[llvm-dev] [cfe-dev] [RFC] remove the llvm.expect intrinsic
Martin J. O'Riordan via llvm-dev
llvm-dev at lists.llvm.org
Fri Apr 22 15:18:14 PDT 2016
Sorry for jumping in so late into this discussion, but I genuinely believe that removing this is a bad idea.
My reason for saying this is going to sound very strange, but I think that we need to understand a bit more about how this is being handled.
A while back one of my customers asked me if there was a method for advising the compiler how an if-statement was likely to resolve, and I said “sure” and told them to use ‘__builtin_expect’.
At the time I thought that the compiler was probably predicting a higher probability or true/false than was actually the case for a particular instance, as my customer was telling me that the compiler was optimising in favour of the less probable case (something the customer knew, but the information could not be determined from the code).
This was fair enough, these things happen and the customer (or profile directed feedback) has more knowledge than the default inferences in the compiler.
They added the ‘__builtin_expect’ to provide their domain specific expert knowledge, and the performance did indeed improve as expected, with the compiler preferring the most likely branch that the programmer had indicated.
The surprise though, was when we experimentally changed the outcome of the ‘__builtin_expect’ to say the exact opposite of what the actual case was. That is, to invert the truth. The program was more performant with the “wrong” truth than it was with no statement of truth. When we told the compiler that a particular outcome was more probable than when in fact it was less, the performance was better than when we said nothing. And when we told it the actual probable outcome, it was more performance still. So telling the outcome of the branch as being more likely true, or more likely false, was better than not telling the compiler anything at all.
I must admit, this was a considerable surprise for me, but it does mean that there is something changing in that area of the code that responds differently when ‘__builtin_expect’ is used versus the inferred probabilities.
It is not something that I have investigated further as it is not a specific area that I can prioritise my efforts, but I think that it is something I have to raise awareness off in the context of this thread where removing this builtin is being proposed. At the moment I have a lot of programs that are benefitting from the explicit use of this builtin, even when the programmer directed outcome is wrong. So before we can remove this builtin, we need to explain why there is a difference on behaviour when it is present and stating a particular outcome versus it being omitted and the inferred outcome being the same.
From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Xinliang David Li via cfe-dev
Sent: 22 April 2016 22:49
To: Sanjay Patel <spatel at rotateright.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; cfe-dev <cfe-dev at lists.llvm.org>; David Li <davidxl at google.com>; Nick Lewycky <nicholas at mxc.ca>
Subject: Re: [cfe-dev] [llvm-dev] [RFC] remove the llvm.expect intrinsic
Your patch does make the handling of __builtin_unpredictable and __builtin_expect 'unified' in some way.
However given the point brought up by Reid, and the rationale by Richard, as well as consideration of the easiness for enhancement in the future, I think keep the current mechanism is a better approach.
The test cases from Richard are broken with current lowering, but it is just bugs to be fixed.
On Fri, Apr 22, 2016 at 1:50 PM, Sanjay Patel via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> > wrote:
I, of course, thought the ~100 lines added by D19299 was a reasonable trade for the ~800 lines removed in D19300.
David Li (and anyone else following along), do you still like those patches after hearing this objection or should I abandon?
On Fri, Apr 22, 2016 at 11:55 AM, Reid Kleckner <rnk at google.com <mailto:rnk at google.com> > wrote:
Sorry, I didn't realize that was the clang side.
I think it's kind of ugly that the frontend has to pattern match and lower this intrinsic in three places: if, switch, and general case. The existing intrinsic is nice because it directly represents what the user can write, and lets the optimizer decide how to best represent it. As David Li mentioned, we may want to do value profiling in the future, and if we remove this intrinsic, we will have to come and revisit this code.
I think a single early pass to lower this intrinsic is a low cost to pay for that simplicity.
On Fri, Apr 22, 2016 at 10:39 AM, Sanjay Patel <spatel at rotateright.com <mailto:spatel at rotateright.com> > wrote:
Hi Reid -
The intent of D19299 is to remove all Clang refs to llvm.expect. Do you see any holes after applying that patch?
On Fri, Apr 22, 2016 at 11:36 AM, Reid Kleckner <rnk at google.com <mailto:rnk at google.com> > wrote:
Clang still appears to use llvm.expect. I think if you can show that it's trivial to update clang with a patch, then yeah, moving back down to one representation for this sounds reasonable.
On Fri, Apr 22, 2016 at 9:20 AM, Sanjay Patel via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org> > wrote:
I've proposed removing the llvm.expect intrinsic:
The motivation for this change is in:
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.
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.
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.
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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