coor, it is possible to
+  // reach into the data wrapped by the box (the descriptor) therefore when
+  // asking about the memory source of the %5, we are really asking about the
+  // source of the data of box %2.
+  //
+  // When asking about the source of %0 which is the address of the box, we
+  // reach the same source as in the first case: the global @_QMtopEa. Yet one
+  // source refers to the data while the other refers to the address of the box
+  // itself.
+  //
+  // To distinguish between the two, the isData flag has been added, whereby
+  // data is defined as any memory reference that is not a box reference.
+  // Additionally, because it is relied on in HLFIR lowering, we allow querying
+  // on a box SSA value, which is interpreted as querying on its data.
+  //
+  // So in the above example, !fir.ref<f32> and !fir.box<!fir.ptr<!fir.array<?xf32>>> is data, 
+  // while !fir.ref<!fir.box<!fir.ptr<!fir.array<?xf32>>>> is not data.
+
+  // This generally applies to function arguments. In the example below, %arg0
----------------
jdenny-ornl wrote:

```suggestion
  // This also applies to function arguments. In the example below, %arg0
```

https://github.com/llvm/llvm-project/pull/91020