[LLVMdev] Proposal to improve vzeroupper optimization strategy

[Gao, Yunzhong]

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?

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?

If the sin() function was compiled by clang for a platform that supports AVX
instructions, I do not expect it to contain legacy SSE instructions. Is it not
the case for your platform?

I just looked at the library code for our sin() function and I do not see any
legacy SSE instructions (but for license restrictions I cannot share our
library codes; sorry).

> If you want a mechanism to disable vzeroupper generation for particular
> function calls, that might make sense...
> -Eli

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