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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.arch
Subject: Re: Optimization by programmers
Date: Sun, 27 Sep 2026 10:55:29 -0700
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Michael S <already5chosen@yahoo.com> writes:

> On Sun, 27 Sep 2026 18:04:05 +0200
> David Brown <david.brown@hesbynett.no> wrote:
>
>> On 27/09/2026 17:18, Michael S wrote:
>>
>>> Fortran is not the same as C.
>>> My understanding is that in Fortran array indexes are by default
>>> 32-bit.  That couses real cost under wrapv.
>>> In C there is no such thing as default size of array index.  The
>>> programmer can define it as 32-bit and suffer the same performance
>>> degradatioon under wrapv as Fortran does.  Or he can define index as
>>> either ptrdiff_t or size_t and suffere no degradation.
>>
>> You certainly /can/ do that.  But a lot of C code uses "int" for
>> array indexing.  The "ideal" type in many cases would be
>> "int_fast32_t" to say it should be of a big enough size (32-bit is
>> enough range for a great many purposes) but can bigger if that's
>> faster.  On most 64-bit targets, "int_fast32_t" will be 64-bit.
>
> That's not universal.
> For example, on both alive 64-bit Windows targets int_fast32_t is
> 32-bit.
> Godbolt shows the same for clang for MIPS64.  I have no idea what OS it
> targets.
>
> Besides, int_fast32_t is both above my threshold of acceptable ugliness
> and acceptable geekery.

I favor a type equivalent to size_t for use as array index variables.