Julia: Struct with constant field to optimize performance - performance

Question

foo is a callable example struct that will have additional fields later. Since some of these fields hold arrays with changing values it's not an option to make the instance a const G. However, the boolean value b stays constant. Is there a way to tell the compiler that this field will never change in order to enable optimizations? In this example I would like F to be as fast as G and H.
using BenchmarkTools
struct foo
b::Bool
end
function (f::foo)(x)
if f.b
x+1
end
end
F = foo(true)
const G = foo(true)
H(x) = x+1
#btime F(1) # 12.078 ns (0 allocations: 0 bytes)
#btime G(1) # 0.023 ns (0 allocations: 0 bytes)
#btime H(1) # 0.024 ns (0 allocations: 0 bytes)

Answer 1

You are not benchmarking the F correctly. Here is a way to do it:
julia> #btime $F(1)
0.026 ns (0 allocations: 0 bytes)
2
The problem is not that F is a global non-constant variable while G is a constant, so when accessing F its value is not stable within #btime.
Adding a $ in front of the call makes F a local variable during a benchmark and you can then see that it is equally fast.
Also it is probably better to benchmark some larger functions in such cases. Here is a quick example:
julia> function test(x)
s = 0
for i in 1:10^6
s += x(1)
end
s
end
test (generic function with 1 method)
julia> #btime test($F)
35.830 μs (0 allocations: 0 bytes)
2000000
julia> #btime test($G)
35.839 μs (0 allocations: 0 bytes)
2000000
Also you can check with #code_native that F and G end up with identical native code:
julia> #code_native F(1)
.text
; ┌ # REPL[10]:2 within `foo'
cmpb $0, (%rsi)
je L17
; │ # REPL[10]:3 within `foo'
; │┌ # int.jl:53 within `+'
addq $1, %rdx
; │└
movq %rdx, (%rdi)
movb $2, %dl
xorl %eax, %eax
retq
L17:
movb $1, %dl
xorl %eax, %eax
; │ # REPL[10]:3 within `foo'
retq
nopw %cs:(%rax,%rax)
; └
julia> #code_native G(1)
.text
; ┌ # REPL[10]:2 within `foo'
cmpb $0, (%rsi)
je L17
; │ # REPL[10]:3 within `foo'
; │┌ # int.jl:53 within `+'
addq $1, %rdx
; │└
movq %rdx, (%rdi)
movb $2, %dl
xorl %eax, %eax
retq
L17:
movb $1, %dl
xorl %eax, %eax
; │ # REPL[10]:3 within `foo'
retq
nopw %cs:(%rax,%rax)
; └