[PATCH] D83621: [clang][Tooling] Try to avoid file system access if there is no record for the file in compile_commads.json

Mon Jul 13 02:08:03 PDT 2020

ArcsinX added a comment.

In D83621#2146706 <https://reviews.llvm.org/D83621#2146706>, @sammccall wrote:

> Wow, yeah, this seems pretty bad! I'm not very familiar with this code. I'm curious, for "it tooks more than 1 second" - what OS is this on?

Windows.
I faced this problem on a huge project and it takes 7 seconds for `MatchTrie.findEquivalent()` to return.

> My guess is that way back when, the performance of looking up a CDB entry that didn't exist wasn't a big deal, or windows support wasn't a big thing, or this was just an oversight.
> 
> I think it would be slightly cleaner to fix this in the trie itself, either:

Firstly, I wanted to fix this problem in `FileMatchTrie`, but for caching solution we need to update cache at every `instert()` call, but for caching solution inside `JSONCompilationDatabase` we can avoid this, because all inserts already done at `JSONCompilationDatabase::parse()`.

> - don't scan for equivalences if the set of candidates exceeds some size threshold (10 or so)
> - don't scan for equivalences if the node we'd scan under is the root

After such fixes `JSONCompilationDatabase::getCompileCommands()` will return other results than before in some cases.

Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D83621/new/

https://reviews.llvm.org/D83621