This PR aims to fix #237. Atomic.get is a library function that provides atomic reads across domains. On x86, these atomic reads emit an external function call - caml_atomic_load. This function loads a value and emits a read_barrier to prevent re-ordering from happening.

The external call is unnecessary, and on x86, can be translated to plain loads.

TODO:

• Extend Patomic_load with an immediate_or_pointer field
• Modify Cload and Cloadmut to use inline records instead of tuples
• Extend Cload and Cloadmut with is_atomic field
• Translate Patomic_load to Cload {} or Cload {}
• Emit Iloadmut instead of caml_atomic_load

Changes

Extending Patomic_load with an immediate_or_pointer field

Patomic_load is a lambda op, which has been extended with a field to denote whether that load is to a value or an immediate.

| Patomic_load

to

| Patomic_load of {immediate_or_pointer : immediate_or_pointer}

Changing Cload and Cloadmut to use records instead of tuples

Cload and Cloadmut are two cmm operations defined as -

| Cload of memory_chunk * Asttypes.mutable_flag
| Cloadmut

This has now been modified to be -

| Cload of {memory_chunk: memory_chunk; mutability: Asttypes.mutable_flag; is_atomic: bool}
| Cloadmut of {is_atomic:bool}

All occurring values and usage in pattern matching has been updated to the new representation

Emitting Cload or Cloadmut from Patomic_load

The cmmgen.ml file emits cmm from lambda expressions. For this, the pattern match has been modified as -

| Patomic_load ->
     Cop (Cextcall("caml_atomic_load", typ_val, true, None), [transl env arg], dbg)

to

  | Patomic_load {immediate_or_pointer} ->
      let ptr = transl env arg
      in
      ( match immediate_or_pointer with
        | Immediate -> Cop (Cload {memory_chunk=Word_int ; mutability=Mutable ; is_atomic=true} , [ptr; Cconst_int 0], dbg)
        | Pointer -> Cop (Cloadmut {is_atomic=true}, [ptr; Cconst_int 0], dbg) )

Emit Iloadmut instead of caml_atomic_load

In selectgen.ml, the Cload and Cloadmut are translated to iload and iloadmut. In x86, the is_atomic param is ignored.