[llvm-dev] RFC: Support regex based path remapping for llvm-cov

Fāng-ruì Sòng via llvm-dev
Wed, 17 Feb 2021 08:40:16 -0000

I was confused by the option name -path-equivalence because ...
equivalence does not imply the mapping direction, however, in reality
there must be a direction, you either change A to B or B to A (this
affects the output or the bytes recorded in the binary).

On Tue, Feb 16, 2021 at 8:11 PM Choongwoo Han via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> We'd like to contribute something related to -path-equivalence option
of llvm-cov. We want to hear feedbacks from you guys.
>
> In summary, there are three changes.
> 1. Use regular expression to remap paths.
> 2. Do not use `sys::fs::equivalent` to find the same file, which causes a
huge performance issue in our environment.
> 3. Use remapped paths for output directory paths of `llvm-show` instead of
original paths.
>
> Here is some context why we want this feature. We're building a single
binary from multiple machines, so the binary contains different source code
paths for the same file. There is already `-path-equivalence` option for
remapping paths, but it does not meet our requirement because we need n to 1
mapping while the current implementation only supports 1 to 1 mapping. Thus,
currently, we are using a custom llvm-cov build using a regular expression. The
command line looks like this
> $ llvm-cov show
-path-equivalence='[A-Za-z0-9_/\\:]+[\\/]src[\\/][0-9]+[\\/]([A-Za-z]),:/'
...
> Then, /path/to/src/0/f/asdf becomes f:/asdf.
>
> We are using a capture to find a prefix (i.e. captured_string +
filepath.replace(regex, to)), but we can drop this part if it looks complicated
and not intuitive.
>
> Secondly, there is a huge performance degradation in our environment
because of file system accesses in llvm-cov.
>
https://github.com/llvm/llvm-project/blob/d079dbc591899159925a1fe10b081fa0f6bb61bd/llvm/tools/llvm-cov/CodeCoverage.cpp#L251-L253
> ```
> ErrorOr<const MemoryBuffer &>
> CodeCoverageTool::getSourceFile(StringRef SourceFile) {
> ...
>   for (const auto &Files : LoadedSourceFiles)
>     if (sys::fs::equivalent(SourceFile, Files.first))
>       return *Files.second;
> ```
>
> If I rewrite the `sys::fs::equivalent` to a plain string comparison, the
performance is improved a lot. When I tested it with a sample binary, the
running time reduced from 1 minutes to 7 seconds. And, the overall running time
of llvm-cov in our Windows environment reduced from 17 hours to 4 hours.
> Since we are already using the remapped paths to find the same files, we
don't need this kind of strict check to find equivalent files. So, I'd
like to replace this strict comparison to a simple one when this regex-based
remapping is being used.
>
> Lastly, the current implementation is using the original paths to generate
output files. For example, if `-path-equivalence=/tmp,/output -format=html
-output-dir=./outdir` is used, the generated file paths will be
`./outdir/tmp/...`, not `./outdir/output/...`. It does not make sense because
there can be multiple different original paths. Thus, I'd like to use the
remapped file paths to generate the output files.
>
> Since we don't want to break the existing option, I'm thinking
about introducing a new flag adding `-regex` at the end (i.e.
`-path-equivalence-regex`). I want to ask which approach is best for this case
(introducing a new flag or updating the existing behavior).
>
> Please let me know if there is any concern from this proposal.
> I also uploaded a proof of concept to https://reviews.llvm.org/D96696
>
> Thanks,
> Choongwoo Han
>
>
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