[LLVMdev] Distinguish variadic register defines/uses in MCInstrDesc?

[Jim Grosbach]

Hi Christoph,

What's the motivation here? Why are Jakob's previous recommendations insufficient?

Specifically:
Jakob Stoklund Olesen wrote:
> I don't see what makes them so different from other instructions.
> MCInstrDesc provides the information which operands are defines and which uses 
> for all other instruction types and for variadic there are only three possible 
> cases:
> * All variadic registers are uses (push/stm on ARM)
> * All variadic registers are defines (pop/ldm)
> * uses and defines are mixed.
> 
> I think variable_ops on x86 RET can also be modeled as the first case.
> 
> The first two cases don't need any special handling. The start and count of 
> variadic operands is known through MCInstrDesc and 
> MCInstruction::getNumOperands(). So I can handle them like any other 
> instruction if I know which case applies. Only the third case really needs 
> special handling and I couldn't find an instruction that falls into this case.


On Oct 5, 2012, at 8:43 AM, Christoph Grenz <christophg+llvm at grenz-bonn.de> wrote:

> Hallo,
> 
> I worked on how to handle the distinction between variadic defines and uses 
> and my current solution is this:
> 
> I introduce a new dag item in Instruction called VariadicOperandList, which by 
> default is undefined. It keeps a marker variable_* and all operands which are 
> placeholders for variable lists (like 'reglist' on ARM).
> 
> I think it's the cleanest solution to keep them in a seperate dag, because 
> else either register defines would be in the 'ins' list or the parser would 
> become much more complex to change the order of operands to get the variable 
> items in 'outs' to the end of the list.
> 
> I appended my current patches.
> 
> At the moment two tests fail.
> 2011-06-16-TailCallByVal.ll because an instruction alias for a variadic 
> instruction isn't recognized and thumb2-ldm.ll fails with "Trying to add an 
> operand to a machine instr that is already done!"
> 
> Any comments?
> 
> Thanks,
> Christoph Grenz
> 
> Am Donnerstag, 27. September 2012, 01:42:04 schrieb Christoph Grenz:
>> Am Mittwoch, 26. September 2012, 11:18:20 schrieb Jakob Stoklund Olesen:
>>> Hi Christoph,
>>> 
>>> As you noticed, MCInstrDesc doesn't distinguish between variadic uses and
>>> defs. Since variadic instructions will always require some kind of special
>>> handling, it doesn't seem worthwhile to make the model more detailed.
>> 
>> I don't see what makes them so different from other instructions.
>> MCInstrDesc provides the information which operands are defines and which
>> uses for all other instruction types and for variadic there are only three
>> possible cases:
>> * All variadic registers are uses (push/stm on ARM)
>> * All variadic registers are defines (pop/ldm)
>> * uses and defines are mixed.
>> 
>> I think variable_ops on x86 RET can also be modeled as the first case.
>> 
>> The first two cases don't need any special handling. The start and count of
>> variadic operands is known through MCInstrDesc and
>> MCInstruction::getNumOperands(). So I can handle them like any other
>> instruction if I know which case applies. Only the third case really needs
>> special handling and I couldn't find an instruction that falls into this
>> case.
>>> I think your current workaround sounds fine.
>>> 
>>> /jakob
>> 
>> Christoph
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