[LLVMdev] Proposal to improve vzeroupper optimization strategy

[Eli Friedman]

On Fri, Sep 20, 2013 at 2:58 PM, Gao, Yunzhong <
yunzhong_gao at playstation.sony.com> wrote:

>  Hi Eli,****
>
> Thanks for the feedback. Please see below.
> - Gao.****
>
> ** **
>
> From: Eli Friedman [mailto:eli.friedman at gmail.com] ****
>
> Sent: Thursday, September 19, 2013 12:31 PM****
>
> To: Gao, Yunzhong****
>
> Cc: llvmdev at cs.uiuc.edu****
>
> Subject: Re: [LLVMdev] Proposal to improve vzeroupper optimization strategy
> ****
>
> ** **
>
> > This is essentially equivalent to "don't insert vzeroupper anywhere", as
> ****
>
> > far as I can tell. (The case of SSE instructions without a v- prefixed**
> **
>
> > equivalent is rare enough we can separate it from this discussion.)****
>
> ** **
>
> So will you be interested in a patch that disables vzeroupper by default?
>

A patch which adds a switch/LLVM IR function attribute to disable
vzeroupper would be fine.  A patch that disables vzeroupper on your
platform would be fine (assuming the target triple is distinguishable).
 Turning off vzeroupper by default on all platforms is not fine.


> I implemented this possibly over-engineering solution in our local tree to
> work****
>
> around some bad instruction selection issues in LLVM backend. When
> benchmarking****
>
> on our game codes, I noticed that sometimes legacy SSE instructions were**
> **
>
> selected despite existence of AVX equivalent, in which case the vzeroupper
> ****
>
> instruction was needed. And it is much easier to detect existence of
> vzeroupper****
>
> instruction than to detect each single legacy SSE instructions.****
>
> ** **
>
> The instruction selection issues were later fixed in our tree (patches to
> be****
>
> submitted later), at least for the handful of games I tested on. So a
> simple****
>
> change to just disable vzeroupper by default will be acceptable to us as
> well.****
>
> ** **
>
> > The reason we need vzeroupper in the first place is because we can't
> assume****
>
> > other functions won't use legacy SSE instructions; for example, on most*
> ***
>
> > systems, calling sin() will use legacy SSE instructions.  I mean, if you
> can****
>
> > make some unusual guarantee about your platform, it might make sense to*
> ***
>
> > disable vzeroupper generation in general, but it simply doesn't make
> sense****
>
> > on most platforms.****
>
> ** **
>
> I am confused by this point. By "most systems," do you have in mind a
> platform****
>
> where the sin() function was compiled by gcc but the application codes were
> ****
>
> compiled by clang?
>

On, for example, OS X, AVX is not enabled by default, so the sin() function
uses legacy SSE instructions.  Users can still turn on AVX in their
applications.

-Eli
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