James McCoy wrote:
> Recently, there was some work done on defining how to handle converting
> numeric literals in vim script to C integers -- basically, clamp to the
> extremes of the datatype. If it would exceed the max/min value, then
> just treat it as the max/min value.
>
> However, there's nothing defining how arithmetic behaves when it would
> exceed the range of the datatype. The only information in the help is:
>
> Number A 32 or 64 bit signed number. |expr-number| *Number*
> 64-bit Numbers are available only when compiled with the
> |+num64| feature.
>
> Now, in C signed over/underflow is undefined behavior. If you're "lucky",
> the implementation will wrap but it could just as well optimize the code
> under the assumption that the code can't over/underflow.
Yeah, I consider this a bug in the C standard. Undefined behavior is
useless, the only reason I suspect this is done is so that compiler
writers can makey the code < 1% faster. And at the same time require
programmers to handle all the edge cases, which gets really complicated
and everybody forgets about.
> Currently, Vim runs afoul of undefined behavior as can easily be seen
> with a -fsanitize=undefined build by running
>
> if has('num64')
> echo float2nr(pow(2, 62)) * 2
> else
> echo float2nr(pow(2, 30)) * 2
> endif
>
> It would be nice for there to be defined behavior here for the user,
> instead of exposing them to the whims of the compiler implementation.
Unfortunately, that is quite difficult. I propose the compiler standard
gets fixed. This means only the few compilers that exist need to take
care of this, instead of the millions of C programs.
--
Every person is responsible for the choices he makes.
/// Bram Moolenaar -- [email protected] -- http://www.Moolenaar.net \\\
/// sponsor Vim, vote for features -- http://www.Vim.org/sponsor/ \\\
\\\ an exciting new programming language -- http://www.Zimbu.org ///
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