llvm dev - May 2009 - [LLVMdev] Question on tablegen

Hello,

I am trying to create a machine instruction for "extractelement". I
want to translate
r <- extractelement v, 0
to
mov r, v.x

I was looking at the dag I can use and I found vector_extract. The
inputs for this SDnode are a register and a iPtr constant. With that,
I need to create 4 separate def's to extract element 0, 1, 2, and 3
and translate to v.x, v.y, v.z, and v.w. I was wondering if I can use
the dag's 2nd input as an index into a list of strings and form the
assembly instruction, something like !strconcat("mov $dst, v.",
elemnames[$input]). I am still trying to learn the tablegen syntax and
semantics, so how to do this is not clear to me. I will appreciate any
suggestions on how to do this, or pointers to other places where
similar things are done.

Thanks,
Manjunath

One way to do this is to handle this in the AsmPrinter, with
operand modifiers.

For example, on x86 there are instructions with ${dst:call} in
their asm string. The "call" part is interpreted as an operand
modifier. The assembly printer looks for the "call" modifier
on MachineOperand::MO_Immediate operands
(in X86ATTAsmPrinter::printOperand), which lets it perform custom
printing for that type of operand. You could use a modifier which
tells the AsmPrinter to print an immediate as the appropriate
letter.

Dan


On May 5, 2009, at 10:23 PM, Manjunath Kudlur wrote:
> Hello,
>
> I am trying to create a machine instruction for "extractelement".
I
> want to translate
> r <- extractelement v, 0
> to
> mov r, v.x
>
> I was looking at the dag I can use and I found vector_extract. The
> inputs for this SDnode are a register and a iPtr constant. With that,
> I need to create 4 separate def's to extract element 0, 1, 2, and 3
> and translate to v.x, v.y, v.z, and v.w. I was wondering if I can use
> the dag's 2nd input as an index into a list of strings and form the
> assembly instruction, something like !strconcat("mov $dst, v.",
> elemnames[$input]). I am still trying to learn the tablegen syntax and
> semantics, so how to do this is not clear to me. I will appreciate any
> suggestions on how to do this, or pointers to other places where
> similar things are done.
>
> Thanks,
> Manjunath
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Dan,

Thanks a lot. Using a modifier in the assembly string works for this
case. I am trying to solve a related problem. I am trying to print out
a set of "mov" ops for the vector_shuffle node. Since the source of
the "mov" is from one of the sources to vector_shuffle, depending on
the mask, I am not sure what assembly string to emit. For example, if
I have

d <- vector_shuffle s1, s2, <0,3>

I want to emit

mov d.x, s1.x
mov d.y, s2.y

For this, I need some thing like

"mov $d.x, <$src1 or $src2>.<something that depends on the
mask>"

I can use the same modifier trick for which component of the source to
select, but I don't know how to select which of the sources to select
($src1 or $src2) depending on the mask. Do you have any suggestions?

Thanks,
Manjunath

On Wed, May 6, 2009 at 10:10 AM, Dan Gohman <gohman at apple.com>
wrote:
> One way to do this is to handle this in the AsmPrinter, with
> operand modifiers.
>
> For example, on x86 there are instructions with ${dst:call} in
> their asm string. The "call" part is interpreted as an operand
> modifier. The assembly printer looks for the "call" modifier
> on MachineOperand::MO_Immediate operands
> (in X86ATTAsmPrinter::printOperand), which lets it perform custom
> printing for that type of operand. You could use a modifier which
> tells the AsmPrinter to print an immediate as the appropriate
> letter.
>
> Dan
>
>
> On May 5, 2009, at 10:23 PM, Manjunath Kudlur wrote:
>
>> Hello,
>>
>> I am trying to create a machine instruction for
"extractelement". I
>> want to translate
>> r <- extractelement v, 0
>> to
>> mov r, v.x
>>
>> I was looking at the dag I can use and I found vector_extract. The
>> inputs for this SDnode are a register and a iPtr constant. With that,
>> I need to create 4 separate def's to extract element 0, 1, 2, and 3
>> and translate to v.x, v.y, v.z, and v.w. I was wondering if I can use
>> the dag's 2nd input as an index into a list of strings and form the
>> assembly instruction, something like !strconcat("mov $dst,
v.",
>> elemnames[$input]). I am still trying to learn the tablegen syntax and
>> semantics, so how to do this is not clear to me. I will appreciate any
>> suggestions on how to do this, or pointers to other places where
>> similar things are done.
>>
>> Thanks,
>> Manjunath
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>