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> But there could be gaps, especially in cases where Rust semantics mismatch with C in ways that LLVM hasn't exposed yet. Could you elaborate on what potential

> But there could be gaps, especially in cases where Rust semantics mismatch with C in ways that LLVM hasn't exposed yet. Could you elaborate on what potential mismatches there currently are between the semantics of Rust and C? Where particularly do you think there might be issues/dragons hiding in the semantics of a translation?

deleted 6y ago
[deleted]
gpm 6y ago
Integer overflows (signed overflows defined to wrap or panic in rust, are undefined behavior in C). Float integer conversions (overflows are defined to saturate in rust, are undefined behavior in C). Maybe things like how pointer casts are treated in correct (but unsafe) rust vs C. Or similarly in transmute. Generally it wouldn't surprise me if rust was standardizing a memory model that was subtly different from C. I'm sure the list goes on, but "that sort of thing".
- tedunangst 6y ago
  Pretty sure rust wraps on overflow, not saturates, but it's not hard to get that behavior from most C compilers if desired.
  - pornel 6y ago
    float to integer saturates in Rust. `99999.0 as u8` gives 255. `(char)99999.0` is UB in C and LLVM. Rust had to add explicit clamping code to float casts, because LLVM compiled nasal demons otherwise: https://github.com/rust-lang/rust/issues/10184 https://github.com/rust-lang/rust/issues/10184 Rust also has different aliasing rules. Stricter than C for references, but more relaxed than C for raw pointers.
    - tedunangst 6y ago
      Clamping floats is interesting, but seems pretty niche.
- tedunangst 6y ago
  Would have been nice to provide notice of your edits.
  - gpm 6y ago
    I'm not sure if I edited at all, if I did edit it was within a minute or two of my original post. Edit: Now that I am coming back and reading this though I am going to make an edit, and like normal for substantive edits I'm going to mark it clearly... Edit2: Or I can't edit, so let's put it here: A better example than any of the above might be infinite loops, `while (true) {}` (or any similar loop without side effects) is undefined behavior in C, it's defined behavior in rust.
    - tedunangst 6y ago
      Unfortunate timing.

deleted 6y ago

[deleted]

gpm 6y ago

Integer overflows (signed overflows defined to wrap or panic in rust, are undefined behavior in C). Float integer conversions (overflows are defined to saturate in rust, are undefined behavior in C). Maybe things like how pointer casts are treated in correct (but unsafe) rust vs C. Or similarly in transmute. Generally it wouldn't surprise me if rust was standardizing a memory model that was subtly different from C. I'm sure the list goes on, but "that sort of thing".

tedunangst 6y ago
Pretty sure rust wraps on overflow, not saturates, but it's not hard to get that behavior from most C compilers if desired.
- pornel 6y ago
  float to integer saturates in Rust. `99999.0 as u8` gives 255. `(char)99999.0` is UB in C and LLVM. Rust had to add explicit clamping code to float casts, because LLVM compiled nasal demons otherwise: https://github.com/rust-lang/rust/issues/10184 https://github.com/rust-lang/rust/issues/10184 Rust also has different aliasing rules. Stricter than C for references, but more relaxed than C for raw pointers.
  - tedunangst 6y ago
    Clamping floats is interesting, but seems pretty niche.
tedunangst 6y ago
Would have been nice to provide notice of your edits.
- gpm 6y ago
  I'm not sure if I edited at all, if I did edit it was within a minute or two of my original post. Edit: Now that I am coming back and reading this though I am going to make an edit, and like normal for substantive edits I'm going to mark it clearly... Edit2: Or I can't edit, so let's put it here: A better example than any of the above might be infinite loops, `while (true) {}` (or any similar loop without side effects) is undefined behavior in C, it's defined behavior in rust.
  - tedunangst 6y ago
    Unfortunate timing.