Re: [RFC] Mitigating unexpected arithmetic overflow

[Linus Torvalds]

On Thu, 9 May 2024 at 11:08, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
>  Any half-way decent compiler will end up optimizing away the shifts
> and adds for the high bits because they see the assignment to
> 'all_bits'. There's no point in generating high bits that just get
> thrown away.

. it might also actually be a good idea *IF* we were to have some
kind of "implicit cast drops bits" warning, in that the compiler for
that case wouldn't remove the upper bits calculation, but would
trigger a warning if they are non-zero.

So there are actually potential advantages to just always apparently
doing the full 64-bit arithmetic.

Without debug warnings, it's a no-op that the compiler will just skip.
And with some hypothetical debug flag, it would be a "you are now
losing the high bits of the time value when assigning the result to a
limited 32-bit time_t" warning.

             Linus