[Mesa-dev] [PATCH v3 2/6] anv: Add a helper for doing mass allocations

[Jason Ekstrand]

On Fri, Apr 7, 2017 at 3:19 PM, Chris Wilson <chris at chris-wilson.co.uk>
wrote:

> On Fri, Apr 07, 2017 at 02:41:13PM -0700, Jason Ekstrand wrote:
> >    On Fri, Apr 7, 2017 at 1:26 PM, Chris Wilson <[1]
> chris at chris-wilson.co.uk>
> >    wrote:
> >
> >      On Fri, Apr 07, 2017 at 12:55:53PM -0700, Jason Ekstrand wrote:
> >      > +#define _ANV_MULTIALLOC_UPDATE_POINTER(_i) \
> >      > +   if ((_i) < ma->ptr_count) \
> >      > +      *ma->ptrs[_i] = ptr + (uintptr_t)*ma->ptrs[_i]
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(0);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(1);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(2);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(3);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(4);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(5);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(6);
> >      > +   _ANV_MULTIALLOC_UPDATE_POINTER(7);
> >      > +#undef _ANV_MULTIALLOC_UPDATE_POINTER
> >
> >      #define _ANV_MULTIALLOC_UPDATE_POINTER(_i) case _i + 1:
> *ma->ptrs[_i] =
> >      ptr +(uintptr)*ma->ptrs[_i]
> >
> >      switch (ma->ptr_count) {
> >      _ANV_MULTIALLOC_UPDATE_POINTER(7);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(6);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(5);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(4);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(3);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(2);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(1);
> >      _ANV_MULTIALLOC_UPDATE_POINTER(0);
> >      }
> >
> >      #undef _ANV_MULITALLOC_UPDATE_POINTER
> >
> >    If ma->ptr_count is constant, they generate exactly the same code.
> If it
> >    isn't (i.e. if one of the multialloc_adds is predicated), then they
> still
> >    generate basically the same code with the code for the if version
> being
> >    slightly more straightforward.
>
> Took a look at this with https://godbolt.org/g/UwrMk1
>

Weird... That's not at all what I'm seeing with my demo file.  In fact,
when I try to compile your demo file with GCC on my local machine, it
reduces the entire thing down to less than a dozen instrutions.


> clang does much better at seeing the constants, but gcc doesn't and
> generates a large if-ladder for one and a jump table for the switch. icc
> doesn't fare any better than gcc. The if-ladder is going to be worse
> than the computed jump, but best is to file a gcc bug :|
>
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