# Do C++ performance optimisations apply to Julia?

**URL:** <https://discourse.julialang.org/t/do-c-performance-optimisations-apply-to-julia/82567>\
**Category:** Performance\
**Created:** [June 10, 2022, 2:19pm UTC](https://discourse.julialang.org/t/do-c-performance-optimisations-apply-to-julia/82567 "2022-06-10T14:19:35Z")\
**Posts on this page:** 1\
**Showing post:** 30

<div class="post-metadata">

**Author:** ![gbaraldi](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gbaraldi/32/22101_2.png) [@gbaraldi](https://discourse.julialang.org/u/gbaraldi)\
**Post date:** [June 25, 2022, 8:29pm UTC](https://discourse.julialang.org/t/do-c-performance-optimisations-apply-to-julia/82567/30 "2022-06-25T20:29:19Z")

</div>

I have a couple of examples of optimizations.

```julia

#simple LUT

julia> function foo(x)
           if x == 1
               7
           elseif x == 2
               8
           elseif x == 3
               9
           elseif x == 4
               10
           elseif x == 5
               11
           elseif x == 6
               12
           elseif x == 7
               13
           elseif x == 8
               14
           elseif x == 9
               15
           elseif x == 10
               16
           elseif x == 11
               17
           else
               0
           end
       end
foo (generic function with 1 method)

julia> @code_llvm foo(4)
; @ REPL[1]:1 within `foo`
define i64 @julia_foo_102(i64 signext %0) #0 {
top:
; @ REPL[1]:2 within `foo`
  %switch.tableidx = add i64 %0, -1
  %1 = icmp ult i64 %switch.tableidx, 10
  %.not10 = icmp eq i64 %0, 11
  %. = select i1 %.not10, i64 17, i64 0
  %switch.offset = add i64 %0, 6
  %common.ret.op = select i1 %1, i64 %switch.offset, i64 %.
; @ REPL[1] within `foo`
  ret i64 %common.ret.op
}

#jump table julia> function foo(x)
           if x == 1
               x * 7
           elseif x == 2
               x * 8
           elseif x == 3
               x * 9
           elseif x == 4
               x * 10
           elseif x == 5
               x * 11
           elseif x == 16
               x * 12
           elseif x == 71
               x * 13
           elseif x == 8
               x * 14
           elseif x == 9
               x * 15
           elseif x == 10
               x * 16
           elseif x == 11
               x * 17
           else
               x * 0
           end
       end
foo (generic function with 1 method)

julia> @code_llvm foo(4)
; @ REPL[5]:1 within `foo`
define i64 @julia_foo_119(i64 signext %0) #0 {
top:
; @ REPL[5]:2 within `foo`
  switch i64 %0, label %common.ret [
    i64 1, label %L3
    i64 2, label %L7
    i64 3, label %L11
    i64 4, label %L15
    i64 5, label %L19
    i64 16, label %L23
    i64 71, label %L27
    i64 8, label %L31
    i64 9, label %L35
    i64 10, label %L39
    i64 11, label %L43
  ]

common.ret: ; preds = %L43, %L39, %L35, %L31, %L27, %L23, %L19, %L15, %L11, %L7, %L3, %top
  %common.ret.op = phi i64 [7, %L3], [16, %L7], [27, %L11], [40, %L15], [55, %L19], [192, %L23], [923, %L27], [112, %L31], [135, %L35], [160, %L39], [187, %L43], [0, %top]
; @ REPL[5] within `foo`
  ret i64 %common.ret.op

L3: ; preds = %top
  br label %common.ret

L7: ; preds = %top
  br label %common.ret

L11: ; preds = %top
  br label %common.ret

L15: ; preds = %top
  br label %common.ret

L19: ; preds = %top
  br label %common.ret

L23: ; preds = %top
  br label %common.ret

L27: ; preds = %top
  br label %common.ret

L31: ; preds = %top
  br label %common.ret

L35: ; preds = %top
  br label %common.ret

L39: ; preds = %top
  br label %common.ret

L43: ; preds = %top
  br label %common.ret
}

#more sophisticated LUT

julia> @code_llvm foo(a)
; @ REPL[9]:1 within `foo`
define i64 @julia_foo_150(i64 signext %0) #0 {
top:
; @ REPL[9]:2 within `foo`
  %switch.tableidx = add i64 %0, -1
  %1 = icmp ult i64 %switch.tableidx, 10
  br i1 %1, label %switch.lookup, label %L31

switch.lookup: ; preds = %top
  %switch.gep = getelementptr inbounds [10 x i64], [10 x i64]* @switch.table.julia_foo_150, i64 0, i64 %switch.tableidx
  %switch.load = load i64, i64* %switch.gep, align 8
  br label %common.ret

common.ret: ; preds = %L31, %switch.lookup
  %common.ret.op = phi i64 [%., %L31], [%switch.load, %switch.lookup]
; @ REPL[9] within `foo`
  ret i64 %common.ret.op

L31: ; preds = %top
; @ REPL[9]:22 within `foo`
; ┌ @ promotion.jl:477 within `==`
   %.not10 = icmp eq i64 %0, 11
   %. = select i1 %.not10, i64 17, i64 0
   br label %common.ret
; └
}

```

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