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tanzanite7

VC2012: cost of recursive capturing lambda function

0 posts in this topic

Not a burning problem - just curiosity:

Using "static std::function<bool(int64u)> fnAdd = [&amp;](int64u tag)->bool { ... fnAdd(...) ... }" produces the suspected/expected, but unnecessary, inline initialization / shutdown code + other pointless overhead:
[code]
0000000140021BA2 mov eax,dword ptr [$S2 (0141678770h)] // static initialization check overhead ...
0000000140021BA8 and eax,1
0000000140021BAB test eax,eax
0000000140021BAD jne ResGlShader::update+85h (0140021BF5h) // ... end of check.
0000000140021BAF mov eax,dword ptr [$S2 (0141678770h)] // one time overhead ...
0000000140021BB5 or eax,1
0000000140021BB8 mov dword ptr [$S2 (0141678770h)],eax // this whole flag check/update looks completely bonkers - i suspect some crazy cache optimization (bit-packing multiple flags into one 32bit memory location?).
// capture code ... all i need is "this" and one local ("valid"), but it stores also what i am using via this - probably for performance reasons.
0000000140021BBE lea r9,[valid]
0000000140021BC3 mov r8,qword ptr [this]
0000000140021BCB lea rdx,[sources]
0000000140021BD0 lea rcx,[rsp+60h]
// the storing part is moved elsewhere (over a dozen loads and stores) - probably for instruction cache reasons (it should have moved most of the leading garbage there too though)
0000000140021BD5 call <lambda_ccd461a2e723c170260c9697d9a65f88>::<lambda_ccd461a2e723c170260c9697d9a65f88> (0140021EA0h)
0000000140021BDA mov rdx,rax
0000000140021BDD lea rcx,[fnAdd (0141678780h)]
// this is terrible ... introduces "malloc"/exceptions ... ffs. None of that is needed ... argh!
0000000140021BE4 call std::function<bool __cdecl(unsigned __int64)>::function<bool __cdecl(unsigned __int64)><<lambda_ccd461a2e723c170260c9697d9a65f88> > (014003A2E0h)
// destruction at program exit ... there is nothing in lambda function to destroy - dealing with std::function ("free") is probably all it does.
0000000140021BE9 lea rcx,[`ResGlShader::update'::`2'::`dynamic atexit destructor for 'fnAdd'' (01400C6380h)]
0000000140021BF0 call atexit (014003E7F0h)
0000000140021BF5 // ... end of skip

// The "fnAdd" function call has some extra std::function overhead too (argh*2):
// /.../ input: int64u param + "this". Stack pointer for local variable does not need passing. Clean and neat.
0000000140021C0C call std::_Func_class<bool,unsigned __int64,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil>::operator() (0140036E00h)
// ... call will land in (std::function check for valid contained function ... completely redundant):
0000000140036E00 mov qword ptr [rsp+10h],rdx
0000000140036E05 sub rsp,28h
0000000140036E09 mov rcx,qword ptr [rcx+18h]
0000000140036E0D test rcx,rcx
0000000140036E10 je std::_Func_class<bool,unsigned __int64,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil,std::_Nil>::operator()+22h (0140036E22h)
0000000140036E12 mov rax,qword ptr [rcx]
0000000140036E15 lea rdx,[_Vx0]
0000000140036E1A call qword ptr [rax+10h] // call for contained function object ... used as variable and not a fixed constant which it actually is x_x. It will land ...
0000000140036E1D add rsp,28h
0000000140036E21 ret
// ... here:
000000014003D430 mov rdx,qword ptr [rdx]
000000014003D433 add rcx,8
000000014003D437 jmp <lambda_ccd461a2e723c170260c9697d9a65f88>::operator() (0140021EF0h) // finally ...
[/code]

VC does the smart thing on certain conditions (stuff that does not have side effects mainly) and moves such static initializations out of the function body - but it can not do it here. Can i help it?

The "std::function" has many side effects and hence can not be optimized out, but i can not do recursion without it (or did i miss some other workaround that works with VC2012?) !

This is not performance related question (i do not see any situation where it would matter ... if it would matter then i would make a real member function that would have none of the considerable overhead) - it is just for my own educational purposes in regards of lambda functions. So far very pleased with them - just that doing recursion with them is a f* disaster (works fine, just looks like a disaster).

edit:
Well, one can not use "static" often with lambdas, so overhead there is mostly unavoidable - except getting rid of std::function. Giving up for now (aka. using a real function) - but leaving the question open in case a better solution shows up. Edited by tanzanite7
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