On: Why not just do simple C++ RAII in C?

[rcxdude]

I use rust, which already does this.

[fooker → rcxdude]

Signed overflow is officially a 'bug' in rust, it traps in debug mode but silently follows LLVM/platform behavior in release mode.

Huh, doesn't that sound familiar?

[steveklabnik → fooker]

> silently follows LLVM/platform behavior

This is not the case. It's two's compliment overflow.

Also, since we're being pedantic here: it's not actually about "debug mode" or "release mode", it is tied to a flag, and compilers must have that flag on in debug mode. This gives the ability to move release mode to also produce the flag in the future, if it's decided that the overhead is worth it. We'll see if it ever is.

> Huh, doesn't that sound familiar?

Nope, it is completely different from undefined behavior, which gives the compiler license to do anything it wants. These are well defined semantics, the polar opposite of UB.

[fooker → steveklabnik]

>This is not the case. It's two's compliment overflow.

Okay, here is an example showing that rust follows LLVM behavior when the optimizer is turned on. LLVM addition produces poison when signed wrap happens. I'm a little bit puzzled about the vehement responses in the comments wow. I have worked on several compilers (including a few patches to Rust), and this is all common knowledge.

https://godbolt.org/z/r6WTxGjrb

[steveklabnik → fooker]

The Rust output:

  define noundef i32 @square(i32 noundef %x, i32 noundef %y) unnamed_addr #0 !dbg !7 {
    %_0 = add i32 %y, %x, !dbg !12
    ret i32 %_0, !dbg !13
  }

Let's compare like to like, here's one with equivalent C++ code: https://godbolt.org/z/Y4MnGeof4

The C++ output:

  define dso_local noundef i32 @square(int, int)(i32 noundef %0, i32 noundef %1) local_unnamed_addr #0 !dbg !99 {
    tail call void @llvm.dbg.value(metadata i32 %0, metadata !104, metadata !DIExpression()), !dbg !106
    tail call void @llvm.dbg.value(metadata i32 %1, metadata !105, metadata !DIExpression()), !dbg !106
    %3 = add nsw i32 %1, %0, !dbg !107
    ret i32 %3, !dbg !108
  }

> LLVM addition produces poison when signed wrap happens.

https://llvm.org/docs/LangRef.html#add-instruction

> nuw and nsw stand for “No Unsigned Wrap” and “No Signed Wrap”, respectively. If the nuw and/or nsw keywords are present, the result value of the add is a poison value if unsigned and/or signed overflow, respectively, occurs.

Note that Rust produces `add`. The C++ produces `add nsw`. No poison in Rust, poison in C++.

Here is an example of these differences producing different results, due to the differences in behavior: https://godbolt.org/z/Gaonnc985

Rust:

  define noundef zeroext i1 @test() unnamed_addr #0 !dbg !14 {
    ret i1 true, !dbg !15
  }

C++:

  define dso_local noundef zeroext i1 @test()() local_unnamed_addr #0 !dbg !123 {
    tail call void @llvm.dbg.value(metadata i32 undef, metadata !128, metadata !DIExpression()), !dbg !129
    ret i1 false, !dbg !130
  }

This is because in Rust, the wrapping behavior means that this will always be true, but in C++, because it is UB, the compiler assumes it will always be false.

> I'm a little bit puzzled about the vehement responses in the comments wow.

You are claiming that Rust has semantics that it was very, very deliberately designed to not have.

[samatman → steveklabnik]

Rust includes a great deal of undefined behavior, unlocked with the trustme keyword. Ahem, sorry, unsafe. If only...

So if we're going to be pedantic, it's safe Rust which has defined semantics for basically everything. A considerable accomplishment, to be sure.

[steveklabnik → samatman]

While this is true, we’re talking about integer overflow. That’s part of safe Rust. So it’s not really germane to this conversation.