Re: [Tinycc-devel] Request: __attribute__((vector_size))

[Michael Matz]

Hi,

On Mon, 6 Jan 2020, ian wrote:

> Did I miss something ?
> It seemed to me that it was dealing with application side of things.

Sure, Rasmus asked to have this extension in TCC to be able to use it for 
writing libraries or apps; i.e. he asked if TCC could be extended to 
compile code like this:

---------------------
typedef int v4si __attribute__ ((vector_size (16)));
void foo (void) {
  v4si a = {1,2,3,4}, b = {5,6,7,8}, c;
  c = a + b;
  bar(c[0], c[1], c[2], c[3]);
}
---------------------

For that TCC would need to be extended somewhat, and I was alluding to the 
fact that this extension isn't totally trivial if it shouldn't generate 
very low quality code.  If it's okay to generate low quality code and 
not adhere to the psABI for parameter passing of these types then it's not 
too much work.

If you were asking if such extension is really a must have in a 
compiler: no, otherwise it wouldn't be an extension.  It's a nice to have, 
and I can see why Rasmus wants it, but it comes at a non-trivial cost to 
support it in a small compiler.


Ciao,
Michael.

> Like, say, libraries. For the least, as long as compilers does not care
> about maths or physics....
>
> If I'm wrong, please, explain to me where...
>
> ian
>
> Le 06/01/2020 à 21:46, Michael Matz a écrit :
>       Hi,
>
>       On Mon, 6 Jan 2020, ian wrote:
>
>             Is the support for data structures and calculations
>             a real "need to" for a compiler ????
>
>
>       Um, I don't know how to answer this.  As it seems the
>       alternative to calculations is guessing, yes, I do think
>       calculations and data structures are indeed necessary for a
>       compiler, or any program whatsoever.
>
>
>       Ciao,
>       Michael.
>
>
>             regards,
>
>             ian
>
>             Le 06/01/2020 à 20:36, Michael Matz a écrit :
>                   Hello,
>
>                   On Mon, 6 Jan 2020, Rasmus Riiner via
>             Tinycc-devel wrote:
>
>                         See here:
>                        
>             https://gcc.gnu.org/onlinedocs/gcc/Vector-Extensions.html
>
>                         This is one of those things that doesn't
>             come up in
>                         a lot of types of programming, but is
>             indispensable
>                         in some, namely anything to do with 2D
>             or 3D math.
>                         The ability to just write `position +=
>             velocity *
>                         dt;` style statements might be one of
>             the most
>                         important reasons most game developers
>             choose C++
>                         over C. It's certainly the only thing
>             keeping me
>                         from using TCC in the development of my
>             own project.
>
>                         Actually emitting SSE instructions might
>             be beyond
>                         the scope of tcc (Tiny after all),
>             supporting the
>                         expressions though would be a huge help.
>
>
>                   Without proper support in the backends for
>             vector instructions
>                   the generated code for this extension from TCC
>             would be of
>                   abysmal quality. Technically it wouldn't be
>             too much work to
>                   semantically support that, but the values
>             would need to be
>                   committed to memory (instead of vector
>             registers) and regarded
>                   as constant sized arrays (at which point the
>             function call ABI
>                   would be different from the respective native
>             ABI, which, again,
>                   would be hard to fix without proper support
>             for vector insns in
>                   the backends).
>
>                   That extension really doesn't feel very well
>             suited for a simple
>                   compiler like TCC ;-)  But perhaps it can be
>             done
>                   non-intrusively enough.
>
>                         I sat down yesterday in an attempt to do
>             this
>                         myself, but it turns out I'm not good
>             enough for the
>                         job. I couldn't make heads or tails of
>             it. However,
>                         this might be a breeze for someone that
>             knows TCC
>                         like the back of their hand.
>
>                         So, any takers?
>
>
>
>                   Ciao,
>                   Michael.
>
>                  
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