By: Wilco (Wilco.Dijkstra.delete@this.ntlworld.com), July 16, 2013 3:47 am
Room: Moderated Discussions
bakaneko (nyan.delete@this.hyan.wan) on July 16, 2013 2:42 am wrote:
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on July 15, 2013 8:45 pm wrote:
> > bakaneko (nyan.delete@this.hyan.wan) on July 15, 2013 7:47 pm wrote:
> > >
> > > As someone who works for ARM as compiler writer,
> > > why don't you tell us in more detail how Intel
> > > cheated?
> >
> > Exophase's post to anandtech was quoted here earlier, I think. It has the relevant details:
> >
> > http://forums.anandtech.com/showthread.php?t=2330027
> >
> > and quite frankly, while optimizing multiple bit operations into a word is a very
> > valid optimization, the code icc generates there seems a fair bit past that.
> >
> > Sure, it could in theory happen with a really smart compiler and lots of generic optimizations.
> > In practice? It really smells like the compiler actively targeting a very particular code-sequence.
> > IOW, compiler cheating. The timing that Exophase points out makes it look worse.
> >
> > And Wilco is right that it smells pretty bad when AnTuTu seems to be so close to
> > intel, and seem to have bent over backwards using recent versions of icc etc.
> >
> > It's all "explainable". But it doesn't pass the smell test.
>
> It's also all besides the point. I would expect a better
> explanation from someone who claims to know their shit about
> compilers than a logical fallacy ("Intel improved their
> compiler recently", "Intel is cheating because it hits one
> function in a certain benchmark"). Instead he nags like his
> marriage has gone bad.
If you had actually read the whole thread including the links to various articles I posted then you would have found the detailed explanation that makes it obvious that Intel has been cheating AnTuTu. Why should I explain all the details again in every post I make?
> And no, the ICC results aren't that far fetched. Intel
> actually recommends -O3 -xSSSE3_ATOM* with the NDK. Which
> could also explain why Exophase saw optimizations which
> would be counterproductive for bigger programs. (If the
> flags have similar meaning to gcc, where inlining and loop
> unrolling have similar problems.)
Given the ICC results already dropped 20% after minor changes in AnTuTu it seems this optimization is no longer effective. And that proves it was very specific to the actual source code rather than a generic optimization that any compiler does.
> Smelling is good and all, but doesn't mean you should leave
> out verifying your claims.
Exophase, Jim McGregor on EETimes and Jeff Bier on BDTI did already provide all the details. So if you want to know more, just go and read their articles. If afterwards you have specific questions about the optimizations, post them here, and I'll explain.
Btw. I left ARM 7 years ago.
Wilco
> Linus Torvalds (torvalds.delete@this.linux-foundation.org) on July 15, 2013 8:45 pm wrote:
> > bakaneko (nyan.delete@this.hyan.wan) on July 15, 2013 7:47 pm wrote:
> > >
> > > As someone who works for ARM as compiler writer,
> > > why don't you tell us in more detail how Intel
> > > cheated?
> >
> > Exophase's post to anandtech was quoted here earlier, I think. It has the relevant details:
> >
> > http://forums.anandtech.com/showthread.php?t=2330027
> >
> > and quite frankly, while optimizing multiple bit operations into a word is a very
> > valid optimization, the code icc generates there seems a fair bit past that.
> >
> > Sure, it could in theory happen with a really smart compiler and lots of generic optimizations.
> > In practice? It really smells like the compiler actively targeting a very particular code-sequence.
> > IOW, compiler cheating. The timing that Exophase points out makes it look worse.
> >
> > And Wilco is right that it smells pretty bad when AnTuTu seems to be so close to
> > intel, and seem to have bent over backwards using recent versions of icc etc.
> >
> > It's all "explainable". But it doesn't pass the smell test.
>
> It's also all besides the point. I would expect a better
> explanation from someone who claims to know their shit about
> compilers than a logical fallacy ("Intel improved their
> compiler recently", "Intel is cheating because it hits one
> function in a certain benchmark"). Instead he nags like his
> marriage has gone bad.
If you had actually read the whole thread including the links to various articles I posted then you would have found the detailed explanation that makes it obvious that Intel has been cheating AnTuTu. Why should I explain all the details again in every post I make?
> And no, the ICC results aren't that far fetched. Intel
> actually recommends -O3 -xSSSE3_ATOM* with the NDK. Which
> could also explain why Exophase saw optimizations which
> would be counterproductive for bigger programs. (If the
> flags have similar meaning to gcc, where inlining and loop
> unrolling have similar problems.)
Given the ICC results already dropped 20% after minor changes in AnTuTu it seems this optimization is no longer effective. And that proves it was very specific to the actual source code rather than a generic optimization that any compiler does.
> Smelling is good and all, but doesn't mean you should leave
> out verifying your claims.
Exophase, Jim McGregor on EETimes and Jeff Bier on BDTI did already provide all the details. So if you want to know more, just go and read their articles. If afterwards you have specific questions about the optimizations, post them here, and I'll explain.
Btw. I left ARM 7 years ago.
Wilco