How to describe ‘earlyclobber’ explicitly for specific source operand ?

How to describe ‘earlyclobber’ explicitly for specific source operand ?

[Jojo R]

Hi,

	We know gcc supply earlyclobber function to avoid register overlap,

	but it can not describe explicitly for specific source operand, is it right ?

	The vector ISA spec of risc-v describe some register overlap rules here:

	https://github.com/riscv/riscv-v-spec/blob/master/v-spec.adoc#sec-vec-operands

	e.g.

	# Widening signed integer add/subtract, 2*SEW = 2*SEW +/- SEW
	vwadd.wv vd, vs2, vs1, vm # vector-vector

	it means vd can overlap vs2, but can not overlap vs1,
	there are more registers are wasted if gcc can not describe earlyclobber of vd only for vs1,
	also gcc will generate extra REG-REG move instruction :(

	Any suggestions ?

Thanks.

— Jojo

Re: How to describe ‘earlyclobber’ explicitly for specific source operand ?

[Peter Bergner]

On 11/19/21 1:28 AM, Jojo R via Gcc wrote:
> 	We know gcc supply earlyclobber function to avoid register overlap,
> 
> 	but it can not describe explicitly for specific source operand, is it right ?

You add the early clobber to the OUTPUT operand(s) that can clobber any of the
input source operands.  You don't mark the source operands that could be clobbered.

Peter

Re: How to describe ‘earlyclobber’ explicitly for specific source operand ?

[Jojo R]

— Jojo
在 2021年11月20日 +0800 AM6:11，Peter Bergner <bergner@linux.ibm.com>，写道：
> On 11/19/21 1:28 AM, Jojo R via Gcc wrote:
> > We know gcc supply earlyclobber function to avoid register overlap,
> >
> > but it can not describe explicitly for specific source operand, is it right ?
>
> You add the early clobber to the OUTPUT operand(s) that can clobber any of the
> input source operands. You don't mark the source operands that could be clobbered.
Yes, so we need to enhance the early clobber to cover this scene ?
>
> Peter

Re: How to describe ‘earlyclobber’ explicitly for specific source operand ?

[Richard Earnshaw]

On 22/11/2021 06:40, Jojo R via Gcc wrote:
> 
> — Jojo
> 在 2021年11月20日 +0800 AM6:11，Peter Bergner <bergner@linux.ibm.com>，写道：
>> On 11/19/21 1:28 AM, Jojo R via Gcc wrote:
>>> We know gcc supply earlyclobber function to avoid register overlap,
>>>
>>> but it can not describe explicitly for specific source operand, is it right ?
>>
>> You add the early clobber to the OUTPUT operand(s) that can clobber any of the
>> input source operands. You don't mark the source operands that could be clobbered.
> Yes, so we need to enhance the early clobber to cover this scene ?
>>
>> Peter

You can write alternatives that explicitly tie a source to the 
destination, provided that the source and destination are the same size. 
  See the Arm backend for examples.

R.