4 ms·

> C's memory aliasing model does this mean that because you can’t be sure if two vectors don’t overlap in C you therefore can’t auto-vectorize loops?

gumby 4y ago
That's an example. It also constrains some optimizations like reordering.
- titzer 4y ago
  > It also constrains some optimizations like reordering. Ha! As if C compilers really cared about correctness. This is why type-punning and aliasing memory through different typed pointers is fraught with peril, often UB. It's so C compilers can cheat and use type-based alias analysis, breaking programs that have "UB" but would work absolutely fine if the compiler wasn't so aggressive. In general, almost all arguments between language design and optimization for C/C++ are settled with "let's not think too hard about how to help users; go ahead and optimize this and make it their fault (UB) when they observe the hard cases". It's so rude.
  - pornel 4y ago
    This is roughly what happens, but you shouldn't attribute this to rudeness. Compiler users also demand performance. They compare implementations (and languages) and point out when such-and-such is X% slower. They file bugs about "missed" optimizations and "unnecessary" instructions. C is especially awkward here, because compilers are expected to precisely conform the C specification, but at the same time C users don't actually want to program for the abstract machine from the C specification (the one that can't overflow signed integers or shift by a full int width). C users expect their programs to actually behave like programs for a particular hardware they target, without any overhead or discrepancy from "emulating" the imaginary C-spec machine. In face of such conflicting requirements, C compilers will always be forced to balance performance, predictability, conformance, and compatibility.
    - titzer 4y ago
      You make some good points. C was often used in the situation where "Hey, we just built this enormous supercomputer for LLNL, let's make this C program run as fast as possible on this one machine. Compiler team, please do everything possible to get the maximum performane out of this machine." I feel like a lot has followed from this repeating dozens of times over the past few decades. Performance has been the #1 priority, above all else, and it shows.
      - gumby 4y ago
        Not sure if the S-1 is a good example — IIRC it still had a 36-bit word and was mainly intended for pl1 and lisp (I only really saw it from the S-1 Lisp side). Was there evening a C for it? It would have been as bad as running C on a Cray YMP (which I mention above)

gumby 4y ago

That's an example. It also constrains some optimizations like reordering.

titzer 4y ago
> It also constrains some optimizations like reordering. Ha! As if C compilers really cared about correctness. This is why type-punning and aliasing memory through different typed pointers is fraught with peril, often UB. It's so C compilers can cheat and use type-based alias analysis, breaking programs that have "UB" but would work absolutely fine if the compiler wasn't so aggressive. In general, almost all arguments between language design and optimization for C/C++ are settled with "let's not think too hard about how to help users; go ahead and optimize this and make it their fault (UB) when they observe the hard cases". It's so rude.
- pornel 4y ago
  This is roughly what happens, but you shouldn't attribute this to rudeness. Compiler users also demand performance. They compare implementations (and languages) and point out when such-and-such is X% slower. They file bugs about "missed" optimizations and "unnecessary" instructions. C is especially awkward here, because compilers are expected to precisely conform the C specification, but at the same time C users don't actually want to program for the abstract machine from the C specification (the one that can't overflow signed integers or shift by a full int width). C users expect their programs to actually behave like programs for a particular hardware they target, without any overhead or discrepancy from "emulating" the imaginary C-spec machine. In face of such conflicting requirements, C compilers will always be forced to balance performance, predictability, conformance, and compatibility.
  - titzer 4y ago
    You make some good points. C was often used in the situation where "Hey, we just built this enormous supercomputer for LLNL, let's make this C program run as fast as possible on this one machine. Compiler team, please do everything possible to get the maximum performane out of this machine." I feel like a lot has followed from this repeating dozens of times over the past few decades. Performance has been the #1 priority, above all else, and it shows.
    - gumby 4y ago
      Not sure if the S-1 is a good example — IIRC it still had a 36-bit word and was mainly intended for pl1 and lisp (I only really saw it from the S-1 Lisp side). Was there evening a C for it? It would have been as bad as running C on a Cray YMP (which I mention above)