[Codegen] Move `LoweringConfigAttr` to an attribute interface

[Quinn Dawkins]

The #iree_codegen.lowering_config attribute is designed such that different backends will interpret its contents differently. This is at odds with the the current implementation of the attribute which has a fixed set of fields and a number of backend specific member functions.

iree/compiler/src/iree/compiler/Codegen/Dialect/Codegen/IR/IREECodegenAttrs.td

Lines 203 to 224 in a3b74bc

 def IREECodegen_LoweringConfigAttr : 
     AttrDef<IREECodegen_Dialect, "LoweringConfig", []> { 
   let mnemonic = "lowering_config"; 
   let summary = [{drive lowering of an operation within dispatch region}]; 
   let description = [{ 
     Specifies the information that is used by backend compiler to 
     translate an operation to scalar code. The way the information is 
     used is specific to each backend (indeed specific to the pass 
     pipeline used) to compile that operation. 
  
     TODO: Currently there is no verification that the configuration 
     specifies everything needed for a pass-pipeline. The values to set 
     for these parameters is dependent on the pass-pipeline 
     implementation. In future, each pass pipeline could verify that 
     the lowering configuration has all the necessary attributes for 
     the pipeline. 
   }]; 
  
   let assemblyFormat = [{ 
     `<` `tile_sizes` `=` $tilingLevels 
       (`,` `native_vector_size` `=` `[` $nativeVectorSize^ `]`)? `>` 
   }]; 

The addition of fields to this attribute means that all backends and any shared passes need to reason about it (likely just emitting errors). A recent example of the GPU side was the introduction of a special attribute on the TranslationInfo to track the intended target intrinsic for an operation, however translation info applies globally.

iree/compiler/src/iree/compiler/Codegen/Dialect/GPU/IR/IREEGPUAttrs.td

Line 155 in a3b74bc

 def IREEGPU_MmaScheduleAttr : AttrDef<IREEGPU_Dialect, "MMASchedule"> { 

The solution is to create an interface for LoweringConfig attributes that interfaces with common passes and has separate concrete implementations for backends that need it. Today the two primary shared passes are TileAndDistributeToWorkgroups and GenericVectorization

iree/compiler/src/iree/compiler/Codegen/Common/TileAndDistributeToWorkgroupsPass.cpp

Line 94 in a3b74bc

 IREE::Codegen::LoweringConfigAttr rootOpConfig = getLoweringConfig(rootOp); 

iree/compiler/src/iree/compiler/Codegen/Common/GenericVectorization.cpp

Line 235 in a3b74bc

 IREE::Codegen::LoweringConfigAttr loweringConfig = getLoweringConfig(op); 

So the interface methods to start will be

SmallVector<int64_t> getTileSizeVals(unsigned level);
SmallVector<int64_t> getNativeVectorSizeVals();

cc @MaheshRavishankar @hanhanW

[Han-Chung Wang]

cc @banach-space