LLVM Picks Up 3DNow! Improvements In 2017

[phoronix]

Phoronix: LLVM Picks Up 3DNow! Improvements In 2017

As a flashback to the past, hitting the LLVM Git/SVN code today were improvements for those still running with processors supporting AMD's 3DNow! extensions...

http://www.phoronix.com/scan.php?page=news_item&px=LLVM-6.0-3DNow-Improvements

[tildearrow]

"AMD K6 up to Bulldozer CPU"

Bulldozer doesn't have 3DNow!, right?

[FireBurn]

"AMD K6 up to Bulldozer CPU"

Bulldozer doesn't have 3DNow!, right?

I believe the newer processors emulated it or some such

[VikingGe]

Bulldozer doesn't expose 3DNow support, and I highly doubt that the instructions would work. The only remnant of 3DNow that is still supported is the PREFETCHW instruction, which (afaik) has its own CPUID flag nowadays.

Anyway, there's absolutely no reason to ever prefer 3DNow! + MMX over SSE2 on CPUs that support it. What I'd like to know is the motivation behind this work.

[microcode]

Bulldozer doesn't expose 3DNow support, and I highly doubt that the instructions would work. The only remnant of 3DNow that is still supported is the PREFETCHW instruction, which (afaik) has its own CPUID flag nowadays.

Anyway, there's absolutely no reason to ever prefer 3DNow! + MMX over SSE2 on CPUs that support it. What I'd like to know is the motivation behind this work.

Well, it's possible there's some limited benefit to it on some models. Maybe on one of those huge Opteron supercomputers (though I'm not sure how quickly those things phase out hardware these days).

[Adarion]

I believe the newer processors emulated it or some such

No. They don't. Programs will throw a bad opcode or something if you try.
At least I tried exactly that (by accident) several days ago.

Iirc. one single instruction was still kept. But 3dnow / extd. ist gone for quite some time. The FX ones (AM3+) did not support it, most Athlon II models did.
Well, anyway, why not. If it helps on older machines, then let's have it. It shouldn't hurt elsewhere (unless you switch it on for a non capable CPU).

[AJenbo]

The latest cpu with 3dNow is 5 years old at this point. The oldest would be 20!

[Jumbotron]

** BREAKING NEWS ** from the "Better Late Than Never" department. LOL......got to be for some embedded K6 derivative out there. Oh well....late AMD attention is better than no attention at all I suppose.

[chithanh]

Some VIA CPUs also supported 3DNow.

Also I think that certain AMD CPUs (Bobcat?) would still execute 3DNow instructions despite not advertising them. This is similar to the Ryzen/FMA4 situation.

[DanL]

Is this relevant? https://reviews.llvm.org/D39952