https://issues.dlang.org/show_bug.cgi?id=21027
Issue ID: 21027
Summary: Backend: DMD use 'rep stosb' even for ulong arrays
Product: D
Version: D2
Hardware: x86_64
OS: Linux
Status: NEW
Keywords: performance
Severity: normal
Priority: P1
Component: dmd
Assignee: nob...@puremagic.com
Reporter: pro.mathias.l...@gmail.com
Take the following code:
```
alias Content = ulong[256];
void main ()
{
Content v;
}
```
What DMD generates for this is on Linux c86_64 (used `run.dlang.org`):
```
.text._Dmain segment
assume CS:.text._Dmain
_Dmain:
push RBP
mov RBP,RSP
sub RSP,0808h
mov ECX,0800h
mov qword ptr -8[RBP],0
lea RAX,-8[RBP]
mov AL,[RAX]
lea RDI,0FFFFF7F8h[RBP]
rep
stosb
xor EAX,EAX
leave
ret
add [RAX],AL
.text._Dmain ends
```
The best to do here would be to call `memset` or `memcpy`, which is what LDC
does.
The second best would be to use `rep stosd` 0x100 times, as it is faster than
`rep stosb` 0x800 times.
Source:
- Agner Fog, optimizing assembly
(https://www.agner.org/optimize/optimizing_assembly.pdf), 16.9 Strings
instructions (all processors):
> `REP MOVSD` and `REP STOSD` are quite fast if the repeat count is not too
> small. The largest word size (DWORD in 32-bit mode, QWORD in 64-bit mode) is
> preferred. Both source and destination should be aligned by the word size or
> better. In many cases, however, it is faster to use vector registers. Moving
> data in the largest available registers is faster than `REP MOVSD` and `REP
> STOSD` in most cases, especially on older processors. See page 150 for
> details.
Related: https://issues.dlang.org/show_bug.cgi?id=14458
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