llvm dev - Jul 2014 - [LLVMdev] codeGen, instruction write one value to the input register.

Hi Guys,

In  LLVM codegen, 
a typical binary operation instruction is defined something like below:

"  def _rr: NVPTXInst<(outs Int1Regs:$dst), (ins Int1Regs:$a,
Int1Regs:$b),
          "xor.pred \t$dst, $a, $b;",
      [(set Int1Regs:$dst, (OpNode Int1Regs:$a, Int1Regs:$b))]>;
“

which takes two inputs and write the result to the $dst register.

Then how to define a binary instruction which returns two results, one is
written to the $dst register, and the other one is written to one of the inputs?
I mean, to implement something like:
”
Type sincos( Type input,  Type * cosVal)
“  

the instruction will compute sin and cos value of input, return the sin result
and write the cos result to cosVal.
Is there anything special constraints or something I should put onto the cos
register?

Best

Kevin

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On Mon, Jun 16, 2014 at 4:51 PM, kewuzhang <kewu.zhang at amd.com>
wrote:
> Hi Guys,
>
> In  LLVM codegen,
> a typical binary operation instruction is defined something like below:
>
> "  def _rr: NVPTXInst<(outs Int1Regs:$dst), (ins Int1Regs:$a,
Int1Regs:$b),
>           "xor.pred \t$dst, $a, $b;",
>       [(set Int1Regs:$dst, (OpNode Int1Regs:$a, Int1Regs:$b))]>;
> “
>
> which takes two inputs and write the result to the $dst register.
>
> Then how to define a binary instruction which returns two results, one is
> written to the $dst register, and the other one is written to one of the
> inputs? I mean, to implement something like:
> ”
> Type sincos( Type input,  Type * cosVal)
> “
>
> the instruction will compute sin and cos value of input, return the sin
> result and write the cos result to cosVal.
> Is there anything special constraints or something I should put onto the
cos
> register?
>
Hey Kevin,

You might get a good start looking at the AVX2 VGATHER patterns in
llvm/lib/Target/X86/X86InstrSSE.td. Those patterns return two results.
They also make use of the @earlyclobber constraint.

Hope that helps,
Cameron

Tks  Cameron, 

I did some study on it.

Now I am having the same problem as mentioned in this
thread;"http://lists.cs.uiuc.edu/pipermail/llvmdev/2013-January/057654.html”
,
> Looking at the X86 back-end code, it looks like there do exist
> multi-output instructions, but they all use physical (implicit) registers
> for the second destination, not arbitrary register types.  And if I change
> the second destination to be a certain physical register, this problem goes
> away in my code.  Is this just not a supported case currently?  Or am I
> doing something wrong?
Wondering if there is any update about it?

best

kevin
On Jun 16, 2014, at 5:08 PM, Cameron McInally <cameron.mcinally at
nyu.edu> wrote:
> On Mon, Jun 16, 2014 at 4:51 PM, kewuzhang <kewu.zhang at amd.com>
wrote:
>> Hi Guys,
>> 
>> In  LLVM codegen,
>> a typical binary operation instruction is defined something like below:
>> 
>> "  def _rr: NVPTXInst<(outs Int1Regs:$dst), (ins Int1Regs:$a,
Int1Regs:$b),
>>          "xor.pred \t$dst, $a, $b;",
>>      [(set Int1Regs:$dst, (OpNode Int1Regs:$a, Int1Regs:$b))]>;
>> “
>> 
>> which takes two inputs and write the result to the $dst register.
>> 
>> Then how to define a binary instruction which returns two results, one
is
>> written to the $dst register, and the other one is written to one of
the
>> inputs? I mean, to implement something like:
>> ”
>> Type sincos( Type input,  Type * cosVal)
>> “
>> 
>> the instruction will compute sin and cos value of input, return the sin
>> result and write the cos result to cosVal.
>> Is there anything special constraints or something I should put onto
the cos
>> register?
>> 
> 
> Hey Kevin,
> 
> You might get a good start looking at the AVX2 VGATHER patterns in
> llvm/lib/Target/X86/X86InstrSSE.td. Those patterns return two results.
> They also make use of the @earlyclobber constraint.
> 
> Hope that helps,
> Cameron
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