[cfe-dev] [llvm-dev] [RFC] remove the llvm.expect intrinsic

Fri Apr 22 15:21:36 PDT 2016

I like this suggestion

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Mehdi
Amini via llvm-dev
Sent: 22 April 2016 18:41
To: Sanjay Patel <spatel at rotateright.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org
Subject: Re: [llvm-dev] [RFC] remove the llvm.expect intrinsic

On Apr 22, 2016, at 10:39 AM, Mehdi Amini <mehdi.amini at apple.com
<mailto:mehdi.amini at apple.com> > wrote:

On Apr 22, 2016, at 10:27 AM, Sanjay Patel via llvm-dev
<llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:

On Fri, Apr 22, 2016 at 10:39 AM, Philip Reames <listmail at philipreames.com
<mailto:listmail at philipreames.com> > wrote:

On 04/22/2016 09:20 AM, Sanjay Patel via llvm-dev wrote:

I've proposed removing the llvm.expect intrinsic:
http://reviews.llvm.org/D19300

The motivation for this change is in:
http://reviews.llvm.org/D19299

For reference:
1. We created an intrinsic that's only reason for existing is to improve
perf, but the intrinsic can harm optimization by interfering with transforms
in other passes.

I don't follow this at all.  Given expects are eagerly lowered to metadata,
where's the interaction?  In particular, the expect lowering overrules any
metadata on the associated branch/switch.  This is exactly what you'd expect
for a user annotation interacting with PGO data.

I agree that a user annotation should override PGO data.

PGO is also a user input: the user is basically saying "I want the code to
be optimized for *this* use case".

So interestingly I would have thought the opposite: PGO overrides the source
code annotation.

Here are a couple of reasons why:

 - libraries can be used by different client and what is common in one case
might not for another.

 - code evolves, and user can fail to revisit assumption about the common
case

 - the user can be wrong, PGO should not (?).

For this last point, whatever information prevails in the end, it may be
valuable to report to the user some optimization hints about the mismatch
between the PGO measurement and the annotation.

-- 

Mehdi

This is why I initially proposed that builtin_expect() would extend on
'[un]predictable' metadata rather than 'prof' metadata in D19299. 

But I think that's a separate discussion now; we use 'prof' metadata for
this purpose today, and I'm not trying to change that in these patches. If
the user-specified representation of builtin_expect() is overwritten by PGO
data, I think that's a bug independent of the current proposal/patches.

2. To solve that, we created a pass to always transform the intrinsic into
metadata at a very early stage in LLVM. But now every program is paying a
compile-time tax (running the LowerExpectIntrinsic pass) for a feature that
is rarely used.

Er, what cost?  Given this is a single linear pass over the IR - and could
actually be made essentially free by checking to see if the module has any
uses of expect - I'm suspicious of this compile time argument.  Have you
actually seen this in profiles?  

No - I expect the actual overhead is in the noise. The real objection to the
LowerExpectIntrinsic pass is that it is completely unnecessary. This was
raised in the original review. We shouldn't have two mechanisms to represent
exactly the same thing. This is also why my initial implementation for
builtin_unpredictable() was rejected:
http://reviews.llvm.org/D12341

I assumed there was some good reason for LowerExpectIntrinsic to exist, so I
copied that design. As noted in that review, my assumption was wrong. And it
was suggested then that we should remove LowerExpectIntrinsic but nobody had
gotten around to it. With these patches, I'd like to finally fix this.

A possible front-end replacement transformation for a source-level
"builtin_expect()" is in D19299: I think a front-end can always directly
create metadata for this kind of programmer hint rather than using an
intermediate representation (the intrinsic). Likewise, if there's some
out-of-tree IR pass that is creating an llvm.expect, it should be able to
create branch weight metadata directly instead.

This seems like a reasonable proposal.  The expect intrinsic does give us a
mechanism to express value profiling predictions, but we don't appear to
actually use that today.  My bias would be to leave it in place, but I'm not
going to object strongly if the consensus goes the other way.  

Please let me know if you see any problems with this proposal or the
patches.

For reference, here's the original post-commit review thread for
llvm.expect:
https://marc.info/?l=llvm-commits
<https://marc.info/?l=llvm-commits&m=130997676129580&w=4>
&m=130997676129580&w=4

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