[clang-scan-deps] add skip excluded conditional preprocessor block preprocessing optimization

This commit adds an optimization to clang-scan-deps and clang's preprocessor that skips excluded preprocessor
blocks by bumping the lexer pointer, and not lexing the tokens until reaching appropriate #else/#endif directive.
The skip positions and lexer offsets are computed when the file is minimized, directly from the minimized tokens.

On an 18-core iMacPro with macOS Catalina Beta I got 10-15% speed-up from this optimization when running clang-scan-deps on
the compilation database for a recent LLVM and Clang (3511 files).

Differential Revision: https://reviews.llvm.org/D67127

llvm-svn: 371656

1 files changed, 5 insertions, 0 deletions

diff --git a/clang/lib/Lex/Preprocessor.cpp b/clang/lib/Lex/Preprocessor.cpp
index 3abd0c76d0f..82007732a9b 100644
--- a/clang/lib/Lex/Preprocessor.cpp
+++ b/clang/lib/Lex/Preprocessor.cpp
@@ -158,6 +158,11 @@ Preprocessor::Preprocessor(std::shared_ptr<PreprocessorOptions> PPOpts,
 
   if (this->PPOpts->GeneratePreamble)
     PreambleConditionalStack.startRecording();
+
+  ExcludedConditionalDirectiveSkipMappings =
+      this->PPOpts->ExcludedConditionalDirectiveSkipMappings;
+  if (ExcludedConditionalDirectiveSkipMappings)
+    ExcludedConditionalDirectiveSkipMappings->clear();
 }
 
 Preprocessor::~Preprocessor() {