[LLVMdev] alias information on machine instructions

Chris Lattner sabre at nondot.org
Wed May 23 00:23:38 PDT 2007

On Fri, 4 May 2007, Florian Brandner wrote:
> Chris Lattner wrote:
>> There are a couple of ways to do this.  Is your scheduler a prepass
>> scheduler (before regalloc) or a post-pass scheduler (after regalloc)?
> it is a post-pass scheduler, which operates on MachineInstrs. we need to
> run it after register allocation to hide latencies of spill code,
> prolog, and epilog.


>> If you want to extract maximal parallelism, I assume you want a prepass
>> scheduler.  In that case, you should look into the SelectionDAG based
>> schedulers, which do have alias information on them.
> i had a look at the SelectionDAG based schedulers. it seems that
> aliasing loads/stores are chained together by the DAGCombiner. after
> scheduling, when the MachineInstrs are created, the alias information
> cannot be used anymore in the current framework. is this correct?

Right.  The original Value*'s are preserved in the DAG, but dropped when 
MachineInstrs are created.  We could add a machineoperand to capture this 
Value* if desired.

> i don't think that it is a good idea to do alias analysis on
> MachineInstrs, thus i would like to preserve the alias information, and
> annotate the MachineInstrs explicitly. however, this information might
> be invalidated by subsequent passes.

Right, it would have to be added as a Value* operand to MachineInstr. 
This would not be invalidated by later passes, they would just ignore the 
field.  Targets would have to be updated though, as certain pieces of code 
"know" about the expected operands.



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