[llvm-commits] [llvm] r51660 - in /llvm/trunk: include/llvm/Target/TargetInstrDesc.h lib/CodeGen/MachineInstr.cpp lib/Target/Target.td utils/TableGen/CodeGenInstruction.cpp utils/TableGen/CodeGenInstruction.h utils/TableGen/InstrInfoEmitter.cpp

[Evan Cheng]

Fine with me as long as we eventually replace all these  
TargetInstrDesc flags when we have proper instruction cost estimates.

Evan

On May 28, 2008, at 6:26 PM, Chris Lattner wrote:

>
> On May 28, 2008, at 6:22 PM, Evan Cheng wrote:
>
>> Why do we want another flag? In two-address pass, we want to remat  
>> the
>> def instead of copying its value if the instruction is *cheap*.
>>
>> Most of the rematerializable instructions are cheap. But I don't  
>> think
>> we want to remat load from constantpool here, do we?
>>
>> In the long run, we want proper cost information for every  
>> instruction
>> instead of Rematerializable and / or CheapAsMove. Then we can replace
>> the whole mess with proper checks. Until then, I don't see something
>> like CheapAsAMove buys us.
>
> This is specifically trying to decide if the instruction is as cheap
> as a move.  If the instruction is more expensive (e.g. an add or xor)
> it should not have this.
>
> We want to eventually remat adds and xors when register pressure is
> high... these are very different concepts.  Does that seem reasonable?
>
> -Chris
>
>>
>>
>> Evan
>>
>> On May 28, 2008, at 4:56 PM, Chris Lattner wrote:
>>
>>>
>>> On May 28, 2008, at 4:50 PM, Evan Cheng wrote:
>>>
>>>> Please don't add another flag. For now, please check if an
>>>> instruction
>>>> is rematerializable && !mayload.
>>>
>>> Why?
>>>
>>> -Chris
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