[Mesa-dev] [PATCH 3/6] gallivm: optimize gather a bit, by using supplied destination type

[Dave Airlie]

On 12 December 2016 at 10:11,  <sroland at vmware.com> wrote:
> From: Roland Scheidegger <sroland at vmware.com>
>
> By using a dst_type in the the gather interface, gather has some more
> knowledge about how values should be fetched.
> E.g. if this is a 3x32bit fetch and dst_type is 4x32bit vector gather
> will no longer do a ZExt with a 96bit scalar value to 128bit, but
> just fetch the 96bit as 3x32bit vector (this is still going to be
> 2 loads of course, but the loads can be done directly to simd vector
> that way).
> Also, we can now do some try to use the right int/float type. This should
> make no difference really since there's typically no domain transition
> penalties for such simd loads, however it actually makes a difference
> since llvm will use different shuffle lowering afterwards so the caller
> can use this to trick llvm into using sane shuffle afterwards (and yes
> llvm is really stupid there - nothing against using the shuffle
> instruction from the correct domain, but not at the cost of doing 3 times
> more shuffles, the case which actually matters is refusal to use shufps
> for integer values).
> Also do some attempt to avoid things which look great on paper but llvm
> doesn't really handle (e.g. fetching 3-element 8 bit and 16 bit vectors
> which is simply disastrous - I suspect type legalizer is to blame trying
> to extend these vectors to 128bit types somehow, so fetching these with
> scalars like before which is suboptimal due to the ZExt).
>
> Remove the ability for truncation (no point, this is gather, not conversion)
> as it is complex enough already.
>
> While here also implement not just the float, but also the 64bit avx2
> gathers (disabled though since based on the theoretical numbers the benefit
> just isn't there at all until Skylake at least).

Hi Roland,

This breaks the build on big endian machines.

  CC       gallivm/lp_bld_gather.lo
  CC       gallivm/lp_bld_init.lo
gallivm/lp_bld_gather.c: In function 'lp_build_gather_elem_vec':
gallivm/lp_bld_gather.c:238:42: error: 'dst_elem_type' undeclared
(first use in this function)
                             LLVMConstInt(dst_elem_type,
                                          ^
gallivm/lp_bld_gather.c:238:42: note: each undeclared identifier is
reported only once for each function it appears in
gallivm/lp_bld_gather.c: In function 'lp_build_gather':

Dave.