[llvm-bugs] [Bug 34281] New: Bad performance on reparse

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https://bugs.llvm.org/show_bug.cgi?id=34281

            Bug ID: 34281
           Summary: Bad performance on reparse
           Product: clang
           Version: 4.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: libclang
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jmachowinski at gmx.de
                CC: klimek at google.com, llvm-bugs at lists.llvm.org

Hi,
while testing kdevelop 5, I ran into some problems with the code completion.
I debugged it and tracked it down to the reparse step in libclang taking
5 seconds in an i7-5930K.

In my concrete example, I got a cpp file that consists of 173 line of code,
with minor template usage.
This program includes a big amount of headers, that use heavy templating.

The initial parse takes 13 seconds, which is fine. What makes me wonder is
the reparse step taking so long, as everything in the headers should be
cached by the PCH.

I prepared a git repo with an example, if you want to look at the code I used,
or try it yourself.

https://github.com/jmachowinski/clangTest

It should be noted, that I do not set CXTranslationUnit_Incomplete,
as kdevelop needs the flag to be NOT set. Setting the flag speeds up reparse
to 0.5 seconds.

Debug output :

Parse start
Precompiling preamble:   8.2960 (100.0%)   0.2760 (100.0%)   8.5720 (100.0%)  
8.5706 (100.0%)
Parsing ../test/tdot_asguard.cpp:   4.8320 (100.0%)   0.0200 (100.0%)   4.8520
(100.0%)   4.8499 (100.0%)
  ASTContext: expressions, declarations, and types: 254289451
  ASTContext: identifiers: 507904
  ASTContext: selectors: 4096
  ASTContext: side tables: 9053176
  Code completion: cached global results: 0
  SourceManager: content cache allocator: 86016
  SourceManager: malloc'ed memory buffers: 2070147
  SourceManager: mmap'ed memory buffers: 0
  SourceManager: data structures and tables: 124472904
  ExternalASTSource: malloc'ed memory buffers: 0
  ExternalASTSource: mmap'ed memory buffers: 188282552
  Preprocessor: malloc'ed memory: 306782
  Preprocessor: PreprocessingRecord: 838904
  Preprocessor: header search tables: 43
Parse took 13.4559
Cache global code completions for ../test/tdot_asguard.cpp:   0.1280 (100.0%)  
0.1280 (100.0%)   0.1267 (100.0%)
Reparsing ../test/tdot_asguard.cpp:   5.0600 (100.0%)   0.0440 (100.0%)  
5.1040 (100.0%)   5.1043 (100.0%)
  ASTContext: expressions, declarations, and types: 259532331
  ASTContext: identifiers: 2326528
  ASTContext: selectors: 4096
  ASTContext: side tables: 9151480
  Code completion: cached global results: 4063232
  SourceManager: content cache allocator: 111147
  SourceManager: malloc'ed memory buffers: 2082327
  SourceManager: mmap'ed memory buffers: 0
  SourceManager: data structures and tables: 124472904
  ExternalASTSource: malloc'ed memory buffers: 0
  ExternalASTSource: mmap'ed memory buffers: 188282552
  Preprocessor: malloc'ed memory: 8324702
  Preprocessor: PreprocessingRecord: 1775864
  Preprocessor: header search tables: 43
ReParse took 5.10452

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