Re: Segments

On 1/16/2025 9:24 AM, MitchAlsup1 wrote:

On Thu, 16 Jan 2025 17:15:55 +0000, Stephen Fuld wrote:
 

On 1/16/2025 1:11 AM, Terje Mathisen wrote:

Thomas Koenig wrote:

Keith Thompson <Keith.S.Thompson+u@gmail.com> schrieb:

Thomas Koenig <tkoenig@netcologne.de> writes:
[...]

CHERY targets C, which on the one hand, I understand (there's a
ton of C code out there), but trying to retrofit a safe memory
model onto C seems a bit awkward - it might have been better to
target a language which has arrays in the first place, unlike C.

[...]
>
C does have arrays.

>
Sort of - they decay into pointers at first sight.
>
But what I should have written was "multi-dimensional arrays",
with a reasonable way of handling them.
>

Rust provides an interesting data point here:
>
It has Vec<> which is always implemented as a dope vector, i.e. a header
which contains the starting point and current length, along with
allocated size. For multidimendional work, the natural mapping is
Vec<Vec<>>, i.e. similar to classic C arrays of arrays, but with
boundary checking.
>
However, in my own testing I have found that it is often faster to
flatten those multi-dim vectors, and instead use explicit multiplication
to get the actual position:
>
   array[y][x] -> array[y*width + x]
>
Terje

>
I am obviously missing something, but why doesn't the compiler generate
code for that itself?

 The compiler does; but with a dope-vector in view, the compiler
inserts additional checks on the arithmetic and addressing.

OK, so Terje's observation of it being faster doing the calculation himself is due to him not doing these additional checks?
--
  - Stephen Fuld
(e-mail address disguised to prevent spam)