[LLVMdev] Constraints

[Evan Cheng]

On Jun 18, 2008, at 11:41 AM, David Greene wrote:

> On Wednesday 18 June 2008 01:30, Evan Cheng wrote:
>> On Jun 17, 2008, at 1:36 PM, David Greene wrote:
>>> Can someone explain the Constraints system in X86*.td?
>>>
>>> For example:
>>>
>>> let Constraints = "$src1 = $dst"
>>>
>>> This replaces isTwoAddress (according to svn logs), which I gather
>>> is how
>>> two-address instructions used to be marked for X86.
>>
>> You're right. This is the same as isTwoAddress, just more flexible.
>
> Ok, but what are the implications for the rest of llvm?  What does  
> this
> constrait say to later phases?  Is it a requirement that all two- 
> address
> instructions have this constraint or does it just enable more  
> optimization?

It specify the two-address property. So two-address and later passes  
will honor the constraint. It's more a requirement rather than  
enabling more optimizations.

Evan

>
> That is, is functional correctness tied to using this constraint?
>
>>> Except isTwoAddress is still used in X86InstInfo.td.
>>
>> Because I haven't replaced all uses of isTwoAddress with constraints,
>> that's all.
>
> Gotcha.  Thanks.
>
>                                                -Dave
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