[LLVMdev] How to use BlockFrequency in inter-procedural context?

[Das, Dibyendu]

David

Is this true for static heuristics as well ?

If the bb freqs are scaled wrt to the entry block freq and a) use such scaled freqs for the bb's that have calls b) propagate this info topologically over the call graph, how representative will be the info if one just wants to use in a comparative sense ?

-Dibyendu

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________________________________
From: Xinliang David Li<mailto:xinliangli at gmail.com>
Sent: ‎12/‎27/‎2014 10:05 AM
To: ibaev at codeaurora.org<mailto:ibaev at codeaurora.org>
Cc: llvmdev<mailto:llvmdev at cs.uiuc.edu>
Subject: Re: [LLVMdev] How to use BlockFrequency in inter-procedural context?


On Fri, Dec 26, 2014 at 7:12 PM, <ibaev at codeaurora.org<mailto:ibaev at codeaurora.org>> wrote:
The BlockFrequency analysis has been useful for machine block placement,
register allocation and other function-level optimizations. How could we
extend it for use in an inter-procedural (whole-program) context? For
example, if we would like to compare the hotness of two call sites in
different functions, or calculate the hotness of two global variables
referenced in multiple functions.

BlockFrequency can not be used for inter-procedural analysis.


If the ratio of a block BB frequency to the entry block frequency is the
expected number of times the block BB will execute per entry to the
function (according to LLVM Block Frequency Terminology page), would the
multiplication of that ratio to the profile count of the function be a
reasonable approximation of BB total execution count?

The answer depends on how BB frequency is computed. If BB frequency is directly scaled from BB profile count (Execution count), then the answer is 'yes'. In the current implementation, the answer is 'no'.  LLVM only keeps branch probability information from the profile data, and BB frequency is computed from branch probabilities:  There are known limitations of the frequency propagation algorithm to make the resulting frequency 'distorted'. To name a few: loop scale is capped at 4096; branch weight is incremented by 1 (leading to issues such as computed loop trip count as low as half of the actual count); limitation of handling irreducible loops etc. In fact, frequency can not be reliably be used for comparison across different loop nests.

We have plans to address those issues to improve PGO.

thanks,

David





Thanks,
Ivan



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