[PATCH] Make Preprocessor::Lex non-recursive

Richard Smith richard at metafoo.co.uk
Mon Sep 16 15:44:35 PDT 2013

On Mon, Sep 2, 2013 at 5:26 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> Per subject, patch attached which makes Preprocessor::Lex non-recursive.
> Before this patch, Lex() would recurse whenever the current lexer changed
> (e.g. upon entry into a macro).  This patch turns the recursion into a
> loop: the various lex routines now don't return a token when the current
> lexer changes, and at the top level Preprocessor::Lex() now loops until it
> finds a token.  Normally, the recursion doesn't end up being very deep, but
> the recursion depth can explode in edge cases like a bunch of consecutive
> macros which expand to nothing (like in the testcase
> test/Preprocessor/macro_expand_empty.c in this patch).
> To make this work, I made some substantial changes to the way the
> whitespace and empty macro flags are propagated.  It had to be rewritten
> alongside this patch because the previous code to handle this wasn't
> tail-recursive.  The code for this is pretty straightforward: it just adds
> a couple flags to the Lexer class, then updates them whenever we leave a
> macro expansion.  I also fixed a minor bug while I was in the area: the
> previous code didn't work correctly for macros which had tokens in the
> definitions which expanded to nothing.
> I still need to do performance measurements, but I'm not anticipating any
> measurable changes.

This seems reasonable to me. Have you had a chance to do any performance
measurements yet?

Preprocessor::Lex now looks extremely unlikely to be inlined; is there any
benefit in keeping it in the header file?

> -Eli
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