[LLVMdev] RFC: Profiling Enhancements (GSoC)

[Alastair Murray]

Hi all,

In light of the expected removal of ProfileInfo this is a request for 
comments on the next few items that I now plan to work on for GSoC.

Planned tasks:

#0 Add support for determining branch weight metadata by profiling

At the absolute minimum this will require writing a new profile loader 
which will set branch weight metadata based on profiling data.


#1 Optionally use profiling support to heuristics within:
        * Loop unrolling
        * Loop unswitching
        * Inliner

The existing BranchProbabilityInfo, and BlockFrequencyInfo are not used 
in Transforms/*/* at all, just in CodeGen/*.  Is there a reason for 
this, or is it quite possible to use them in Transforms/*/* passes?  The 
original plan was to test these using ProfileEstimator, now the static 
heuristics within BranchProbability could be used instead until branch 
weight metadata is successfully maintained through CFG altering passes.


#2 Make the profiler more JIT friendly

The ProfileInfo based profiling support is not very friendly towards PGO 
within a JIT context.  The main issue is that profiling data must go via 
the filing system.  This will be addressed by providing a new 
libprofile_rt.so runtime library for JITs to use.  This is planned to be 
demonstrated and documented by adding an extra chapter to the 
Kaleidoscope tutorial.  (There is also an issue of profiling a single 
function rather than a whole program.)

(For the record, there are also future plans to work on #3 profiling the 
targets of indirect function calls (highly likely) and #4 path profiling 
(not nearly as likely), but as it is not yet clear when these will be 
worked on I'll seek comment on these at a future date.)

Regards,
Alastair.