[LLVMdev] Adding MachineOperands that are not part of MCInstrDesc.

[Jonas Paulsson]

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
>>
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