SIMD/intrinsincs questions

[Don]

Walter Bright wrote:
> Don wrote:
>> The bad news: The DMD back-end is a state-of-the-art backend from the 
>> late 90's. Despite its age, its treatment of integer operations is, in 
>> general, still quite respectable.
> 
> Modern compilers don't do much better. The point of diminishing returns 
> was clearly reached.

Yup. The only integer operation modern compilers still don't do well is 
-- array operations!

>> However, it _never_ generates SSE instructions. Ever. However, array 
>> operations _are_ detected, and they become to calls to library 
>> functions which use SSE if available. That's not bad for moderately 
>> large arrays -- 200 elements or so -- but of course it's completely 
>> non-optimal for short arrays.
>>
>> The good news: Now that static arrays are passed by value, introducing 
>> inline SSE support for short arrays suddenly makes a lot of sense -- 
>> there can be a big performance benefit for a small backend change; it 
>> could be done without introducing SSE anywhere else. Most importantly, 
>> it doesn't require any auto-vectorisation support.
> 
> What the library functions also do is have a runtime switch based on the 
> capabilities of the processor, switching to operations tailored to that 
> processor. To generate the code directly, assuming the existence of SSE, 
> is to mean the code will only run on modern chips. Whether or not this 
> is a problem depends on your application.

I'd say it's not a problem to use MMX or even SSE1. It's really, really 
difficult to find a processor that doesn't support them. I've tried. 
I've really tried. I don't think many are still around: they all have 
motherboards which require really small hard disks that you can no 
longer buy. Certainly no-one is putting new software on them.
Earlier this year I had to install Windows3.1 (!!!) on an ancient PC at 
work, to support an ancient but expensive bit of lab equipment. Even it 
was a Pentium II. Getting the spare parts for it was a nightmare*; we 
had to ship them from 600km away. Hard disks just don't last that long.

SSE2 is a different story, since AMD never made a 32 bit CPU with SSE2.

*Actually it was more of a horror comedy. It was hard to take it seriously.