> On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote: > > On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com <mailto:dnovillo at google.com>> wrote: > > I've created a few bugzilla issues with details of some of the things I'll be looking into. I'm not yet done wordsmithing the overall design document. I'll try to finish it by early next week at the latest. > > The document is available at > > https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing <https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing> > > There are several topics covered. Ideally, I would prefer that we discuss each topic separately. The main ones I will start working on are the ones described in the bugzilla links we have in the doc. > > This is just a starting point for us. I am not at all concerned with implementing exactly what is proposed in the document. In fact, if we can get the same value using the existing support, all the better. > > OTOH, any other ideas that folks may have that work better than this are more than welcome. I don't have really strong opinions on the matter. I am fine with whatever works.Thanks for the detailed write-up on this. Some of the issues definitely need to be addressed. I am concerned, though, that some of the ideas may be leading toward a scenario where we have essentially two completely different ways of representing profile information in LLVM IR. It is great to have two complementary approaches to collecting profile data, but two representations in the IR would not make sense. The first issue raised is that profile execution counts are not represented in the IR. This was a very intentional decision. I know it goes against what other compilers have done in the past. It took me a while to get used to the idea when Andy first suggested it, so I know it seems awkward at first. The advantage is that branch probabilities are much easier to keep updated in the face of compiler transformations, compared to execution counts. We are definitely missing the per-function execution counts that are needed to be able to compare relative “hotness” across functions, and I think that would be a good place to start making improvements. In the long term, we should keep our options open to making major changes, but before we go there, we should try to make incremental improvements to fix the existing infrastructure. Many of the other issues you raise seem like they could also be addressed without major changes to the existing infrastructure. Let’s try to fix those first. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150305/8b67ebd8/attachment.html>
On 03/05/2015 08:29 AM, Bob Wilson wrote:> >> On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com >> <mailto:dnovillo at google.com>> wrote: >> >> On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com >> <mailto:dnovillo at google.com>> wrote: >> >> I've created a few bugzilla issues with details of some of the >> things I'll be looking into. I'm not yet done wordsmithing the >> overall design document. I'll try to finish it by early next week >> at the latest. >> >> >> The document is available at >> >> https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing >> <https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing> >> >> There are several topics covered. Ideally, I would prefer that we >> discuss each topic separately. The main ones I will start working on >> are the ones described in the bugzilla links we have in the doc. >> >> This is just a starting point for us. I am not at all concerned with >> implementing exactly what is proposed in the document. In fact, if we >> can get the same value using the existing support, all the better. >> >> OTOH, any other ideas that folks may have that work better than this >> are more than welcome. I don't have really strong opinions on the >> matter. I am fine with whatever works. > > Thanks for the detailed write-up on this. Some of the issues > definitely need to be addressed. I am concerned, though, that some of > the ideas may be leading toward a scenario where we have essentially > two completely different ways of representing profile information in > LLVM IR. It is great to have two complementary approaches to > collecting profile data, but two representations in the IR would not > make sense. > > The first issue raised is that profile execution counts are not > represented in the IR. This was a very intentional decision. I know it > goes against what other compilers have done in the past. It took me a > while to get used to the idea when Andy first suggested it, so I know > it seems awkward at first. The advantage is that branch probabilities > are much easier to keep updated in the face of compiler > transformations, compared to execution counts. We are definitely > missing the per-function execution counts that are needed to be able > to compare relative “hotness” across functions, and I think that would > be a good place to start making improvements. In the long term, we > should keep our options open to making major changes, but before we go > there, we should try to make incremental improvements to fix the > existing infrastructure. > > Many of the other issues you raise seem like they could also be > addressed without major changes to the existing infrastructure. Let’s > try to fix those first.After reading the document, I agree with Bob's perspective here. I would strongly recommend that you start with the optimizations than can be implemented within the current framework. The current infrastructure gives a fairly reasonable idea of relative hotness within a function. There's a lot to be done to exploit that information (even in the inliner!) without resorting to cross function analysis. If, after most of those have been implemented, we need more fundamental changes we could consider them. Starting with a fundamental rewrite of the profiling system within LLVM seems like a mistake. At a meta level, as someone who uses LLVM for JITing I would be opposed to a system that assumed consistent profiling counts across function boundaries and gave up on relative hotness information. At least if I'm understanding your proposal, this would *completely break* a multi-tiered JIT. In practice, you generally stop collecting instrumentation profiling once something is compiled at a high enough tier. When compiling it's caller, you'll get very deceptive results if you rely on the execution counts to line up across functions. On the other hand, merging two relative hotness profiles by scaling based on the hotness of the callsite works out quite well in practice. You can use some information about global hotness to make decisions, but those decisions need to be resilient to such systematic under-counting. Philip -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150306/eb75230f/attachment.html>
Xinliang David Li
2015-Mar-07 03:00 UTC
[LLVMdev] RFC - Improvements to PGO profile support
Bob, Philip, thanks for the feedback. Diego is planning to give more detailed reply next Monday. There seem to be some misunderstanding about the proposals, so I will just give some highlights here: 1) The proposal is not intending to fundamentally change the current framework, but to enhanced the framework so that a) more profile information is preserved b) block/edge count/frequency becomes faster to compute b) profile information becomes faster to access and update (inter-procedurally) 2) Changes to profile APIs and profile client code will be minimized, except that we will add IPA clients (once Chandler's pass manager change is ready) 3) The proposed change does *not* give up relative hotness as mentioned by Philiip. All clients that relies on relative hotness are not affected -- except that the data is better and more reliable 4) With real profile data available, current infrastructure does *not* provide reasonable hotness (e.g., you can try comparing the BBs that execute the same number times, but in loops with different depths in the same function and see how big the diff is), let alone fast updating. I am reasonably confident that the proposal 1) does not affect compilations using static profile (with branch prediction) 2) strictly better for -fprofile-instr-use optimizations. The area I am not so sure is the JIT, but I am really interested in knowing the details and propose solutions for you if the current proposal does not work for you (which I doubt -- because if the current framework works, the new one should work too :) ). I am looking forward to more detailed discussions next week! We shall sit down together and discuss changes, rationale, concerns one by one -- with concrete examples. thanks, David On Fri, Mar 6, 2015 at 5:49 PM, Philip Reames <listmail at philipreames.com> wrote:> > On 03/05/2015 08:29 AM, Bob Wilson wrote: > > > On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote: > > On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com> wrote: > >> I've created a few bugzilla issues with details of some of the things I'll >> be looking into. I'm not yet done wordsmithing the overall design document. >> I'll try to finish it by early next week at the latest. > > > The document is available at > > https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing > > There are several topics covered. Ideally, I would prefer that we discuss > each topic separately. The main ones I will start working on are the ones > described in the bugzilla links we have in the doc. > > This is just a starting point for us. I am not at all concerned with > implementing exactly what is proposed in the document. In fact, if we can > get the same value using the existing support, all the better. > > OTOH, any other ideas that folks may have that work better than this are > more than welcome. I don't have really strong opinions on the matter. I am > fine with whatever works. > > > Thanks for the detailed write-up on this. Some of the issues definitely need > to be addressed. I am concerned, though, that some of the ideas may be > leading toward a scenario where we have essentially two completely different > ways of representing profile information in LLVM IR. It is great to have two > complementary approaches to collecting profile data, but two representations > in the IR would not make sense. > > The first issue raised is that profile execution counts are not represented > in the IR. This was a very intentional decision. I know it goes against what > other compilers have done in the past. It took me a while to get used to the > idea when Andy first suggested it, so I know it seems awkward at first. The > advantage is that branch probabilities are much easier to keep updated in > the face of compiler transformations, compared to execution counts. We are > definitely missing the per-function execution counts that are needed to be > able to compare relative “hotness” across functions, and I think that would > be a good place to start making improvements. In the long term, we should > keep our options open to making major changes, but before we go there, we > should try to make incremental improvements to fix the existing > infrastructure. > > Many of the other issues you raise seem like they could also be addressed > without major changes to the existing infrastructure. Let’s try to fix those > first. > > After reading the document, I agree with Bob's perspective here. > > I would strongly recommend that you start with the optimizations than can be > implemented within the current framework. The current infrastructure gives > a fairly reasonable idea of relative hotness within a function. There's a > lot to be done to exploit that information (even in the inliner!) without > resorting to cross function analysis. If, after most of those have been > implemented, we need more fundamental changes we could consider them. > Starting with a fundamental rewrite of the profiling system within LLVM > seems like a mistake. > > At a meta level, as someone who uses LLVM for JITing I would be opposed to a > system that assumed consistent profiling counts across function boundaries > and gave up on relative hotness information. At least if I'm understanding > your proposal, this would *completely break* a multi-tiered JIT. In > practice, you generally stop collecting instrumentation profiling once > something is compiled at a high enough tier. When compiling it's caller, > you'll get very deceptive results if you rely on the execution counts to > line up across functions. On the other hand, merging two relative hotness > profiles by scaling based on the hotness of the callsite works out quite > well in practice. You can use some information about global hotness to make > decisions, but those decisions need to be resilient to such systematic > under-counting. > > Philip
On Thu, Mar 5, 2015 at 11:29 AM, Bob Wilson <bob.wilson at apple.com> wrote:> > On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote: > > On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com> > wrote: > > I've created a few bugzilla issues with details of some of the things I'll >> be looking into. I'm not yet done wordsmithing the overall design document. >> I'll try to finish it by early next week at the latest. >> > > The document is available at > > > https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing > <https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing> > > There are several topics covered. Ideally, I would prefer that we discuss > each topic separately. The main ones I will start working on are the ones > described in the bugzilla links we have in the doc. > > This is just a starting point for us. I am not at all concerned with > implementing exactly what is proposed in the document. In fact, if we can > get the same value using the existing support, all the better. > > OTOH, any other ideas that folks may have that work better than this are > more than welcome. I don't have really strong opinions on the matter. I am > fine with whatever works. > > > Thanks for the detailed write-up on this. Some of the issues definitely > need to be addressed. I am concerned, though, that some of the ideas may be > leading toward a scenario where we have essentially two completely > different ways of representing profile information in LLVM IR. It is great > to have two complementary approaches to collecting profile data, but two > representations in the IR would not make sense. >Yeah, I don't think I'll continue to push for a new MD_count attribute. If we were to make MD_prof be a "real" execution count, that would be enough. Note that by re-using MD_prof we are not changing its meaning at all. The execution count is still a weight and the ratio is still branch probability. All that we are changing are the absolute values of the number and increasing its data type width to remove the 32bit limitation.> The first issue raised is that profile execution counts are not > represented in the IR. This was a very intentional decision. I know it goes > against what other compilers have done in the past. It took me a while to > get used to the idea when Andy first suggested it, so I know it seems > awkward at first. The advantage is that branch probabilities are much > easier to keep updated in the face of compiler transformations, compared to > execution counts. >Sorry. I don't follow. Updating counts as the CFG is transformed is not difficult at all. What examples do you have in mind? The big advantage of making MD_prof an actual execution count is that it is a meaningful metric wrt scaling and transformation. Say, for instance, that we have a branch instruction with two targets with counts {100, 300} inside a function 'foo' that has entry count 2. The edge probability for the first edge (count 100) is 100/(100+300) = 25%. If we inline foo() inside another function bar() at a callsite with profile count == 1, the cloned branch instruction gets its counters scaled with the callsite count. So the new branch has counts {100 * 1 / 2, 300 * 1 / 2} {50, 150}. But the branch probability did not change. Currently, we are cloning the branch without changing the edge weights. This scaling is not difficult at all and can be incrementally very quickly. We cannot afford to recompute all frequencies on the fly because it would be detrimental to compile time. If foo() itself has N callees inlined into it, each inlined callee needs to trigger a re-computation. When foo() is inlined into bar(), the frequencies will need to be recomputed for foo() and all N callees inlined into foo().> We are definitely missing the per-function execution counts that are > needed to be able to compare relative “hotness” across functions, and I > think that would be a good place to start making improvements. In the long > term, we should keep our options open to making major changes, but before > we go there, we should try to make incremental improvements to fix the > existing infrastructure. >Right, and that's the core of our proposal. We don't really want to make major infrastructure changes at this point. The only thing I'd like to explore is making MD_prof a real count. This will be useful for the inliner changes and it would also make incremental updates easier, because the scaling that needs to be done is very straightforward and quick. Note that this change should not modify the current behaviour we get from profile analysis. Things that were hot before should continue to be hot now.> Many of the other issues you raise seem like they could also be addressed > without major changes to the existing infrastructure. Let’s try to fix > those first. >That's exactly the point of the proposal. We definitely don't want to make major changes to the infrastructure at first. My thinking is to start working on making MD_prof a real count. One of the things that are happening is that the combination of real profile data plus the frequency propagation that we are currently doing is misleading the analysis. For example (thanks David for the code and data). In the following code: int g; __attribute__((noinline)) void bar() { g++; } extern int printf(const char*, ...); int main() { int i, j, k; g = 0; // Loop 1. for (i = 0; i < 100; i++) for (j = 0; j < 100; j++) for (k = 0; k < 100; k++) bar(); printf ("g = %d\n", g); g = 0; // Loop 2. for (i = 0; i < 100; i++) for (j = 0; j < 10000; j++) bar(); printf ("g = %d\n", g); g = 0; // Loop 3. for (i = 0; i < 1000000; i++) bar(); printf ("g = %d\n", g); g = 0; } When compiled with profile instrumentation, frequency propagation is distorting the real profile because it gives different frequency to the calls to bar() in the 3 different loops. All 3 loops execute 1,000,000 times, but after frequency propagation, the first call to bar() gets a weight of 520,202 in loop #1, 210,944 in loop #2 and 4,096 in loop #3. In reality, every call to bar() should have a weight of 1,000,000. Thanks. Diego. -------------- next part -------------- An HTML attachment was scrubbed... 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Duncan P. N. Exon Smith
2015-Mar-12 21:42 UTC
[LLVMdev] RFC - Improvements to PGO profile support
> On 2015-Mar-10, at 10:14, Diego Novillo <dnovillo at google.com> wrote: > > > > On Thu, Mar 5, 2015 at 11:29 AM, Bob Wilson <bob.wilson at apple.com> wrote: > >> On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com> wrote: >> >> On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo <dnovillo at google.com> wrote: >> >> I've created a few bugzilla issues with details of some of the things I'll be looking into. I'm not yet done wordsmithing the overall design document. I'll try to finish it by early next week at the latest. >> >> The document is available at >> >> https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing >> >> There are several topics covered. Ideally, I would prefer that we discuss each topic separately. The main ones I will start working on are the ones described in the bugzilla links we have in the doc. >> >> This is just a starting point for us. I am not at all concerned with implementing exactly what is proposed in the document. In fact, if we can get the same value using the existing support, all the better. >> >> OTOH, any other ideas that folks may have that work better than this are more than welcome. I don't have really strong opinions on the matter. I am fine with whatever works. > > Thanks for the detailed write-up on this. Some of the issues definitely need to be addressed. I am concerned, though, that some of the ideas may be leading toward a scenario where we have essentially two completely different ways of representing profile information in LLVM IR. It is great to have two complementary approaches to collecting profile data, but two representations in the IR would not make sense. > > Yeah, I don't think I'll continue to push for a new MD_count attribute. If we were to make MD_prof be a "real" execution count, that would be enough. Note that by re-using MD_prof we are not changing its meaning at all. The execution count is still a weight and the ratio is still branch probability. All that we are changing are the absolute values of the number and increasing its data type width to remove the 32bit limitation. > > > > The first issue raised is that profile execution counts are not represented in the IR. This was a very intentional decision. I know it goes against what other compilers have done in the past. It took me a while to get used to the idea when Andy first suggested it, so I know it seems awkward at first. The advantage is that branch probabilities are much easier to keep updated in the face of compiler transformations, compared to execution counts. > > Sorry. I don't follow. Updating counts as the CFG is transformed is not difficult at all. What examples do you have in mind? The big advantage of making MD_prof an actual execution count is that it is a meaningful metric wrt scaling and transformation. > > Say, for instance, that we have a branch instruction with two targets with counts {100, 300} inside a function 'foo' that has entry count 2. The edge probability for the first edge (count 100) is 100/(100+300) = 25%. > > If we inline foo() inside another function bar() at a callsite with profile count == 1, the cloned branch instruction gets its counters scaled with the callsite count. So the new branch has counts {100 * 1 / 2, 300 * 1 / 2} = {50, 150}. But the branch probability did not change. Currently, we are cloning the branch without changing the edge weights. > > This scaling is not difficult at all and can be incrementally very quickly. We cannot afford to recompute all frequencies on the fly because it would be detrimental to compile time. If foo() itself has N callees inlined into it, each inlined callee needs to trigger a re-computation. When foo() is inlined into bar(), the frequencies will need to be recomputed for foo() and all N callees inlined into foo(). > > > We are definitely missing the per-function execution counts that are needed to be able to compare relative “hotness” across functions, and I think that would be a good place to start making improvements. In the long term, we should keep our options open to making major changes, but before we go there, we should try to make incremental improvements to fix the existing infrastructure. > > Right, and that's the core of our proposal. We don't really want to make major infrastructure changes at this point. The only thing I'd like to explore is making MD_prof a real count. This will be useful for the inliner changes and it would also make incremental updates easier, because the scaling that needs to be done is very straightforward and quick. > > Note that this change should not modify the current behaviour we get from profile analysis. Things that were hot before should continue to be hot now. > > > Many of the other issues you raise seem like they could also be addressed without major changes to the existing infrastructure. Let’s try to fix those first. > > That's exactly the point of the proposal. We definitely don't want to make major changes to the infrastructure at first. My thinking is to start working on making MD_prof a real count. One of the things that are happening is that the combination of real profile data plus the frequency propagation that we are currently doing is misleading the analysis. > > For example (thanks David for the code and data). In the following code: > > int g; > __attribute__((noinline)) void bar() { > g++; > } > > extern int printf(const char*, ...); > > int main() > { > int i, j, k; > > g = 0; > > // Loop 1. > for (i = 0; i < 100; i++) > for (j = 0; j < 100; j++) > for (k = 0; k < 100; k++) > bar(); > > printf ("g = %d\n", g); > g = 0; > > // Loop 2. > for (i = 0; i < 100; i++) > for (j = 0; j < 10000; j++) > bar(); > > printf ("g = %d\n", g); > g = 0; > > > // Loop 3. > for (i = 0; i < 1000000; i++) > bar(); > > printf ("g = %d\n", g); > g = 0; > } > > When compiled with profile instrumentation, frequency propagation is distorting the real profile because it gives different frequency to the calls to bar() in the 3 different loops. All 3 loops execute 1,000,000 times, but after frequency propagation, the first call to bar() gets a weight of 520,202 in loop #1, 210,944 in loop #2 and 4,096 in loop #3. In reality, every call to bar() should have a weight of 1,000,000.(Sorry for the delay responding; I've been on holiday.) There are two things going on here. Firstly, the loop scales are being capped at 4096. I propagated this approximation from the previous version of BFI. If it's causing a problem (which it looks like it is), we should drop it and fix what breaks. You can play around with this by commenting out the `if` statement at the end of `computeLoopScale()` in BlockFrequencyInfoImpl.cpp. For example, without that logic this testcase gives: Printing analysis 'Block Frequency Analysis' for function 'main': block-frequency-info: main - entry: float = 1.0, int = 8 - for.cond: float = 51.5, int = 411 - for.body: float = 50.5, int = 403 - for.cond1: float = 5051.0, int = 40407 - for.body3: float = 5000.5, int = 40003 - for.cond4: float = 505001.0, int = 4040007 - for.body6: float = 500000.5, int = 4000003 - for.inc: float = 500000.5, int = 4000003 - for.end: float = 5000.5, int = 40003 - for.inc7: float = 5000.5, int = 40003 - for.end9: float = 50.5, int = 403 - for.inc10: float = 50.5, int = 403 - for.end12: float = 1.0, int = 8 - for.cond13: float = 51.5, int = 411 - for.body15: float = 50.5, int = 403 - for.cond16: float = 500051.0, int = 4000407 - for.body18: float = 500000.5, int = 4000003 - for.inc19: float = 500000.5, int = 4000003 - for.end21: float = 50.5, int = 403 - for.inc22: float = 50.5, int = 403 - for.end24: float = 1.0, int = 8 - for.cond26: float = 500001.5, int = 4000011 - for.body28: float = 500000.5, int = 4000003 - for.inc29: float = 500000.5, int = 4000003 - for.end31: float = 1.0, int = 8 (Now we get 500000.5 for all the inner loop bodies.) Secondly, instrumentation-based profiling intentionally fuzzes the profile data in the frontend using Laplace's Rule of Succession (look at `scaleBranchWeight()` in CodeGenPGO.cpp). For example, "loop 1" (which isn't affected by the 4096 cap) should give a loop scale of 500000.5, not 1000000. (The profile data says 1000000/10000 for the inner loop, 10000/100 for the middle, and 100/1 for the outer loop. Laplace says that we should fuzz these branch weights to 1000001/10001, 10001/101, and 101/2, which works out to 1000001/2 == 500000.5 total.)
On 03/10/2015 10:14 AM, Diego Novillo wrote:> > > On Thu, Mar 5, 2015 at 11:29 AM, Bob Wilson <bob.wilson at apple.com > <mailto:bob.wilson at apple.com>> wrote: > > >> On Mar 2, 2015, at 4:19 PM, Diego Novillo <dnovillo at google.com >> <mailto:dnovillo at google.com>> wrote: >> >> On Thu, Feb 26, 2015 at 6:54 PM, Diego Novillo >> <dnovillo at google.com <mailto:dnovillo at google.com>> wrote: >> >> I've created a few bugzilla issues with details of some of >> the things I'll be looking into. I'm not yet done >> wordsmithing the overall design document. I'll try to finish >> it by early next week at the latest. >> >> >> The document is available at >> >> https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing >> <https://docs.google.com/document/d/15VNiD-TmHqqao_8P-ArIsWj1KdtU-ElLFaYPmZdrDMI/edit?usp=sharing> >> >> There are several topics covered. Ideally, I would prefer that we >> discuss each topic separately. The main ones I will start working >> on are the ones described in the bugzilla links we have in the doc. >> >> This is just a starting point for us. I am not at all concerned >> with implementing exactly what is proposed in the document. In >> fact, if we can get the same value using the existing support, >> all the better. >> >> OTOH, any other ideas that folks may have that work better than >> this are more than welcome. I don't have really strong opinions >> on the matter. I am fine with whatever works. > > Thanks for the detailed write-up on this. Some of the issues > definitely need to be addressed. I am concerned, though, that some > of the ideas may be leading toward a scenario where we have > essentially two completely different ways of representing profile > information in LLVM IR. It is great to have two complementary > approaches to collecting profile data, but two representations in > the IR would not make sense. > > > Yeah, I don't think I'll continue to push for a new MD_count > attribute. If we were to make MD_prof be a "real" execution count, > that would be enough. Note that by re-using MD_prof we are not > changing its meaning at all. The execution count is still a weight and > the ratio is still branch probability. All that we are changing are > the absolute values of the number and increasing its data type width > to remove the 32bit limitation.Independent of everything else, relaxing the 32 bit restriction is clearly a good idea. This would make a great standalone patch.> > > > The first issue raised is that profile execution counts are not > represented in the IR. This was a very intentional decision. I > know it goes against what other compilers have done in the past. > It took me a while to get used to the idea when Andy first > suggested it, so I know it seems awkward at first. The advantage > is that branch probabilities are much easier to keep updated in > the face of compiler transformations, compared to execution counts. > > > Sorry. I don't follow. Updating counts as the CFG is transformed is > not difficult at all. What examples do you have in mind? The big > advantage of making MD_prof an actual execution count is that it is a > meaningful metric wrt scaling and transformation. > > Say, for instance, that we have a branch instruction with two targets > with counts {100, 300} inside a function 'foo' that has entry count 2. > The edge probability for the first edge (count 100) is 100/(100+300) = > 25%. > > If we inline foo() inside another function bar() at a callsite with > profile count == 1, the cloned branch instruction gets its counters > scaled with the callsite count. So the new branch has counts {100 * 1 > / 2, 300 * 1 / 2} = {50, 150}. But the branch probability did not > change. Currently, we are cloning the branch without changing the edge > weights. > > This scaling is not difficult at all and can be incrementally very > quickly. We cannot afford to recompute all frequencies on the fly > because it would be detrimental to compile time. If foo() itself has N > callees inlined into it, each inlined callee needs to trigger a > re-computation. When foo() is inlined into bar(), the frequencies will > need to be recomputed for foo() and all N callees inlined into foo().It really sounds like your proposal is to essentially eagerly compute scaling rather than lazyily compute it on demand. Assuming perfect implementations for both (with no rounding losses), the results should be the same. Is that a correct restatement? I'm going to hold off on responding to why that's a bad idea until you confirm, because I'm not sure I follow what you're trying to say. :) Also, trusting exact entry counts is going to be somewhat suspect. These are *highly* susceptible to racy updates, overflow, etc... Anything which puts too much implicit trust in these numbers is going to be problematic.> > > We are definitely missing the per-function execution counts that > are needed to be able to compare relative “hotness” across > functions, and I think that would be a good place to start making > improvements. In the long term, we should keep our options open to > making major changes, but before we go there, we should try to > make incremental improvements to fix the existing infrastructure. > > > Right, and that's the core of our proposal. We don't really want to > make major infrastructure changes at this point. The only thing I'd > like to explore is making MD_prof a real count. This will be useful > for the inliner changes and it would also make incremental updates > easier, because the scaling that needs to be done is very > straightforward and quick. > > Note that this change should not modify the current behaviour we get > from profile analysis. Things that were hot before should continue to > be hot now.I have no objection to adding a mechanism for expressing an entry count. I am still very hesitant about the proposals with regards to redefining the current MD_prof. I'd encourage you to post a patch for the entry count mechanism, but not tie its semantics to exact execution count. (Something like "the value provided must correctly describe the relative hotness of this routine against others in the program annoatated with the same metadata. It is the relative scaling that is important, not the absolute value. In particular, the value need not be an exact execution count.")> > > Many of the other issues you raise seem like they could also be > addressed without major changes to the existing infrastructure. > Let’s try to fix those first. > > > That's exactly the point of the proposal. We definitely don't want to > make major changes to the infrastructure at first. My thinking is to > start working on making MD_prof a real count. One of the things that > are happening is that the combination of real profile data plus the > frequency propagation that we are currently doing is misleading the > analysis.I consider this a major change. You're trying to redefine a major part of the current system. Multiple people have spoken up and objected to this change (as currently described). Please start somewhere else.> > For example (thanks David for the code and data). In the following code: > > int g; > __attribute__((noinline)) void bar() { > g++; > } > > extern int printf(const char*, ...); > > int main() > { > int i, j, k; > > g = 0; > > // Loop 1. > for (i = 0; i < 100; i++) > for (j = 0; j < 100; j++) > for (k = 0; k < 100; k++) > bar(); > > printf ("g = %d\n", g); > g = 0; > > // Loop 2. > for (i = 0; i < 100; i++) > for (j = 0; j < 10000; j++) > bar(); > > printf ("g = %d\n", g); > g = 0; > > > // Loop 3. > for (i = 0; i < 1000000; i++) > bar(); > > printf ("g = %d\n", g); > g = 0; > } > > When compiled with profile instrumentation, frequency propagation is > distorting the real profile because it gives different frequency to > the calls to bar() in the 3 different loops. All 3 loops execute > 1,000,000 times, but after frequency propagation, the first call to > bar() gets a weight of 520,202 in loop #1, 210,944 in loop #2 and > 4,096 in loop #3. In reality, every call to bar() should have a weight > of 1,000,000.Duncan responded to this. My conclusion from his response: this is a bug, not a fundamental issue. Remove the max scaling factor, switch the counts to 64 bits and everything should be fine. If you disagree, let's discuss.> > > Thanks. Diego. > > > _______________________________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150324/2caf9dc3/attachment.html>