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A small comparison
Nov 20, 2013
bearophile
Nov 21, 2013
jerro
Nov 21, 2013
bearophile
Nov 22, 2013
Kai Nacke
Dec 13, 2013
Kai Nacke
```I've found a post about a functional library for LuaJIT code:

http://rtsisyk.github.io/luafun/intro.html

This dynamically typed Lua code:

require "fun" ()
n = 100
x = sum(map(function(x) return x^2 end, take(n, tabulate(math.sin))))
-- calculate sum(sin(x)^2 for x in 0..n-1)
print(x)
50.011981355266

Gets compiled to:

->LOOP:
0bcaffd0  movsd [rsp+0x8], xmm7
0bcaffd6  addsd xmm4, xmm5
0bcaffda  ucomisd xmm6, xmm1
0bcaffde  jnb 0x0bca0028        ->6
0bcaffe4  addsd xmm6, xmm0
0bcaffe8  addsd xmm7, xmm0
0bcaffec  fld qword [rsp+0x8]
0bcafff0  fsin
0bcafff2  fstp qword [rsp]
0bcafff5  movsd xmm5, [rsp]
0bcafffa  mulsd xmm5, xmm5
0bcafffe  jmp 0x0bcaffd0        ->LOOP

So I've written a similar D version using Phobos:

void main() {
import std.stdio, std.algorithm, std.range;
enum n = 100;
immutable double r = n.iota.map!(x => x.sin ^^ 2).reduce!q{a + b};
r.writeln;
}

LDC2 compiles it to (32 bit):

LBB0_1:
fxch
fstpt   44(%esp)
fld %st(0)
fstpt   (%esp)
fld1
faddp   %st(1)
fstpt   32(%esp)
calll   __D3std4math3sinFNaNbNfeZe
subl    \$12, %esp
fmul    %st(0), %st(0)
fldt    44(%esp)
faddp   %st(1)
fstpt   44(%esp)
fldt    32(%esp)
fldt    44(%esp)
decl    %esi
fxch
jne LBB0_1

As you see the D version doesn't use the fsin instruction as LuaJIT, it calls a library function.

If I import the sin from core.stdc.math ldc2 produces (notice the usage of xmm registers):

LBB0_1:
movsd   %xmm1, 32(%esp)
movsd   %xmm0, 40(%esp)
movsd   %xmm1, (%esp)
calll   _sin
fstpl   48(%esp)
movsd   48(%esp), %xmm0
mulsd   %xmm0, %xmm0
movsd   40(%esp), %xmm1
addsd   %xmm0, %xmm1
movsd   %xmm1, 40(%esp)
movsd   32(%esp), %xmm1
movsd   40(%esp), %xmm0
addsd   LCPI0_0, %xmm1
decl    %esi
jne LBB0_1

I have also written a normal for loop in both C and D. This is C code:

#include "stdio.h"
#include "math.h"

int main() {
double r = 0.0;
int i;
for (i = 0; i < 100; i++) {
const double aux = sin(i);
r += aux * aux;
}
printf("%f\n", r);
return 0;
}

GCC compiles it to:

.L3:
cvtsi2sd    %ebx, %xmm0
call    sin
.L2:
mulsd   %xmm0, %xmm0
addl    \$1, %ebx
cmpl    \$100, %ebx
addsd   8(%rsp), %xmm0
movsd   %xmm0, 8(%rsp)
jne .L3

The Intel compiler compiles it to this (this is even vectorized, sin2 and mulpd work on two doubles at a time):

..B1.2:                         # Preds ..B1.8 ..B1.7
cvtdq2pd  %xmm8, %xmm0                                  #8.32
call      __svml_sin2                                   #8.28
mulpd     %xmm0, %xmm0                                  #9.20
addb      \$2, %r12b                                     #7.5
paddd     %xmm9, %xmm8                                  #5.14
addpd     %xmm0, %xmm10                                 #9.9
cmpb      \$100, %r12b                                   #7.5
jb        ..B1.2        # Prob 99%                      #7.5

In scientific code it's common to have small kernels that take most of the run-time of a program, so it's important to shave every instructions from such loops.

Do you think Phobos/LDC2 are doing well enough here?

Bye,
bearophile
```
```> void main() {
>     import std.stdio, std.algorithm, std.range;
>     enum n = 100;
>     immutable double r = n.iota.map!(x => x.sin ^^ 2).reduce!q{a + b};
>     r.writeln;
> }

If I use core.stdc.math.sin and compile the code with:

gdc c.d -o c -O3 -fno-bounds-check -frelease

I get:

404e80:	cvtsi2sd %ebx,%xmm0
404e84:	callq  403070 <sin@plt>
404e89:	mulsd  %xmm0,%xmm0
404e8d:	add    \$0x1,%ebx
404e90:	cmp    \$0x64,%ebx
404e93:	addsd  0x28(%rsp),%xmm0
404e99:	movsd  %xmm0,0x28(%rsp)
404e9f:	jne    404e80 <_Dmain+0x60>

This is almost exactly the same code as

> .L3:
>     cvtsi2sd    %ebx, %xmm0
>     call    sin
> .L2:
>     mulsd   %xmm0, %xmm0
>     addl    \$1, %ebx
>     cmpl    \$100, %ebx
>     addsd   8(%rsp), %xmm0
>     movsd   %xmm0, 8(%rsp)
>     jne .L3
```
```jerro:

> If I use core.stdc.math.sin and compile the code with:
>
> gdc c.d -o c -O3 -fno-bounds-check -frelease
>
> I get:
>
>   404e80:	cvtsi2sd %ebx,%xmm0
>   404e84:	callq  403070 <sin@plt>
>   404e89:	mulsd  %xmm0,%xmm0
>   404e8d:	add    \$0x1,%ebx
>   404e90:	cmp    \$0x64,%ebx
>   404e93:	addsd  0x28(%rsp),%xmm0
>   404e99:	movsd  %xmm0,0x28(%rsp)
>   404e9f:	jne    404e80 <_Dmain+0x60>
>
> This is almost exactly the same code as
>
>> .L3:
>>    cvtsi2sd    %ebx, %xmm0
>>    call    sin
>> .L2:
>>    mulsd   %xmm0, %xmm0
>>    addl    \$1, %ebx
>>    cmpl    \$100, %ebx
>>    addsd   8(%rsp), %xmm0
>>    movsd   %xmm0, 8(%rsp)
>>    jne .L3

I guess the back-end is very similar, and it's able to optimize the code well :-)

What's the difference between std.math.sin and core.stdc.math.sin?

Bye,
bearophile
```
```Hi bearophile!

On Wednesday, 20 November 2013 at 22:12:29 UTC, bearophile wrote:
>
> In scientific code it's common to have small kernels that take most of the run-time of a program, so it's important to shave every instructions from such loops.
>
> Do you think Phobos/LDC2 are doing well enough here?

The difference between std.math.sin and core.stdc.math.sin is that std.math.sin is mapped to the LLVM intrinsic llvm.sin while core.stdc.math.sin is the sine function from the C library.

I would expect that the LLVM intrinsic is replaced with fsin (at least with -m32). I am unsure why this does not happen.

My understanding is that the auto-vectorizer should kick-in here (see http://llvm.org/docs/Vectorizers.html). I really need to go deeper here to understand what's happening in LLVM.

For sure, Phobos/LDC2 should do better here!!!

Regards,
Kai
```
```On Friday, 22 November 2013 at 21:44:28 UTC, Kai Nacke wrote:
> I would expect that the LLVM intrinsic is replaced with fsin (at least with -m32). I am unsure why this does not happen.

The answer is simple: it is yet not implemented. Just look at the slides from last LLVM developer meeting: http://www.llvm.org/devmtg/2013-11/slides/Rotem-Vectorization.pdf

Regards,
Kai
```