tree 375d86b91c0481f281eaf6ce5b74de3c4c5f261e
parent 30e45d19167a8681cf17f64e2db47b227dbf8574
author Balazs Benics <balazs.benics@sigmatechnology.se> 1638782417 +0100
committer Copybara-Service <copybara-worker@google.com> 1638782777 -0800

[analyzer] Ignore flex generated files

Some projects [1,2,3] have flex-generated files besides bison-generated
ones.
Unfortunately, the comment `"/* A lexical scanner generated by flex */"`
generated by the tools is not necessarily at the beginning of the file,
thus we need to quickly skim through the file for this needle string.

Luckily, StringRef can do this operation in an efficient way.

That being said, now the bison comment is not required to be at the very
beginning of the file. This allows us to detect a couple more cases
[4,5,6].

Alternatively, we could say that we only allow whitespace characters
before matching the bison/flex header comment. That would prevent the
(probably) unnecessary string search in the buffer. However, I could not
verify that these tools would actually respect this assumption.

Additionally to this, e.g. the Twin project [1] has other non-whitespace
characters (some preprocessor directives) before the flex-generated
header comment. So the heuristic in the previous paragraph won't work
with that.
Thus, I would advocate the current implementation.

According to my measurement, this patch won't introduce measurable
performance degradation, even though we will do 2 linear scans.

I introduce the ignore-bison-generated-files and
ignore-flex-generated-files to disable skipping these files.
Both of these options are true by default.

[1]: https://github.com/cosmos72/twin/blob/master/server/rcparse_lex.cpp#L7
[2]: https://github.com/marcauresoar/make-examples/blob/22362cdcf9dd7c597b5049ce7f176621e2e9ac7a/sandbox/count-words/lexer.c#L6
[3]: https://github.com/vladcmanea/2nd-faculty-year-Formal-Languages---Automata-assignments/blob/11abdf64629d9eb741438ba69f04636769d5a374/lab1/lex.yy.c#L6

[4]: https://github.com/KritikaChoudhary/System-Software-Lab/blob/47f5b2cfe2a2738fd54eae9f8439817f6a22034e/B_yacc/1/y1.tab.h#L2
[5]: https://github.com/VirtualMonitor/VirtualMonitor/blob/71d1bf9b1e7b392a7bd0c73dc217138dc5865651/src/VBox/Additions/x11/x11include/xorg-server-1.8.0/parser.h#L2
[6]: https://github.com/bspaulding/DrawTest/blob/3f773ceb13de14275429036b9cbc5aa19e29bab9/Framework/OpenEars.framework/Versions/A/Headers/jsgf_parser.h#L2

Reviewed By: xazax.hun

Differential Revision: https://reviews.llvm.org/D114510

GitOrigin-RevId: 9873ef409c4a937c566e20e7a88af049d212ce03