Ups. Forgot the actual examples.

Am 17.12.2016 um 14:01 schrieb Christoph Höger:
> Dear all,
> 
> find attached two simple runge-kutta iteration schemes. One is written
> in C, the other in OCaml. I compared the runtime of both and gcc (-O2)
> produces an executable that is roughly 30% faster (to be more precise:
> 3.52s vs. 2.63s). That is in itself quite pleasing, I think. I do not
> understand however, what causes this difference. Admittedly, the
> generated assembly looks completely different, but both compilers inline
> all functions and generate one big loop. Ocaml generates a lot more
> scaffolding, but that is to be expected.
> 
> There is however an interesting particularity: OCaml generates 6 calls
> to cos, while gcc only needs 3 (and one direct jump). Surprisingly,
> there are also calls to cosh, acos and pretty much any other
> trigonometric function (initialization of constants, maybe?)
> 
> However, the true culprit seems to be an excess of instructions between
> the different calls to cos. This is what happens between the first two
> calls to cos:
> 
> gcc:
> jmpq   400530 <cos@plt>
> nop
> nopw   %cs:0x0(%rax,%rax,1)
> 
> sub    $0x38,%rsp
> movsd  %xmm0,0x10(%rsp)
> movapd %xmm1,%xmm0
> movsd  %xmm2,0x18(%rsp)
> movsd  %xmm1,0x8(%rsp)
> callq  400530 <cos@plt>
> 
> ocamlopt:
> 
> callq  401a60 <cos@plt>
> mulsd  (%r12),%xmm0
> movsd  %xmm0,0x10(%rsp)
> sub    $0x10,%r15
> lea    0x25c7b6(%rip),%rax
> cmp    (%rax),%r15
> jb     404a8a <dlerror@plt+0x2d0a>
> lea    0x8(%r15),%rax
> movq   $0x4fd,-0x8(%rax)
> 
> movsd  0x32319(%rip),%xmm1
> 
> movapd %xmm1,%xmm2
> mulsd  %xmm0,%xmm2
> addsd  0x0(%r13),%xmm2
> movsd  %xmm2,(%rax)
> movapd %xmm1,%xmm0
> mulsd  (%r12),%xmm0
> addsd  (%rbx),%xmm0
> callq  401a60 <cos@plt>
> 
> 
> Is this caused by some underlying difference in the representation of
> numeric values (i.e. tagged ints) or is it reasonable to attack this
> issue as a hobby experiment?
> 
> 
> thanks for any advice,
> 
> Christoph
> 


-- 
Christoph Höger

Technische Universität Berlin
Fakultät IV - Elektrotechnik und Informatik
Übersetzerbau und Programmiersprachen

Sekr. TEL12-2, Ernst-Reuter-Platz 7, 10587 Berlin

Tel.: +49 (30) 314-24890
E-Mail: christoph.hoeger@tu-berlin.de