llvm dev - May 2015 - [LLVMdev] Sub-Register extraction

Hi everyone,

I'm working on a architecture where 2 32-bits registers can be use as one
64-bits register for some instructions.
We have some SIMD instructions with those registers.

I have a pattern to transform the "extractelt" node into a
"EXTRACT_SUBREG" node.
My problem is that when LLVM is trying to get the high sub-register of a vector,
it's doing a shift by 32 and then copy the low 32 bits sub-register. While
it could just have copy the high 32 bits sub-register directly.

I found that the "EXTRACT_SUBREG" node is lowered into a
"COPY" node. But I cannot manage to find where this node is lower into
a shift + a copy.

Thank you all in advance,
Romaric

Hi Romaric,
> On May 22, 2015, at 7:31 AM, Romaric Jodin <rjodin at kalray.eu>
wrote:
> 
> Hi everyone,
> 
> I'm working on a architecture where 2 32-bits registers can be use as
one 64-bits register for some instructions.
> We have some SIMD instructions with those registers.
> 
> I have a pattern to transform the "extractelt" node into a
"EXTRACT_SUBREG" node.
> My problem is that when LLVM is trying to get the high sub-register of a
vector, it's doing a shift by 32 and then copy the low 32 bits sub-register.
While it could just have copy the high 32 bits sub-register directly.
> 
> I found that the "EXTRACT_SUBREG" node is lowered into a
"COPY" node. But I cannot manage to find where this node is lower into
a shift + a copy.
This sounds like you’re missing some patterns to lower what you want.

If that is the case, the “bad” codegen would occur right after instruction
selection.

Anyway, if you want to check when the pattern is generated, you can use (-mllvm)
-print-machineinstrs.

Hope that helps.

Cheers,
-Quentin
> 
> Thank you all in advance,
> Romaric
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