[llvm-dev] [LLVMdev] Path forward on profile guided inlining?

Philip Reames via llvm-dev llvm-dev at lists.llvm.org
Thu Dec 10 16:00:20 PST 2015

Given I didn't get any response to my original query, I chose not to 
invest time in this at the time.  I am unlikely to get time for this in 
the near future.

On 12/07/2015 03:13 PM, Easwaran Raman wrote:
> (Resending after  removing llvmdev at cs.uiuc.edu 
> <mailto:llvmdev at cs.uiuc.edu> and using llvm-dev at lists.llvm.org 
> <mailto:llvm-dev at lists.llvm.org>)
> On Mon, Dec 7, 2015 at 3:08 PM, Easwaran Raman <eraman at google.com 
> <mailto:eraman at google.com>> wrote:
>     Hi Philip,
>      Is there any update on this? I've been sending patches to get rid
>     of the callee hotness based inline hints from the frontend and
>     move the logic to the inliner. The next step is to use the
>     callsite hotness instead. I also want to focus on the
>     infrastructure to enable this and what I've been experimenting
>     with is similar to your two alternative approaches:
>         Alternate Approaches:
>         1) We could just recompute BFI explicitly in the inliner right
>         before passing the result to ICA for the purposes of
>         prototyping. If this was off by default, this might be a
>         reasonable scheme for investigation.  This could likely never
>         be enabled for real uses.
>         2) We could pre-compute BFI once per function within a given
>         SCC and then try to keep it up to date during inlining.  If we
>         cached the call frequencies for the initial call sites, we
>         could adjust the visit order to minimize the number of times
>         we need to recompute a given functions block frequencies. 
>         (e.g. we can look at all the original call sites within a
>         function before looking at newly inlined ones)
>     My proposal is very similar (perhaps identical) to your option 2
>     above. I don't understand the part where you talk about adjusting
>     the visit order to minimize BFI computation.
>     BFI computation:  BFI for a function is computed on demand and
>     cached.
The problem here is that the old pass manager does not have a place/way 
to cache the function level analysis.  You could do this within the 
inliner itself, but the function passes run by the SCC pass manager will 
invalidate everything.  That's the problem I was trying to solve.
>     Update: When 'bar' gets inlined into 'foo', the BFI for 'foo' is
>     updated. Let OldBB in 'bar' gets cloned as NewBB in 'foo'. 
>     NewBB's block frequency can be incrementally computed from OldBB's
>     block frequency, entry block frequency of 'bar' and the frequency
>     of the block containing the 'foo' -> 'bar' callsite. Even when the
>     new CGSCC level BFI analysis is in place, this incremental update
>     is useful to minimize computation.
>     Invalidation:  Once inlining is completed in an SCC (at the end of
>     runOnSCC), the entries for functions in that SCC are invalidated
>     since other passes run by the CGSCC pass manager (including those
>     run by the function pass manager run under CGSCC pass manager)
>     might affect the computed BFI for the functions in the SCC.
This sounds like you're essentially planning to extend the old pass 
manager with function level analysis support in the SCC pass manager.  I 
advise against this given the fact that Chandler is actively working on 
the new one and getting patches reviewed for the old one is 
"challenging" at best.  If you take this approach, make sure Chandler - 
who is pretty much the only active reviewer in this area - is on-board 
with your approach before you start.
>     When the new PM infrastructure and a CGSCC based BFI analysis is
>     in place, the transition should be easy assuming it will provide
>     getBFI(Function *) and invalidateBFI(Function *) interfaces. BFI
>     for a function is computed at most twice in this approach. Thoughts?
>     Thanks,
>     Easwaran
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