llvm dev - Feb 2021 - [EXTERNAL] Re: RFC: Support regex based path remapping for llvm-cov

Awesome! It seems like this will address our issue as well. I'll try it.

Still, I want to address the performance issue though. I'll just submit
separate patches for them later.

Thanks,

________________________________
From: Petr Hosek
Sent: Monday, February 15, 2021 22:50
To: Choongwoo Han
Cc: llvm-dev at lists.llvm.org
Subject: [EXTERNAL] Re: [llvm-dev] RFC: Support regex based path remapping for
llvm-cov

We have a similar issue. We use a distributed compilation system where different
TUs may be compiled on different machines and the compilation directory would
differ for each TU.

Our solution is to make the compiler output deterministic regardless of the
compilation directory by storing compilation directory separately from
filenames, which can be manually overridden, and by using relative paths if
those were passed to the compiler. This is similar to debug info handling, see
https://blog.llvm.org/2019/11/deterministic-builds-with-clang-and-lld.html.

This solution was implemented in https://reviews.llvm.org/D95753 and I hope to
land this within the next day or two. Would this address your problem as well?

On Mon, Feb 15, 2021 at 10:25 AM Choongwoo Han via llvm-dev <llvm-dev at
lists.llvm.org<mailto: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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