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pcwalton's profile
Patrick Walton
Patrick Walton
Patrick Walton
@pcwalton

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Patrick Walton

@pcwalton

Research engineer at Mozilla

San Francisco, CA
pcwalton.github.io
Joined November 2009

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    1. Sean Barrett‏ @nothings 22 Nov 2019
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      Replying to @pcwalton

      Wait, what havoc does the C lexer hack cause? I've never found it a big deal in a proper compiler, and in tools I've always been able to get away with pseudo-parsing C without tracking typenames. (I'm sure it's different in C++, and I'm also 100% behind 'x:int' regardless.)

      1 reply 0 retweets 1 like
    2. James Widman‏ @JamesWidman 22 Nov 2019
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      Replying to @nothings @pcwalton

      parentheses around the declarator in the declaration of a function parameter

      1 reply 0 retweets 0 likes
    3. James Widman‏ @JamesWidman 22 Nov 2019
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      Replying to @JamesWidman @nothings @pcwalton

      (or is it an unnamed parameter whose type is a pointer-to-function type? depends on name lookup!)

      1 reply 0 retweets 0 likes
    4. Sean Barrett‏ @nothings 22 Nov 2019
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      Replying to @JamesWidman @pcwalton

      example please

      1 reply 0 retweets 0 likes
    5. James Widman‏ @JamesWidman 22 Nov 2019
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      Replying to @nothings @pcwalton

      see 'int (n)' in f() and h() in:https://godbolt.org/z/aN34vm 

      1 reply 0 retweets 1 like
    6. James Widman‏ @JamesWidman 22 Nov 2019
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      Replying to @JamesWidman @nothings @pcwalton

      (i guess i shouldn't have said "around the declarator", since the redundant parentheses in the first 'int(n)' are part of the declarator)

      1 reply 0 retweets 0 likes
    7. James Widman‏ @JamesWidman 22 Nov 2019
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      Replying to @JamesWidman @nothings @pcwalton

      btw i'd tend to call it the "expression parser hack", since Ritchie re-used the expression parser as the declarator parser (and "lexing" usually refers to the phase where preprocessing tokens are determined, which is before '#' directives are executed)

      1 reply 1 retweet 1 like
    8. Sean Barrett‏ @nothings 22 Nov 2019
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      Replying to @JamesWidman @pcwalton

      So, I'm not sure there's any significant quantity of code that is at risk for being misparsed. Like, only case here is tools that don't properly parse, and the only ambiguity is h(), since C doesn't allow unnamed parameters in definitions, only declarations. So where's the havoc?

      3 replies 0 retweets 0 likes
    9. Patrick Walton‏ @pcwalton 22 Nov 2019
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      Replying to @nothings @JamesWidman

      IMO the biggest problem in C is that declaration order has to be significant because otherwise you can't parse properly. This means that e.g. the AST of a file can depend on what it #includes.

      2 replies 0 retweets 4 likes
    10. Patrick Walton‏ @pcwalton 22 Nov 2019
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      Replying to @pcwalton @nothings @JamesWidman

      This means that C has to reparse header files over and over, leading to compilation time problems. Precompiled headers are hard to implement correctly--on macOS they were broken for a long time, so projects often disabled them.

      1 reply 0 retweets 4 likes
      Patrick Walton‏ @pcwalton 22 Nov 2019
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      Replying to @pcwalton @nothings @JamesWidman

      The classic way to implement precompiled headers in GCC is literally to parse a bunch of headers, then core dump. Then you can start from that core dump when compiling your code. That's what macOS used to do. (Yes, really.)

      10:15 AM - 22 Nov 2019
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      • Gibson Fahnestock Ankur Arora Florian Gilcher ∠(・.-)―〉 →◎ Brendan G Bohannon Juha Komulainen Sean Barrett Jed Davis 🏳️‍🌈 Matías N. Goldberg 🏳️‍⚧️::🦀::😷::Diana says 🍾+🔥=🥓 A🐷AB
      3 replies 1 retweet 9 likes
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        2. Rich Felker‏ @RichFelker 25 Nov 2019
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          Replying to @pcwalton @nothings @JamesWidman

          This is an abomination and prevents building GCC as PIE or running GCC on nommu hosts. I encountered that and was so angry. Is it fixed yet?

          1 reply 0 retweets 0 likes
        3. Rich Felker‏ @RichFelker 25 Nov 2019
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          Replying to @RichFelker @pcwalton and

          So much 80s era lisp machine philosophy cruft in the gnu ecosystem.

          1 reply 0 retweets 1 like
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        1. Constructive lawlessness‏ @TheEpsylon 24 Nov 2019
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          Replying to @pcwalton @nothings @JamesWidman

          this is extremely cursed, thanks for the knowledge, I hate it

          0 replies 0 retweets 0 likes
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        1. Brendan G Bohannon‏ @cr88192 25 Nov 2019
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          Replying to @pcwalton @nothings @JamesWidman

          Yeah. In other languages, it is possible to infer that two adjacent identifiers starts a declaration, but in C at best all this can do is make a guess... (can be correct "most of the time", still needs typedef's to make a determination; mostly useful as a parser optimization).

          0 replies 0 retweets 0 likes
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