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[XRay] Account: recursion detection

Authored by lebedev.ri on Jul 27 2020, 12:15 AM.

Description

[XRay] Account: recursion detection

Summary:
Recursion detection can be non-trivial. Currently, the state-of-the-art for LLVM,
as far as i'm concerned, is D72362 [clang-tidy] misc-no-recursion: a new check.
However, it is quite limited:

  • It does very basic call-graph based analysis, in the sense it will report even dynamically-unreachable recursion.
  • It is inherently limited to a single TU
  • It is hard to gauge how problematic each recursion is in practice.

Some of that can be addressed by adding clang analyzer-based check,
then it would at least support multiple TU's.

However, we can approach this problem from another angle - dynamic run-time analysis.
We already have means to capture a run-time callgraph (XRay, duh),
and there are already means to reconstruct it within llvm-xray tool.

This proposes to add a -recursive-calls-only switch to the account tool.
When the switch is on, when re-constructing callgraph for latency reconstruction,
each time we enter/leave some function, we increment/decrement an entry for the function
in a "recursion depth" map. If, when we leave the function, said entry was at 1,
then that means the function didn't call itself, however if it is at 2 or more,
then that means the function (possibly indirectly) called itself.

If the depth is 1, we don't account the time spent there,
unless within this call stack the function already recursed into itself.
Note that we don't pay for recursion depth tracking when recursive-calls-only is not on,
and the perf impact is insignificant (+0.3% regression)

The overhead of the option is actually negative, around -5.26% user time on a medium-sized (3.5G) XRay log.
As a practical example, that 3.5G log is a capture of the entire middle-end opt pipeline
at -O3 for RawSpeed unity build. There are total of 5500 functions in the log,
however -recursive-calls-only says that 269, or 5%, are recursive.

Having this functionality could be helpful for recursion eradication.

Reviewers: dberris, mboerger

Reviewed By: dberris

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84582

Details

Committed
lebedev.riJul 27 2020, 12:15 AM
Reviewer
dberris
Differential Revision
D84582: [XRay] Account: recursion detection
Parents
rGeed333149d17: [NewPM] NFC. remove obsolete TODO comment
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