Const bindings not optimized in local scope functions

[mbauman]

I feel like I've seen this issue reported before (functions hidden behind let declarations have a performance penalty), but I can't find it now. And I'm not sure that it was reduced to a minimal example.

julia> const x = 10
       f() = x*x
       @code_llvm f()

define i64 @julia_f_22102() {
top:
  ret i64 100
}

julia> g = let
           const x = 10
           g() = x*x
       end
       @code_llvm g()

define %jl_value_t* @julia_g_22119(%jl_value_t*, %jl_value_t**, i32) {
top:
  %3 = getelementptr inbounds %jl_value_t* %0, i64 1, i32 0
  %4 = load %jl_value_t** %3, align 8
  %5 = getelementptr inbounds %jl_value_t* %4, i64 1, i32 0
  %6 = load %jl_value_t** %5, align 8
  %7 = getelementptr inbounds %jl_value_t* %6, i64 0, i32 0
  %8 = load %jl_value_t** %7, align 8
  %9 = icmp eq %jl_value_t* %8, null
  br i1 %9, label %err, label %ok2

err:                                              ; preds = %top
  call void @jl_undefined_var_error(%jl_value_t* inttoptr (i64 13193094808 to %jl_value_t*))
  unreachable

ok2:                                              ; preds = %top
  %10 = bitcast %jl_value_t* %8 to i64*
  %11 = load i64* %10, align 16
  %12 = mul i64 %11, %11
  %13 = call %jl_value_t* @jl_box_int64(i64 signext %12)
  ret %jl_value_t* %13
}

It seems like we're missing a relatively simple optimization here — is Julia simply forgetting that x is const in this case? Or is this more complicated than I'm making it out to be?

Use-case: over in Benchmarks.jl, we'd like to support running @benchmark macro within a for loop. This involves defining a local benchmarking function(s). In order to prevent benchmarking dynamic dispatch, we assign any non-constant bindings in the expression to temporary constant bindings. For the most part, this works very well. But we'd prefer to avoid strange performance effects like these.

[yuyichao]

I think this is a combination of #11159 (or more general #3440) and #5148.

[mbauman]

Right, of course, it's definitely at least #5148. Thanks @yuyichao.

julia> let
       const x = 1
       x += 1
       end
2