Re: [PATCH] left shift of negative value do_binop src/arith_yacc.c:111:12

On 20/08/2025 15:49, Steffen Nurpmeso wrote:
> Harald van Dijk wrote in
>   <[email protected]>:
>   |This is what I was referring to with "subtly changing behaviour that
>   |differed across architectures". This behaved differently across
>   |architectures, taking only the low bits of the shift count is what x86
>   |does but is not universal, and I did not think it would be sensible to
>   |implement that on other platforms where it never worked that way. The
>   |behaviour I picked is what would have been the result if excessive shift
>   |counts hadn't been explicitly undefined, it's simply that e.g. a shift
>   |by 123 is equivalent to 123 shifts by 1.
> 
> Ok, but i am not with you regarding that then.  (Do you can name
> "other platforms"?)

It has been about six years since I looked at that so I don't remember 
exactly. I think it was most likely 32-bit ARM where $(( 1 << 65 )) 
would result in 0.

>                      I think the other shells are not either.
> As a little bit laxe one regarding all that i want to point to
> the email [1] (thread start [2], if you really want and do not
> read it anyway), especially
> 
>    I hear and understand what you're saying. I think what I'm
>    trying to point out, is that in C, as it was originally
>    implemented, in expressions "a + b", "a >> 1", "++a", C
>    "does what the machine does".  That's a very different thing
>    from having rational, safe, predictable language semantics
>    for operations on types - but it was also a strength, and a
>    simple way to describe what C would do, deferring to machine
>    semantics. I believe one place in C89/C90 where this is stated
>    explicitly, as "do what the machine does"

If I had seen any way to implement that, I would have. But I saw no such 
way, so went with the behaviour that I felt was easiest to justify.

Cheers,
Harald van Dijk