Re: FP magic in std.math.pow

[Seb via Digitalmars-d-learn]

On Sunday, 31 July 2016 at 22:45:16 UTC, Stefan Koch wrote:

On Sunday, 31 July 2016 at 22:38:59 UTC, Seb wrote:

Consider this short program:

void main()
{
alias S = float;
S s1 = 0x1.24c92ep+5;
S s2 = -0x1.1c71c8p+0;

[...]


It's an anoying feature.
The reason this is not implemented in dmd is that pow does not 
map to a simple cpu instruction on x86.


I will have another shot at fixing this once the CTFE stuff is 
in.


It's about 1000 instructions with std.math.pow.
Yeah any improvement of the FP magic in DMD would be highly 
appreciated ;-)

Re: FP magic in std.math.pow

[Stefan Koch via Digitalmars-d-learn]

On Sunday, 31 July 2016 at 22:38:59 UTC, Seb wrote:

Consider this short program:

void main()
{
alias S = float;
S s1 = 0x1.24c92ep+5;
S s2 = -0x1.1c71c8p+0;

[...]


It's an anoying feature.
The reason this is not implemented in dmd is that pow does not 
map to a simple cpu instruction on x86.


I will have another shot at fixing this once the CTFE stuff is in.

FP magic in std.math.pow

[Seb via Digitalmars-d-learn]

Consider this short program:

void main()
{
alias S = float;
S s1 = 0x1.24c92ep+5;
S s2 = -0x1.1c71c8p+0;

import std.math : std_pow = pow;
import core.stdc.stdio : printf;
import core.stdc.math: powf;

printf("std: %a\n", std_pow(s1, s2));
printf("pow: %a\n", s1 ^^ s2);
printf("pow: %a\n", powf(s1, s2));

version(LDC)
{
import ldc.intrinsics : llvm_pow;
printf("ldc: %a\n", llvm_pow(s1, s2));
}
}

std: 0x1.2c155ap-6
pow: 0x1.2c155ap-6
powf: 0x1.2c1558p-6

As you can see below the C powf compiles to the assembly powf and 
LDC compiles to powf too. The output of std.math.pow is rather 
large, hence not listed.


1) Is this a bug in Phobos or just a very annoying "feature"?
2) I thought that DMD was decoupled from Phobos? So I was very 
astonished to see that it's not (see [1])


[1] 
https://github.com/dlang/dmd/blob/master/src/expression.d#L14781


```
.text._Dmainsegment
assume  CS:.text._Dmain
_Dmain:
pushRBP
mov RBP,RSP
sub RSP,010h
movss   XMM0,FLAT:.rodata[00h][RIP]
movss   -8[RBP],XMM0
movss   XMM1,FLAT:.rodata[00h][RIP]
movss   -4[RBP],XMM1
movss   XMM1,-4[RBP]
movss   XMM0,-8[RBP]
call  powf@PC32
cvtss2sdXMM0,XMM0
mov EDI,offset FLAT:.rodata@32
mov AL,1
call  printf@PC32
xor EAX,EAX
leave
ret
0f1f
add 0[RCX],AL
.text._Dmainends
```