[PATCH] D54826: [Support/FileSystem] Add sub-second precision for atime/mtime of sys::fs::file_status on unix platforms

[Aaron Ballman via Phabricator via llvm-commits]

aaron.ballman added a comment.

In D54826#1307383 <https://reviews.llvm.org/D54826#1307383>, @akyrtzi wrote:

> > The general changes to the support libraries are usually facilitated because of missing functionality that is required somewhere else and it being the most logical place to add it. If so please attach related revisions to this one (through "Edit Related Revisions") so it's more clear why this was added
>
> There have been enhancements to file_status that added more info without the requirement you stated (attach related revisions to this one), like:
>  https://reviews.llvm.org/D31110
>  https://reviews.llvm.org/D18456
>
> This patch makes no changes to the existing API so there's no specific place that I can point to, but it improves accuracy of *every* site that calls getLastModification() and uses its value for comparisons, in and out-of-tree. It addresses 'false-negatives' (mistakenly considering that the mod time did not change because the change happened within 1 second) and 'false-positives' (when considering same mod time as 'changed' so as to be conservative against sub-second changes).


Is it going to be an issue that the Windows side of things has a more wild idea of file timestamp resolution? NTFS has a theoretical max precision of 100ns intervals, though according to MSDN, the access time on NTFS has a resolution of 1 hour, which is better than FAT file systems, where the resolution is 1 day. It seems odd to rely on ns resolution for access time, as that seems like it's going to be highly platform and filesystem dependent.

> For some examples:
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/Frontend/PrecompiledPreamble.cpp#L486
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/Driver/Driver.cpp#L1132
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/Driver/ToolChains/Clang.cpp#L2623
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/Serialization/ModuleManager.cpp#L156
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/Index/IndexUnitWriter.cpp#L252
>  https://github.com/apple/swift-clang/blob/95cdf7c9af0dc102a3fbda4ae1f6265026ff2e30/lib/DirectoryWatcher/DirectoryWatcher-linux.inc.h#L33
> 
> Apart from improving accuracy of mod time comparisons inside clang and its libraries themselves, mod times originating from clang libraries get reported up to our indexing system where we could take advantage of sub-second accuracy, for the same kind of benefits.




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