Re: [llvm-commits] [llvm] r44960 - in /llvm/trunk: include/llvm/Target/ lib/CodeGen/ lib/Target/ lib/Target/ARM/ lib/Target/Alpha/ lib/Target/IA64/ lib/Target/Mips/ lib/Target/PowerPC/ lib/Target/Sparc/ lib/Target/X86/ test/CodeGen/X86/

[Evan Cheng]

On Dec 12, 2007, at 5:37 PM, Evan Cheng wrote:

>
> On Dec 12, 2007, at 5:07 PM, Chris Lattner wrote:
>
>> On Dec 12, 2007, at 3:12 PM, Evan Cheng wrote:
>>>    isLoad = false;
>>> -  if (tii_->isTriviallyReMaterializable(MI)) {
>>> -    isLoad = MI->getInstrDescriptor()->Flags & M_LOAD_FLAG;
>>> +  const TargetInstrDescriptor *TID = MI->getInstrDescriptor();
>>> +  if ((TID->Flags & M_IMPLICIT_DEF_FLAG) ||
>>> +      tii_->isTriviallyReMaterializable(MI)) {
>>> +    isLoad = TID->Flags & M_LOAD_FLAG;
>>>      return true;
>>>    }
>>
>> Hi Evan,
>>
>> At one point were discussed eliminating
>> TII::isTriviallyReMaterializable.  The argument is that target
>> implementations shouldn't have to know about algorithms, they should
>> just describe properties of the target, and the algorithm should
>> figure out if it can make the xform from that info.
>
> What do you mean? The targets don't know about the remat algorithm.
> It's just the spiller making use of a some property of the  
> instructions.

Ok, you are talking about isTriviallyReMaterializable, not  
M_IMPLICIT_DEF_FLAG. Right now it is defined as this:

   bool isTriviallyReMaterializable(MachineInstr *MI) const {
     return (MI->getInstrDescriptor()->Flags & M_REMATERIALIZIBLE) &&
            isReallyTriviallyReMaterializable(MI);
   }

X86::isReallyTriviallyReMaterializable() is:
...
   case X86::MMX_MOVD64rm:
   case X86::MMX_MOVQ64rm:
     // Loads from constant pools are trivially rematerializable.
     return MI->getOperand(1).isRegister() && MI->getOperand 
(2).isImmediate() &&
            MI->getOperand(3).isRegister() && MI->getOperand 
(4).isConstantPoolIndex() &&
            MI->getOperand(1).getReg() == 0 &&
            MI->getOperand(2).getImmedValue() == 1 &&
            MI->getOperand(3).getReg() == 0;
   }

The targets is only describing properties of the instruction because  
we are not able to describe addressing mode in a generic way. Seems  
like we are not too far off from your "pipe dream". :-)

Evan

>
> Evan
>
>>
>> Is this a pipe dream? :)
>>
>> -Chris
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