[flang-commits] [flang] [flang] add tbaa tags to global variables (PR #68727)

[Tom Eccles via flang-commits]

================
@@ -406,7 +406,7 @@ AliasAnalysis::Source AliasAnalysis::getSource(mlir::Value v) {
         attributes.set(Attribute::Pointer);
     }
 
-  if (type == SourceKind::Global)
+  if (type == SourceKind::Global || type == SourceKind::Direct)
----------------
tblah wrote:

So `SourceKind::Direct` isn't necessarily a global?

The use case for the tbaa alias analysis is we want to be able to add tags to global arrays. These can be boxed. The specific case I care about is
```
module mod
real, dimension(:), allocatable :: glbl

subroutine func(arg)
  real, intent(in), dimension(:) :: arg
  ! glbl can't alias with arg
end subroutine
end module
```

The generated code looks something like
```
fir.global @_QMmodEglbl : !fir.box<!fir.heap<!fir.array<?xf32>>> { [...]}

func.func @_QMmodPfunc([...]) {
  [...]
  %glbl_addr = fir.address_of(@_QmodEa) : !fir.ref<!fir.box<!fir.heap<!fir.array<?xf32>>>>
  %decl = fir.declare %glbl_addr [...]
  [...]
  %box = fir.load %decl
  %addr = fir.box_addr %box : (!fir.box<!fir.heap<!fir.array<?xf32>>>) -> !fir.heap<!fir.array<?xf32>>
  [...]
  %element = fir.array_coor %addr(%shape) %index
  %val = fir.array_coor %element
```

What I want to happen here is to notice that `%val`  comes from a global variable and to tag that load as such. This is a lot clearer from the fortran source than the FIR so maybe we need to store more information somewhere.

As I understand it, you're saying that we can't know much about this variable because the global variable is boxed. Why is the box special?

https://github.com/llvm/llvm-project/pull/68727