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
Differential Revision: https://reviews.llvm.org/D125179
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");