[llvm-commits] CVS: llvm/lib/Target/ARM/ARMInstrInfo.cpp ARMInstrInfo.h ARMTargetMachine.cpp ARMTargetMachine.h

[Evan Cheng]

On May 16, 2007, at 10:09 AM, Chris Lattner wrote:

> On May 16, 2007, at 1:18 AM, Evan Cheng wrote:
>>> But even then I am not sure if that eliminate the need to have a
>>> virtual function isPredicatable. Conditional branches probably would
>>> require special handling. Also, while it isn't possible now, it's
>>> conceivable some instruction predicate field may not be 'always'
>>> coming into the if-conversion pass. In that case, it would require a
>>> target hook to examine the field anyway.
>>>
>>
>> cmov is another class of  M_PREDICATED ops that need to be dealt with
>> differently. We'll need a hook to check if something is already
>> predicated with non-always predicate.
>
> Yes, I think this should be the distinction between M_PREDICATED (the
> instr already has a predicate field, but may be set to 'always') vs
> M_PREDICABLE (the instr may or may not have a predicate field, but
> PredicateInstruction can change it into one that does).

I dunno if that's the right solution. To me, whether something is  
"predicated" is a dynamic property. It's dependent on the value of  
its predicated field.


>
> Another thing that occurred to me is that predicates have
> implications for livevar analysis (specifically, the scavenger).
> Consider this code:
>
> R3 = add [lt] ...
> R3 = sub [ge] ...
>
> Note that the add is not dead. :)

Yeah, this is a problematic issue. For now, I think the if-converter  
has to be a pre-emit pass. That means it will have to do some basic  
CFG xform (i.e. remove dead blocks) unless we can move branch folding  
past it. :-(


>
>> Come to think of it, perhaps we should not have selects if the target
>> uses if-conversion.
>
> That would be very nice.  Alternatively, we could have the isel
> "know" about predicated moves and generate them?

I think the right solution is to transform select's back to control  
flow. However, this requires doing the ugly hack to do the lowering  
at scheduling time, right?

Evan

>
> The isel does not know anything about moves at this point.
>
> -Chris
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