[LLVMdev] Is there pass to break down <4 x float> to scalars

[Liu Xin]

Hi, Richard,

I think we are solving a same problem. I am working on shader language
too.  I am not satisfied with current binaries because vector operations
are kept in llvm opt.

glsl shader language has an operation called "swizzle". It can select
sub-components of a vector. If a shader only takes components "xy" for a
vec4. it's certainly wasteful to generate 4 operations for a scalar
processor.

i think a good solution for llvm is in codegen. Many compiler has codegen
optimizer. A DSE is good enough.

Which posted patch about TBAA? you have yet another solution except
decompose-vectors?


thanks,
--lx



On Fri, Oct 25, 2013 at 6:06 PM, Richard Sandiford <
rsandifo at linux.vnet.ibm.com> wrote:

> Liu Xin <navy.xliu at gmail.com> writes:
> > Hi, LLVM community,
> >
> > I write some code in hand using LLVM IR. for simplicity, I write them in
> <4
> > x float>. now I found some stores for elements are useless.
> >
> > for example, If I store {0.0, 1.0, 2.0, 3.0} to a <4 x float> %a. maybe
> > only %a.xy is alive in my program.  our target doesn't feature SIMD
> > instruction, which means we have to lower vector to many  scalar
> > instructions. I found llvm doesn't have DSE in codegen , right?
> >
> >
> > Is there a pass which can break down vector operation to scalars?
>
> I wanted the same thing for SystemZ, which doesn't have vectors,
> in order to improve the llvmpipe code.  FWIW, here's what I have locally.
>
> It is able to decompose loads and stores, but I found in the llvmpipe case
> that this made things worse with TBAA, because DAGCombiner::GaterAllAliases
> has some fairly strict limits.  So I disabled that by default; use
> -decompose-vector-load-store to reenable.
>
> The main motivation for z was instead to get InstCombine to rewrite
> things like scalarised selects.
>
> I haven't submitted it yet because it's less of a win than the TBAA
> DAGCombiner patch I posted, so I didn't want to distract from that.
> It would also need some TargetTransformInfo hooks to decide which
> vectors should be decomposed.
>
> Thanks,
> Richard
>
>
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