[PATCH] Coverage: improve efficiency of the counter propagation to the expansion regions.

[Justin Bogner]

Alex Lorenz <arphaman at gmail.com> writes:
> This patch reduces the complexity of the two inner loops in order to
> speed up the loading of coverage data for very large functions
> (>100kloc, >15k regions, >5k files).
>
> http://reviews.llvm.org/D4708
>
> Files:
>   lib/ProfileData/CoverageMappingReader.cpp
>
> Index: lib/ProfileData/CoverageMappingReader.cpp
> ===================================================================
> --- lib/ProfileData/CoverageMappingReader.cpp
> +++ lib/ProfileData/CoverageMappingReader.cpp
> @@ -251,15 +251,20 @@
>    // from the expanded file.
>    // Perform multiple passes to correctly propagate the counters through
>    // all the nested expansion regions.
> +  SmallVector<CounterMappingRegion *, 8> ExpansionRegions;

How often are there 8 or less expansion regions? If we're talking about
more than 15k regions I don't see this vector staying small very often.

>    for (unsigned Pass = 1, S = VirtualFileMapping.size(); Pass < S; ++Pass) {
> -    for (auto &I : MappingRegions) {
> -      if (I.Kind == CounterMappingRegion::ExpansionRegion) {
> -        for (const auto &J : MappingRegions) {
> -          if (J.FileID == I.ExpandedFileID) {
> -            I.Count = J.Count;
> -            break;
> -          }
> -        }
> +    ExpansionRegions.clear();
> +    ExpansionRegions.resize(VirtualFileMapping.size(), nullptr);

No point in doing this resize every time through the loop - the size
doesn't change.

I also don't find it obvious that you're using resize for the side
effect of zero filling the vector here. That probably deserves a
comment at least.

> +    for (auto &R : MappingRegions) {
> +      if (R.Kind != CounterMappingRegion::ExpansionRegion)
> +        continue;
> +      assert(!ExpansionRegions[R.ExpandedFileID]);
> +      ExpansionRegions[R.ExpandedFileID] = &R;
> +    }
> +    for (auto &R : MappingRegions) {
> +      if (ExpansionRegions[R.FileID]) {
> +        ExpansionRegions[R.FileID]->Count = R.Count;
> +        ExpansionRegions[R.FileID] = nullptr;

I don't think this is quite right. You don't null out th
ExpansionRegions that weren't used. Is that intentional?

Overall, I suspect there's a better algorithm for propagating these
counters. Have you thought about other approaches?


>        }
>      }
>    }