[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 15 11:22:42 PDT 2017
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
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.
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
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
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
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.
- 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.
LLVM Compiler Development
IBM Toronto Software Lab
Office: (905) 413-4077 C2-407/8200/Markham
Email: gyiu at ca.ibm.com
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the llvm-dev