The reason is fairly straightforward (though of course it should be
properly optimized). Assignments always yield the RHS so `foo2` is
equivalent to
function foo3()
result::Int = 0
return 0
end
To see this you can do:
julia> @code_typed foo2()
1-element Array{Any,1}:
:($(Expr(:lambda, {}, {{:result},{{:result,Int64,18}},{}}, :(begin #
none, line 2:
result = top(typeassert)(0,Int)::Int64
return 0
end::Int64))))
while for some reason the compiler doesn't know that the value is constant
in the case of foo (though it should).
On Wed, Apr 9, 2014 at 7:06 PM, Rak Rok <rak...@gmail.com> wrote:
> I think the following code is more illustrative of the issue:
>
> julia> function foo()
>> result::Int = 0
>> result
>> end
>> foo (generic function with 1 method)
>> julia> code_native(foo, ())
>> .section __TEXT,__text,regular,pure_instructions
>> Filename: none
>> Source line: 2
>> push RBP
>> mov RBP, RSP
>> movabs RAX, 140263599429192
>> Source line: 2
>> mov RAX, QWORD PTR [RAX]
>> Source line: 3
>> pop RBP
>> ret
>> julia> function foo2()
>> result::Int = 0
>> end
>> foo2 (generic function with 1 method)
>> julia> code_native(foo2, ())
>> .section __TEXT,__text,regular,pure_instructions
>> Filename: none
>> Source line: 2
>> push RBP
>> mov RBP, RSP
>> xor EAX, EAX
>> Source line: 2
>> pop RBP
>> ret
>
>
> For some reason in the first case Julia loads the 0 from memory whereas in
> the second case it simply xors EAX (instead of RAX, oddly, but not sure if
> it matters here).
>
> Rak.
>
>
>
> On Wed, Apr 9, 2014 at 6:32 PM, Rak Rok <rak...@gmail.com> wrote:
>
>> Excellent! Thanks guys, this is a TON better!
>>
>> I find it a little bit strange that it fetches the value of 0 from some
>> area in memory instead of just say xor RAX, RAX. I'm guessing it doesn't
>> know that the value is actually numeric 0? Is that because zero(T) isn't
>> being inlined fully perhaps?
>>
>> If it could realize that zero(T) is actually 0, it would probably avoid
>> the add altogether and simply do a mov into RAX.
>>
>> Interestingly, the code generated for Uints implies that that it knows
>> that zero(T) is actually just 0 and we get:
>> julia> code_native(f, (Uint,))
>> .section __TEXT,__text,regular,pure_instructions
>> Filename: none
>> Source line: 3
>> push RBP
>> mov RBP, RSP
>> test RDI, RDI
>> je 9
>> Source line: 3
>> imul RDI, RDI
>> jmpq 2
>> xor EDI, EDI
>> Source line: 6
>> mov RAX, RDI
>> pop RBP
>> ret
>>
>> Which is a little strange in its own way, since the xor EDI, EDI is
>> unecessary (and the jmpq 2 as well), but is at least better in that it
>> doesn't load a 0 from memory. Any clues?
>>
>> Thanks for your help guys,
>> Rak.
>>
>> On Thursday, April 3, 2014 5:24:27 PM UTC-4, Matt Bauman wrote:
>>>
>>> Talk about a perfect storm of fixes and enhancements! With the latest
>>> commit by Arch Robison, the native code is remarkably similar to clang's
>>> output -- just load, test, and multiply and add. It's funny to read folks
>>> in the issue talking about how much faster the 0.3 version is... when the
>>> final result ended up being something like > 50 million fold faster than
>>> 0.2. So very impressive.
>>>
>>> You should complain about strange LLVM code more often, Rak. Here's the
>>> native code now:
>>>
>>> julia> code_native(f, (Int,))
>>> .section __TEXT,__text,regular,pure_instructions
>>> Filename: none
>>> Source line: 2
>>> push RBP
>>> mov RBP, RSP
>>> movabs RAX, 140508429342280
>>> Source line: 2
>>> mov RAX, QWORD PTR [RAX]
>>> test RDI, RDI
>>> jle 7
>>> Source line: 3
>>> imul RDI, RDI
>>> add RAX, RDI
>>> Source line: 6
>>> pop RBP
>>> ret
>>>
>>> On Wednesday, April 2, 2014 9:26:44 PM UTC-4, andrew cooke wrote:
>>>>
>>>>
>>>> thanks! it's just possible this will fix a performance issue of mine
>>>> :o)
>>>>
>>>> On Wednesday, 2 April 2014 16:57:36 UTC-3, Steven G. Johnson wrote:
>>>>>
>>>>> Just filed this as an issue:
>>>>> https://github.com/JuliaLang/julia/issues/6382
>>>>>
>>>> julia> code_native(f, (Uint,))
>> .section __TEXT,__text,regular,pure_instructions
>> Filename: none
>> Source line: 3
>> push RBP
>> mov RBP, RSP
>> test RDI, RDI
>> je 9
>> Source line: 3
>> imul RDI, RDI
>> jmpq 2
>> xor EDI, EDI
>> Source line: 6
>> mov RAX, RDI
>> pop RBP
>> ret
>>
>>
>
