Re:Towards fearless SIMD, 7 years later
by eru on Sunday March 30, 2025 @11:32PM
(#43530611)
> It is the reason why the standard purposely leaves some areas undefined.
No. It is _one_ of the reasons, not _the_ reason.
For example, signed integer overflow. If all you were worried about was the behaviour of the underlying hardware, you would declare this _implementation defined_. (With the vast majority of implementations today picking twos-complement.)
Instead, even compilers that target two-complement machines leave this deliberately undefined, so they can target exploit this corner for performance gains, without actually have to properly prove that leaving out certain checks would be sound under twos-complement.
No. It is _one_ of the reasons, not _the_ reason.
For example, signed integer overflow. If all you were worried about was the behaviour of the underlying hardware, you would declare this _implementation defined_. (With the vast majority of implementations today picking twos-complement.)
Instead, even compilers that target two-complement machines leave this deliberately undefined, so they can target exploit this corner for performance gains, without actually have to properly prove that leaving out certain checks would be sound under twos-complement.