[llvm-dev] [RFC] Enhance Partial Inliner by using a general outlining scheme for cold blocks
Xinliang David Li via llvm-dev
llvm-dev at lists.llvm.org
Thu Aug 24 12:04:31 PDT 2017
On Thu, Aug 24, 2017 at 10:40 AM, Graham Yiu <gyiu at ca.ibm.com> wrote:
> Hi David,
>
> So I've began doing some implementation on the outlining portion of the
> code. Currently, I got the partial inliner to outline cold regions (single
> entry, single exit) of the code, based solely on the existence of
> ProfileSummaryInfo (ie. profiling data). However, I have some concerns on
> how this will co-exist with the existing code that peels early returns.
>
> The control flow looks something like this:
>
> // New Code: find cold regions to outline
> if (!computeOutliningInfoForColdRegions()) {
> // If we can't find any cold regions, then fall-back to early return
> peeling
> if (!computeOutliningInfo) {
> return nullptr;
> }
> }
> // Try to outline the identified regions
> // Then try to inline the cloned function
>
> My concern is during inlining, if we fail to inline the cloned function,
> we give up and discard all cloned and outlined functions. But with these
> two types of outlining we're doing, it's possible to attempt to inline the
> cloned function that has outlined cold regions, and if we cannot do so, try
> to inline a different clone that has peeled early returns (ie. the way we
> have it today). This would require us to clone the original function twice
> and modify one based on cold region outlining and the other early return
> peeling, with the latter being our fall-back option if we fail to inline
> the first clone.
>
> What are your thoughts?
>
>
I expect computeOutliningInfoForColdRegions can produce a super set of
outlinable regions to the current 'pattern matching' approach. In other
words, most of the cases currently caught by 'computeOutlineInfo' should be
caught by the new algorithm, so why not ditching the current
'computeOutlningInfo' completely?
My suggestion was to enhance the pass to 1) support outlining multiple
regions; and 2) add a mode to do function outlining only (not the inlining
part). The second is important can be used before the regular inliner
pass. With the new pass manager and profile aware inlining, the inliner
won't undo the outline decision, but in meantime becomes more powerful due
to the reduced hot function size.
David
> Graham Yiu
> LLVM Compiler Development
> IBM Toronto Software Lab
> Office: (905) 413-4077 C2-707/8200/Markham
> Email: gyiu at ca.ibm.com
>
> [image: Inactive hide details for Graham Yiu---08/15/2017 08:04:28
> PM---Hey David, Yes, we'll need to consider the effect on live range]Graham
> Yiu---08/15/2017 08:04:28 PM---Hey David, Yes, we'll need to consider the
> effect on live ranges for regions we want to outline. In
>
> From: Graham Yiu/Toronto/IBM
> To: Xinliang David Li <xinliangli at gmail.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>
> Date: 08/15/2017 08:04 PM
> Subject: Re: [llvm-dev] [RFC] Enhance Partial Inliner by using a general
> outlining scheme for cold blocks
> ------------------------------
>
>
> Hey David,
>
> Yes, we'll need to consider the effect on live ranges for regions we want
> to outline. In my experience, outlining live-exit regions seem to cause the
> most harm as we ruin chances to keep data in registers as you were alluding
> to. It's unclear, however, what the exact effect of outlining regions with
> live-entries would be.
>
> I'll probably try to avoid regions that are not single entry & single exit
> at least initially, to simplify the transformation and analysis. Are
> multi-exit regions common in your experience?
>
> And of course, I agree, we should reuse as much of the current partial
> inlining infrastructure as possible. I'll likely run some ideas by you as I
> begin to make changes.
>
> Cheers,
>
> Graham Yiu
> LLVM Compiler Development
> IBM Toronto Software Lab
> Office: (905) 413-4077 C2-407/8200/Markham
> Email: gyiu at ca.ibm.com
>
>
> [image: Inactive hide details for Xinliang David Li ---08/15/2017 05:36:07
> PM---Hi Graham, Making partial inlining more general is some]Xinliang
> David Li ---08/15/2017 05:36:07 PM---Hi Graham, Making partial inlining
> more general is something worth doing. Regarding your implementat
>
> From: Xinliang David Li <xinliangli at gmail.com>
> To: Graham Yiu <gyiu at ca.ibm.com>
> Cc: llvm-dev <llvm-dev at lists.llvm.org>
> Date: 08/15/2017 05:36 PM
> Subject: Re: [llvm-dev] [RFC] Enhance Partial Inliner by using a general
> outlining scheme for cold blocks
> ------------------------------
>
>
>
> Hi Graham, Making partial inlining more general is something worth doing.
> Regarding your implementation plan, I have some suggestions here:
>
> *) Function outlining introduces additional runtime cost: passing of live
> in values, returning of live out values (via memory), glue code in the
> caller to handle regions without a single exit block etc. The cost
> analysis needs to factor in those carefully
> *) Remove the limitation that there is only *one* outlined routine.
> Instead, the algorithm can compute multiple single-entry/single exit or
> single entry/multiple exit regions (cold ones) in the routine, and outline
> each region into its own function. The benefit include
> 1) simplify the design and implementation and most of the existing code
> can be reused;
> 2) provide more flexibility to allow most effective outlining;
> 3) reduced runtime overhead of making calls to the outline functions.
>
> thanks,
>
> David
>
> On Tue, Aug 15, 2017 at 11:22 AM, Graham Yiu via llvm-dev <
> *llvm-dev at lists.llvm.org* <llvm-dev at lists.llvm.org>> wrote:
>
> Hello,
>
> My team and I are looking to do some enhancements in the partial
> inliner in opt. Would appreciate any feedback that folks might have.
>
> # Partial Inlining in LLVM opt
>
> ## Summary
>
> ### Background
>
> Currently, the partial inliner searches the first few blocks of the
> callee and looks for a branch to the return block (ie. early return). If
> found, it attempts to outline the rest of the slow (or heavy) code so the
> inliner will be able to inline the fast (or light) code. If no early
> returns are found, the partial inliner will give up. As far as I can tell,
> BlockFrequency and BranchProbability information is only used when
> attempting to inline the early return code, and not used to determine
> whether to outline the slow code.
>
> ### Proposed changes
>
> In addition to looking for early returns, we should utilize profile
> information to outline blocks that are considered cold. If we can
> sufficiently reduce the size of the original function via this type of
> outlining, inlining should be able to inline the rest of the hot code.
>
> ## Details
>
> With the presence of profile information, we have a view of what code
> is infrequently executed and make better decisions on what to outline.
> Early return blocks that are infrequently executed should still be included
> as candidates for outlining, but will be treated just like any other cold
> blocks. Without profiling information, however, we should remain
> conservative and only partial inline in the presence of an early return in
> the first few blocks of a function (ie. peel the early return out of the
> function).
>
> To find cold regions to outline, we will traverse the CFG to find
> edges deemed 'cold' and look at the blocks dominated by the successor node.
> If, for some reason, that block has more than one predecessor, then we will
> skip this candidate as there should be a node that dominates this successor
> that has a single entry point. The last node in the dominance vector should
> also have a single successor. If it does not, then further investigation of
> the CFG is necessary to see when/how this situation occurs.
>
> We will need several heuristics to make sure we only outline in cases
> where we are confident it will result in a performance gain. Things such as
> threshold on when a branch is considered cold, the minimum number of times
> the predecessor node has to be executed in order for an edge to be
> considered (confidence factor), and the minimum size of the region to be
> outlined (can use inlining cost analysis like we currently do) will require
> some level of tuning.
>
> Similar to the current implementation, we will attempt to inline the
> leftover (hot) parts of the code, and if for some reason we cannot then we
> discard the modified function and its outlined code.
>
> ### Code changes
>
> The current Partial Inlining code first clones the function of
> interest and looks for a single set of blocks to outline. It then creates a
> function with the set the blocks, and saves the outlined function and
> outline callsite information as part of the function cloning container. In
> order to outline multiple regions of the function, we will need to change
> these containers to keep track of a list of regions to outline. We will
> also need to update the cost analysis to take into account multiple
> outlined functions.
>
> When a ProfileSummary is available, then we should skip the code that
> looks for early returns and go into new code that looks for cold regions to
> outline. When ProfileSummary is not available, then we can fall back to the
> existing code and look for early returns only.
>
> ### Tuning
>
> - The outlining heuristics will need to determine if a set of cold
> blocks is large enough to warrant the overhead of a function call. We also
> don't want the inliner to attempt to inline the outlined code later.
> - The threshold for determining whether a block is cold will also need
> to be tuned. In the case that profiling information is not accurate, we
> will pay the price of the additional call overhead for executing cold code.
> - The confidence factor, which can be viewed as the minimum number of
> times the predecessor has to be executed in order for an edge to be
> considered cold, should also be taken into account to avoid outlining code
> paths we have little information on.
>
> Graham Yiu
> LLVM Compiler Development
> IBM Toronto Software Lab
> Office: *(905) 413-4077* <(905)%20413-4077> C2-407/8200/Markham
> Email: *gyiu at ca.ibm.com* <gyiu at ca.ibm.com>
>
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