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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. Gary Bernhardt‏ @garybernhardt 21 Jun 2018
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      Are there any compilers built on LLVM that can compile hello world in under, say, 50 ms? That is: does LLVM itself necessarily add a big startup cost, or are compilers with long boot times adding that cost elsewhere?

      7 replies 1 retweet 20 likes
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    2. Gary Bernhardt‏ @garybernhardt 21 Jun 2018
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      Clang is a counterexample. Somehow I didn't think of that. So it's nothing inherent to LLVM. Maybe some kind of scaling factor that LLVM adds to complex languages, though? There certainly seems to be a correlation between LLVM as a backend and slow startup.

      6 replies 0 retweets 5 likes
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    3. Joe Groff‏ @jckarter 21 Jun 2018
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      Replying to @garybernhardt

      A lot of higher-level languages that rely on optimizing lots of library code like C++/Rust/Swift tend to take the "throw lots of IR at LLVM and let the optimizer sort it out" approach, which definitely stresses super-linear parts of the optimizer pipeline

      1 reply 0 retweets 6 likes
    4. Gary Bernhardt‏ @garybernhardt 21 Jun 2018
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      Replying to @jckarter

      That makes sense. So that constant startup showing up for Rust but not C is just library size. But OCaml is presumably avoiding that penalty in another way, because it's the fastest "hello world" compiler I've seen.

      3 replies 0 retweets 3 likes
    5. Joe Groff‏ @jckarter 21 Jun 2018
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      Replying to @garybernhardt

      OCaml has its own tailor-made IR that's at a better fit abstraction-wise that it presumably does a lot of processing on before it lowers to asm/llvm. Now that Rust has MIR it could conceivably do similar

      1 reply 0 retweets 2 likes
    6. Gary Bernhardt‏ @garybernhardt 21 Jun 2018
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      Replying to @jckarter

      Right, this makes a lot of sense, thanks!

      1 reply 0 retweets 0 likes
    7. steveklabnik‏ @steveklabnik 21 Jun 2018
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      Replying to @garybernhardt @jckarter

      co-signed all of this thread, and yeah, we've done some work to be able to swap out llvm for https://github.com/cretonne/cretonne … in the future, if that makes sense for us.

      1 reply 0 retweets 1 like
    8. Joe Groff‏ @jckarter 21 Jun 2018
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      Replying to @steveklabnik @garybernhardt

      Part of me wonders whether there's room for an "IR construction kit" to make purpose-built IRs easier to develop. Give you a basic SSA structure, common analyses like inlining, use-def, dominance, call graph, etc., let the user provide their base instruction set and custom passes

      2 replies 0 retweets 3 likes
    9. Patrick Walton‏ @pcwalton 21 Jun 2018
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      Replying to @jckarter @steveklabnik @garybernhardt

      Sadly that won’t work too well for Rust…Rust MIR is deliberately non-SSA, so that we can borrow check it MIR and easily map back to AST for error reporting. (The theory is that this should be OK for optzns since idiomatic Rust is “mostly SSA” already.)

      1 reply 0 retweets 3 likes
    10. Joe Groff‏ @jckarter 21 Jun 2018
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      Replying to @pcwalton @steveklabnik @garybernhardt

      Interesting, why do you need a non-SSA representation? We used SSA for Swift SIL and can do the equivalent of borrow-checking on it with source information preserved

      1 reply 0 retweets 1 like
      Patrick Walton‏ @pcwalton 21 Jun 2018
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      Replying to @jckarter @steveklabnik @garybernhardt

      I think @nikomatsakis can elaborate further… I’m guessing we could do it, but the benefits haven’t yet been proven to outweigh the complexity + build time costs. (We’re only beginning to implement MIR optzns anyway.)

      3:04 PM - 21 Jun 2018 from South Beach, San Francisco
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