[PATCH] D135801: [clang][Lexer] Speedup HeaderSearch when there are many HeaderMaps

[Troy Johnson via Phabricator via cfe-commits]

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HeaderSearch already uses a caching system to avoid duplicate searches, but the initial cold miss can take a long time if a build system has supplied thousands of HeaderMaps. For this case, the SearchDirs vector begins with those HeaderMaps, so a cache miss requires testing if the sought filename is present in each of those maps. Instead, we can consolidate the keys of those HeaderMaps into one StringMap and then each cache miss can skip directly to the correct HeaderMap or continue its search beyond the initial sequence of HeaderMaps. In testing on TUs with ~15000 SearchDirs where the initial 99% are HeaderMaps, time spent in Clang was reduced by 15%. This patch is expected to be neutral for SearchDir vectors that do not begin with HeaderMaps.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D135801

Files:
  clang/include/clang/Lex/HeaderMap.h
  clang/include/clang/Lex/HeaderSearch.h
  clang/lib/Lex/HeaderSearch.cpp

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