[llvm-dev] [RFC] Enhance Partial Inliner by using a general outlining scheme for cold blocks
Graham Yiu via llvm-dev
llvm-dev at lists.llvm.org
Tue Aug 29 10:24:17 PDT 2017
Hi Kader,
I agree with you, if we were going to only do outlining for some functions
and not immediately attempt to inline, it should be an independent pass.
The partial inliner should do what its name suggests and attempt to inline
or bail.
I haven't looked through the CodeExtractor at all. I imagine I'll have to
go through it at some point. I'd also be interested in something that does
an analysis before code extraction that tells me how many live ranges I'm
going to be killing or how many symbols I'm going to be taking the address
of by extracting a specific region of code. Not sure if that currently
exists.
Graham Yiu
LLVM Compiler Development
IBM Toronto Software Lab
Office: (905) 413-4077 C2-707/8200/Markham
Email: gyiu at ca.ibm.com
From: keita abdoul-kader <abdoulk.keita at gmail.com>
To: Xinliang David Li <xinliangli at gmail.com>
Cc: Graham Yiu <gyiu at ca.ibm.com>, llvm-dev
<llvm-dev at lists.llvm.org>
Date: 08/29/2017 12:15 PM
Subject: Re: [llvm-dev] [RFC] Enhance Partial Inliner by using a general
outlining scheme for cold blocks
I second the fact that a way to outline specific function regions
independently of the partial inliner sound very useful. I am not sure
however if we would want a mode within the partialInliner or something
completely independent.
As a general question, does anybody has a clear idea of what are the
constraints on the region CodeExtractor is currently able to handle ?
Going through the code, it looks like the only requirement is for the
header to dominate all the BB in the region ;
On Sat, Aug 26, 2017 at 9:52 AM, Xinliang David Li via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
On Thu, Aug 24, 2017 at 12:47 PM, Graham Yiu <gyiu at ca.ibm.com> wrote:
Hi David,
The only reason I can see to use the 'pattern matching' part as a
fall-back is in case we cannot inline the (what I'm assuming would be) a
much bigger hot-path-only cloned function for whatever reason. What I'm
assuming here is that after cold-region outlining, we may still have a
large portion of the original function body to attempt to inline,
whereas the pattern matching method will only contain a few basic
blocks, giving a better chance to inline something.
With profile data, the overhead of outlining a cold region can be
estimated more accurately. (With the new PM), the threshold of inlining a
hot callsite is also much higher. Without profile, the pattern matching
method won't work too well in general even though it can enable more more
inlining because the call overhead introduced to call the outlined
function may outweigh the benefit of inlining the caller.
What ever region that can be found by the pattern matching method should
be identified by the new method as well. If there are multiple (but
mutually exclusive) candidate regions found, the cost analysis heuristic
should pick the best candidate region for outlining .
For your (2) point, I think we'll have to be careful here. Without a
sense of how 'likely' we're going to inline the new function, we'll have
to make sure our outlining of cold regions will not degrade the
performance of the function in 99.xx% of the cases, as it's unclear how
much performance we'll gain from just outlining (without inlining to
increase the odds of some performance gain). My initial thought was to
ditch the new function and its outlined children if we cannot
immediately inline it.
The outlining only mode is useful to enable more aggressive inlining for
the regular inlining pass. Slightly different heuristics can be applied
here. For instance it can prefer largest candidate region (to maximiize
the chance to inline the caller). The outlined region does not need to be
super cold and leave it to the inliner to do more deeper analysis and
decide to inline it right back in.
David
Graham Yiu
LLVM Compiler Development
IBM Toronto Software Lab
Office: (905) 413-4077 C2-707/8200/Markham
Email: gyiu at ca.ibm.com
Inactive hide details for Xinliang David Li ---08/24/2017 03:05:06
PM---On Thu, Aug 24, 2017 at 10:40 AM, Graham Yiu <gyiu at ca.iXinliang
David Li ---08/24/2017 03:05:06 PM---On Thu, Aug 24, 2017 at 10:40 AM,
Graham Yiu <gyiu at ca.ibm.com> wrote: > Hi David,
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: 08/24/2017 03:05 PM
Subject: Re: [llvm-dev] [RFC] Enhance Partial Inliner by using a general
outlining scheme for cold blocks
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
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
rangeGraham 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
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> 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 C2-407/8200/Markham
Email: gyiu at ca.ibm.com
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