add specialized codegen for jl_get_current_task

[JeffBezanson]

The code_llvm for this seems to be nastier than I'd expect:

julia> @code_llvm current_task()

;  @ task.jl:95 within `current_task'
define nonnull %jl_value_t addrspace(10)* @jsys1_current_task_15548(%jl_value_t addrspace(10)*, %jl_value_t addrspace(10)**, i32) #0 {
top:
  %3 = alloca %jl_value_t addrspace(10)**, align 8
  store volatile %jl_value_t addrspace(10)** %1, %jl_value_t addrspace(10)*** %3, align 8
  %thread_ptr = call i8* asm "movq %fs:0, $0", "=r"()
  %ptls_i8 = getelementptr i8, i8* %thread_ptr, i64 -15712
  %ptls = bitcast i8* %ptls_i8 to %jl_value_t***
  %4 = getelementptr %jl_value_t**, %jl_value_t*** %ptls, i64 826
  %5 = bitcast %jl_value_t*** %4 to %jl_value_t addrspace(10)**
  %6 = load %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)** %5, align 8
  ret %jl_value_t addrspace(10)* %6
}

Why is this using a less efficient calling convention, and what's with the volatile store?

[yuyichao]

The signature was #11306 (IIRC the logic looks for non jl_value_t* types and isn't triggered on empty argument) The logic there was to reduce allocation without considering jlcall frame allocation.

I believe the volatile store is there to help the debugger work by making sure the jlcall frame address is always available.

[chethega]

Bump. FWIW, my patch in #34852 generates

julia> @code_llvm current_task()

;  @ task.jl:120 within `current_task'
define nonnull %jl_value_t addrspace(10)* @julia_current_task_16641() {
top:
  %thread_ptr = call i8* asm "movq %fs:0, $0", "=r"()
  %ptls_i8 = getelementptr i8, i8* %thread_ptr, i64 -15712
  %ptls = bitcast i8* %ptls_i8 to %jl_value_t***
  %0 = getelementptr %jl_value_t**, %jl_value_t*** %ptls, i64 826
  %1 = bitcast %jl_value_t*** %0 to %jl_value_t addrspace(10)**
  %2 = load %jl_value_t addrspace(10)*, %jl_value_t addrspace(10)** %1, align 8
  ret %jl_value_t addrspace(10)* %2
}

I have no idea why, since your patch looks almost the same as mine, maybe something else got rid of the store volatile?

[JeffBezanson]

Rebasing on master fixes it, so something else must have changed it.