llvm dev - Apr 2015 - [LLVMdev] Adding MachineOperands that are not part of MCInstrDesc.

Hi,

I wonder if it is okay to add an operand that is not part of MCInstrDesc 
to an MI after isel?

There are things going on like instruction combining, rematerialization 
etc, which may call MI with TI->get(opcode). If an MI would get replaced 
with a new instance of itself in this way, any previously added operands 
would get lost, as they are not part of the MCInstrDesc.

RegAlloc, does this, so I assume it is safe to add operands post-ra. I 
wonder if it also safe to add phys-reg defs/uses directly after DAG 
isel? Would it be a bug if they would be removed by a transformation?

Thanks,

Jonas Paulsson

On Wed, Apr 22, 2015 at 04:48:54PM +0200, Jonas Paulsson
wrote:
> Hi,
> 
> I wonder if it is okay to add an operand that is not part of
> MCInstrDesc to an MI after isel?
> 
> There are things going on like instruction combining,
> rematerialization etc, which may call MI with TI->get(opcode). If an
> MI would get replaced with a new instance of itself in this way, any
> previously added operands would get lost, as they are not part of
> the MCInstrDesc.
> 
> RegAlloc, does this, so I assume it is safe to add operands post-ra.
> I wonder if it also safe to add phys-reg defs/uses directly after
> DAG isel? Would it be a bug if they would be removed by a
> transformation?
> 
You can add implicit uses and defs.  You may get lucky and adding explicit defs
and uses will work, but I would not recommend doing this.  If you really need
to do this, you could make the instruction variadic or create two instruction
defs,
one with n operands and one with n + 1.

-Tom
> Thanks,
> 
> Jonas Paulsson
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi,

So what about post-ra - is it safe there?

thanks,

Jonas

On 2015-04-22 17:12, Tom Stellard wrote:
> On Wed, Apr 22, 2015 at 04:48:54PM +0200, Jonas Paulsson wrote:
>> Hi,
>>
>> I wonder if it is okay to add an operand that is not part of
>> MCInstrDesc to an MI after isel?
>>
>> There are things going on like instruction combining,
>> rematerialization etc, which may call MI with TI->get(opcode). If an
>> MI would get replaced with a new instance of itself in this way, any
>> previously added operands would get lost, as they are not part of
>> the MCInstrDesc.
>>
>> RegAlloc, does this, so I assume it is safe to add operands post-ra.
>> I wonder if it also safe to add phys-reg defs/uses directly after
>> DAG isel? Would it be a bug if they would be removed by a
>> transformation?
>>
> You can add implicit uses and defs.  You may get lucky and adding explicit
defs
> and uses will work, but I would not recommend doing this.  If you really
need
> to do this, you could make the instruction variadic or create two
instruction defs,
> one with n operands and one with n + 1.
>
> -Tom
>
>> Thanks,
>>
>> Jonas Paulsson
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev