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It's interesting that booleans are stored as 0 or -1 [1]. I wonder what drove that decision. I just checked, and Rust's LLVM stores booleans as 0 or 1. [1] htt

It's interesting that booleans are stored as 0 or -1 [1]. I wonder what drove that decision. I just checked, and Rust's LLVM stores booleans as 0 or 1. [1] https://cranelift.readthedocs.io/en/latest/ir.html#boolean-types https://cranelift.readthedocs.io/en/latest/ir.html#boolean-t...

sunfish 8y ago
A typical Rust bool would use `b1`, which doesn't have an observable representation.
- sunfish 8y ago
  Also, the [bint instruction](https://cranelift.readthedocs.io/en/latest/ir.html#inst-bint https://cranelift.readthedocs.io/en/latest/ir.html#inst-bint) converts a bool value to an integer 0 or 1.
- steveklabnik 8y ago
  We did, eventually, after years, define it as a full byte though. https://github.com/rust-lang/rust/pull/46156 https://github.com/rust-lang/rust/pull/46156
KMag 8y ago
Well, for one, if true is -1 (using two's complement), then a bunch of conditionals and bit-twiddling become more simple. For instance: x = some_bool ? a : b; can be compiled as a branchless: x = (some_bool & a) | (~some_bool & b); This is why Forth also represents true as -1.
- pcwalton 8y ago
  Also why SIMD fairly universally uses -1 for true. C arguably picked the wrong semantics here. Hardware picks up the slack, however, with the setcc instructions on x86, csinc on AArch64, etc.
- eridius 8y ago
  That would only be true if `a` is guaranteed to be a non-zero value though, right? e.g. `x = some_bool ? 0 : 1` would not be able to be represented that way.
  - repsilat 8y ago
    It works for me. Note that `|` is a bitwise-or (not that it really matters, it just removes a branch) const True = -1 const False = 0 const ternary = (condition, left, right) => \ (condition & left) | (~condition & right) ternary(True, 0, 1) // 0 ternary(False, 0, 1) // 1
    - eridius 8y ago
      Oh right, because the right-hand side evaluates to 0 as well in this case. Makes sense.
cpeterso 8y ago
Boolean true is -1 in Visual Basic, too: https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/data-types/boolean-data-type https://docs.microsoft.com/en-us/dotnet/visual-basic/languag...
- perl4ever 8y ago
  It was traditional in earlier BASICs too; I think I remember it being the case in QuickBASIC and AmigaBASIC. I assume someone once upon a time thought it was a nice thing for ~x to be equal to !x.

sunfish 8y ago

A typical Rust bool would use `b1`, which doesn't have an observable representation.

sunfish 8y ago
Also, the [bint instruction](https://cranelift.readthedocs.io/en/latest/ir.html#inst-bint https://cranelift.readthedocs.io/en/latest/ir.html#inst-bint) converts a bool value to an integer 0 or 1.
steveklabnik 8y ago
We did, eventually, after years, define it as a full byte though. https://github.com/rust-lang/rust/pull/46156 https://github.com/rust-lang/rust/pull/46156

KMag 8y ago

Well, for one, if true is -1 (using two's complement), then a bunch of conditionals and bit-twiddling become more simple. For instance: x = some_bool ? a : b; can be compiled as a branchless: x = (some_bool & a) | (~some_bool & b); This is why Forth also represents true as -1.

pcwalton 8y ago
Also why SIMD fairly universally uses -1 for true. C arguably picked the wrong semantics here. Hardware picks up the slack, however, with the setcc instructions on x86, csinc on AArch64, etc.
eridius 8y ago
That would only be true if `a` is guaranteed to be a non-zero value though, right? e.g. `x = some_bool ? 0 : 1` would not be able to be represented that way.
- repsilat 8y ago
  It works for me. Note that `|` is a bitwise-or (not that it really matters, it just removes a branch) const True = -1 const False = 0 const ternary = (condition, left, right) => \ (condition & left) | (~condition & right) ternary(True, 0, 1) // 0 ternary(False, 0, 1) // 1
  - eridius 8y ago
    Oh right, because the right-hand side evaluates to 0 as well in this case. Makes sense.

cpeterso 8y ago

Boolean true is -1 in Visual Basic, too: https://docs.microsoft.com/en-us/dotnet/visual-basic/language-reference/data-types/boolean-data-type https://docs.microsoft.com/en-us/dotnet/visual-basic/languag...

perl4ever 8y ago
It was traditional in earlier BASICs too; I think I remember it being the case in QuickBASIC and AmigaBASIC. I assume someone once upon a time thought it was a nice thing for ~x to be equal to !x.