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
to reflect the new license.
We understand that people may be surprised that we're moving the header
entirely to discuss the new license. We checked this carefully with the
Foundation's lawyer and we believe this is the correct approach.
Essentially, all code in the project is now made available by the LLVM
project under our new license, so you will see that the license headers
include that license only. Some of our contributors have contributed
code under our old license, and accordingly, we have retained a copy of
our old license notice in the top-level files in each project and
repository.
llvm-svn: 351636
NFC here, this just raises some platform specific ifdef hackery
out of a class and creates proper platform-independent typedefs
for the relevant things. This allows these typedefs to be
reused in other places without having to reinvent this preprocessor
logic.
llvm-svn: 334294
Summary:
This is the third of a multi-part change to implement subcommands for
the `llvm-xray` tool.
Here we define the `account` subcommand which does simple function call
accounting, generating basic statistics on function calls we find in an
XRay log/trace. We support text output and csv output for this
subcommand.
This change also supports sorting, summing, and filtering the top N
results.
Part of this tool will later be turned into a library that could be used
for basic function call accounting.
Depends on D24376.
Reviewers: dblaikie, echristo
Subscribers: mehdi_amini, dberris, beanz, llvm-commits
Differential Revision: https://reviews.llvm.org/D24377
llvm-svn: 291749
Roman Lebedev