I felt I was making that exact point 
If x were passed as a reference in Julia, it would behave just like the C++ program. Edit Bad example. If I want that behavior in Julia, I explicitly need to pass a Ref to modx and dereference it,
while in C++ I can write a function that accepts a reference unbeknownst to the caller with all the hideous side effects.
Again, no, that’s because x[] = is not pure assignment but a syntactic sugar for setindex! which is an in-place operation. The difference you’re showing is in the semantic of assignment between C/C++ and Julia.
In a sense I may agree with you that there isn’t much difference with passing by reference for mutable objects. My point was that there aren’t different evaluation strategies, it’s always pass-by-sharing, strategy which is fairly popular in dynamic languages, not even specific to Julia or other niche languages. I tend to summaries it with “mutable objects can be mutated and immutable ones can’t” (and in fact the use of Ref suggested above meant to wrap an immutable object inside a mutable container to simulate the ability to mutate it), but for example this is very different from fortran where you can modify pretty much everything (unless limited by in/out scoping probably?). In Julia it’s fairly easy to know whether a type is mutable or not, for example I find that more opaque in python (probably all classes are mutable?)
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Hmm, I’ve been unable to confirm this. Looking at the code below, you find that the only case where anything other than a chain of conditions is evaluated is in the boundary case, where the value is determined before the function is compiled and so it can just return a single value every time. I’m not seeing any switch-like behaviour.
function switch(n)
m = n ≤ 0.1 ? 0.0 :
n ≤ 0.6 ? 1.0 :
n ≤ 0.8 ? 1.1 :
n ≤ 1.0 ? 1.2 :
n ≤ 1.3 ? 1.35 :
2.2 #=otherwise=#
m
end
let
n = 1.1
@code_llvm switch(n) # Long
end
begin
const a = 1.1
@code_llvm switch(a) # Long
end
function boundary()
a = 1.1
switch(a)
end
@code_llvm boundary()
Output:
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:2 within `switch`
; Function Attrs: uwtable
define double @julia_switch_1237(double %0) #0 {
top:
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:3 within `switch`
; ┌ @ float.jl:447 within `<=`
%1 = fcmp ugt double %0, 1.000000e-01
; └
br i1 %1, label %L4, label %L21
L4: ; preds = %top
; ┌ @ float.jl:447 within `<=`
%2 = fcmp ugt double %0, 6.000000e-01
; └
br i1 %2, label %L7, label %L21
L7: ; preds = %L4
; ┌ @ float.jl:447 within `<=`
%3 = fcmp ugt double %0, 8.000000e-01
; └
br i1 %3, label %L10, label %L21
L10: ; preds = %L7
; ┌ @ float.jl:447 within `<=`
%4 = fcmp ugt double %0, 1.000000e+00
; └
%5 = fcmp ugt double %0, 1.300000e+00
%. = select i1 %5, double 2.200000e+00, double 1.350000e+00
%value_phi3 = select i1 %4, double %., double 1.200000e+00
br label %L21
L21: ; preds = %L10, %L7, %L4, %top
%value_phi = phi double [ 0.000000e+00, %top ], [ 1.000000e+00, %L4 ], [ 1.100000e+00, %L7 ], [ %value_phi3, %L10 ]
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:9 within `switch`
ret double %value_phi
}
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:2 within `switch`
; Function Attrs: uwtable
define double @julia_switch_1239(double %0) #0 {
top:
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:3 within `switch`
; ┌ @ float.jl:447 within `<=`
%1 = fcmp ugt double %0, 1.000000e-01
; └
br i1 %1, label %L4, label %L21
L4: ; preds = %top
; ┌ @ float.jl:447 within `<=`
%2 = fcmp ugt double %0, 6.000000e-01
; └
br i1 %2, label %L7, label %L21
L7: ; preds = %L4
; ┌ @ float.jl:447 within `<=`
%3 = fcmp ugt double %0, 8.000000e-01
; └
br i1 %3, label %L10, label %L21
L10: ; preds = %L7
; ┌ @ float.jl:447 within `<=`
%4 = fcmp ugt double %0, 1.000000e+00
; └
%5 = fcmp ugt double %0, 1.300000e+00
%. = select i1 %5, double 2.200000e+00, double 1.350000e+00
%value_phi3 = select i1 %4, double %., double 1.200000e+00
br label %L21
L21: ; preds = %L10, %L7, %L4, %top
%value_phi = phi double [ 0.000000e+00, %top ], [ 1.000000e+00, %L4 ], [ 1.100000e+00, %L7 ], [ %value_phi3, %L10 ]
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:9 within `switch`
ret double %value_phi
}
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:22 within `boundary`
; Function Attrs: uwtable
define double @julia_boundary_1241() #0 {
top:
; @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\switch_statement.jl:24 within `boundary`
ret double 1.350000e+00
}
3 Likes
I thought it existed because of this comment from Jeff in a issue about adding or not a switch keyword for Julia. However, in a duplicate Jeff admits he is not sure if LLVM had really done it at some point.
1 Like
On 1.8 I tested and it emits a jump table sometimes.
2 Likes
Edit: Marking this as the solution as it best summarises or “answers” the original question. Feel kinda weird doing it but I think for discoverability it might help.
While writing the article I mentioned in the OP, I ended up not being satisfied with the end result of many of the conclusions and I think a much more interesting article can be written about a subset of the tips. I’m going to focus on that subset for the final article.
With that said, I thought it might be useful or interesting to those who stumble upon this thread in the future to read a nicely formatted document with code examples so I’ll leave the link here to an unlisted draft article on Forem:
Also, there is a GitHub repository containing all of the code snippets.
Thanks to everyone who contributed in this thread!
3 Likes
Can you post an example of the code?
I saw this example from this old thread which I have been able to recreate:
const CODES = (2, 3, 5, 7, 11)
const RCODES = reverse(CODES)
f(x) = 0
f(x, y, ys...) = x == y ? 1 + length(ys) : f(x, ys...)
findcode3(x) = f(x, RCODES...)
@code_llvm findcode3(3)
Additionally, I can edit the code to get close to what I wanted originally as we can see that there’s a table lookup happening in the LLVM:
const RETURNVALUES = (0, 1, 5, 10, 11, 13)
const RRETURNVALUES = reverse(RETURNVALUES)
h(x) = RETURNVALUES[0]
h(x, y, ys...) = x == y ? RRETURNVALUES[length(ys)+1] : h(x, ys...)
hindcode3(x) = h(x, RCODES...)
hindcode3(3)
@code_llvm hindcode3(3)
; Function Attrs: uwtable
define i64 @julia_hindcode3_1356(i64 signext %0) #0 {
top:
; ┌ @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 within `h`
%switch.tableidx = add i64 %0, -2
%1 = icmp ult i64 %switch.tableidx, 10
br i1 %1, label %switch.hole_check, label %L16
L16: ; preds = %switch.hole_check, %top
; │ @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 within `h` @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23
%2 = call nonnull {}* @j_h_1358(i64 signext %0) #0
call void @llvm.trap()
unreachable
switch.hole_check: ; preds = %top
; │ @ c:\Users\jmmsm\Documents\GitHub\speedy_julia\examples\ss_try2.jl:23 within `h`
%switch.maskindex = trunc i64 %switch.tableidx to i16
%switch.shifted = lshr i16 555, %switch.maskindex
%3 = and i16 %switch.shifted, 1
%switch.lobit.not = icmp eq i16 %3, 0
br i1 %switch.lobit.not, label %L16, label %switch.lookup
switch.lookup: ; preds = %switch.hole_check
%switch.gep = getelementptr inbounds [10 x i64], [10 x i64]* @switch.table.julia_hindcode3_1356, i64 0, i64 %switch.tableidx
%switch.load = load i64, i64* %switch.gep, align 8
ret i64 %switch.load
; └
}
I have a couple of examples of optimizations.
#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
; └
}
2 Likes