[llvm-commits] Fwd: [llvm] r48223 - in /llvm/trunk/lib: CodeGen/LowerSubregs.cpp CodeGen/SelectionDAG/ScheduleDAG.cpp Target/Target.td Target/X86/X86ISelDAGToDAG.cpp Target/X86/X86InstrInfo.h Target/X86/X86InstrInfo.td Target/X86/X86RegisterInfo.h Target/X86/X86RegisterInfo.td

[Chris Lattner]

On Mar 14, 2008, at 2:47 PM, Evan Cheng wrote:

>
> On Mar 14, 2008, at 2:09 PM, Chris Lattner wrote:
>
>>>>> vr1027 = implicit_def
>>>>> vr1027 = insert_subreg vr1027, vr1025, x86_subreg32bit
>>>>>
>>>>> vr1027 and vr1025 live intervals conflict.
>>>>
>>>> the register allocator should, but doesn't, treat implicit_def
>>>> specially already.  Live ranges that are defined by an implicit_def
>>>> should just be dropped making this a moot point :)
>>>
>>> Yes, that would be a useful optimization except 1) it's yet another
>>> target specific hook or target instruction property since isel will
>>> turn ISD::UNDEF into a target instruction,
>>
>> If you make it a "target-independent instruction", then:
>>
>> a) targets don't need to specify how to match undef
>> b) we get rid of the existing "is implicit_def" flag
>> c) we eliminate the need to have one "implicit_def instruction" per
>> register class.
>
> Ok. This *should* work.

Cool.  This part can be done independently of the regalloc/modeling  
changes too.

>>> However, I'm suggesting that somewhere in regalloc (live interval
>> analysis?) a LiveInterval for y is created.  Currently it is [2,4].
>> I'm suggesting the LiveInterval analysis (or something else in RA
>> land) sees that the defining instruction for the live interval is an
>> implicit def.  As such, it would just discard the live range and nuke
>> the instruction.  The LiveInterval would still exist, it would just  
>> be
>> empty.   Because it is empty, it can't conflict with anything.
>
> Zero length liveinterval is an interesting idea. There is a
> possibility this make break some assumptions in the backend. We'll
> see. I'll poke at this.

I don't think it will eliminate the need to do all handling of it.   
Imagine:

x = implicit_def
...
y = x
...

In theory, it would be nice to zap the y live range when coalescing x/ 
y.  This could be done by seeing that x has no value numbers.  This is  
an (probably unimportant) optimization though.

-Chris