[PATCH] D125179: [clangd] Skip (most) predefined macros when scanning for preamble patching.

[Sam McCall via Phabricator via cfe-commits]

sammccall created this revision.
sammccall added a reviewer: kadircet.
Herald added subscribers: usaxena95, arphaman.
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sammccall requested review of this revision.
Herald added subscribers: cfe-commits, MaskRay, ilya-biryukov.
Herald added a project: clang-tools-extra.

This is unneccesary work.
With this change, we skip generating and lexing ~10k of predefines twice.

A dumb benchmark of building a preamble for an empty file in a loop shows:

- before: 1.90ms/run
- after: 1.36ms/run

So this should be worth 0.5ms for each AST build and code completion.

There can be a functional difference, but it's very minor.
If the preamble contains e.g. `#ifndef __llvm__ ... #endif` then before we would
not take it. After this change we will take the branch (single-file mode takes
all branches with unknown conditions) and so gather different directives.

However I think this is negligible:

- this is already true of non-builtin macros (from included headers). We've had no complaints.
- this affects the baseline and modified in the same way, so only makes a difference transiently when code guarded by such an #ifdef is being edited


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D125179

Files:
  clang-tools-extra/clangd/Preamble.cpp


Index: clang-tools-extra/clangd/Preamble.cpp
===================================================================
--- clang-tools-extra/clangd/Preamble.cpp
+++ clang-tools-extra/clangd/Preamble.cpp
@@ -302,6 +302,7 @@
     return error("compiler instance had no inputs");
   // We are only interested in main file includes.
   Clang->getPreprocessorOpts().SingleFileParseMode = true;
+  Clang->getPreprocessorOpts().UsePredefines = false;
   PreprocessOnlyAction Action;
   if (!Action.BeginSourceFile(*Clang, Clang->getFrontendOpts().Inputs[0]))
     return error("failed BeginSourceFile");


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