jansvoboda11 added a comment.
In D136019#3860557 <https://reviews.llvm.org/D136019#3860557>, @ributzka wrote:
> We could invoke clang with the `-stats` option and compare the result against
> the expected number of stat calls.
The `-stats-file=` option only tells you how many times Clang requested
`DirectoryEntry` object from `FileManager` (i.e. calling
`FileManager::getDirectory(StringRef)`). This function **only** touches the
real file system once and caches information internally. Checking this doesn't
really help us catch regressions like this one. It's a cheap operation and it's
pretty common and reasonable for this number to change between commits.
What's actually expensive about this regression is calling
`HeaderSearch::loadSubdirectoryModuleMaps(DirectoryLookup &)` multiple times,
since `Dir.haveSearchedAllModuleMaps()` never changes (we only mutate the local
`DirectoryLookup` copy). That function accesses the real filesystem via LLVM's
VFS and **that** is actually the expensive operation:
llvm::vfs::FileSystem &FS = FileMgr.getVirtualFileSystem();
for (llvm::vfs::directory_iterator Dir = FS.dir_begin(DirNative, EC), DirEnd;
Dir != DirEnd && !EC; Dir.increment(EC)) {
//
}
It would be nice to have statistics for these as well, but we don't have any
infrastructure for that ATM.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D136019/new/
https://reviews.llvm.org/D136019
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