MLIR

Multi-Level IR Compiler Framework

MLIR Passes

This document describes the available MLIR passes and their contracts.

Affine control lowering (-lower-affine)

Convert operations related to affine control into a graph of blocks using operations from the standard dialect.

Loop statements are converted to a subgraph of blocks (initialization, condition checking, subgraph of body blocks) with loop induction variable being passed as the block argument of the condition checking block. Conditional statements are converted to a subgraph of blocks (chain of condition checking with short-circuit logic, subgraphs of ‘then’ and ‘else’ body blocks). affine.apply operations are converted into sequences of primitive arithmetic operations that have the same effect, using operands of the index type. Consequently, named maps and sets may be removed from the module.

For example, %r = affine.apply (d0, d1)[s0] -> (d0 + 2*d1 + s0)(%d0, %d1)[%s0] can be converted into:

%d0 = <...>
%d1 = <...>
%s0 = <...>
%0 = constant 2 : index
%1 = muli %0, %d1
%2 = addi %d0, %1
%r = addi %2, %s0

Input invariant

  • no Tensor types;

These restrictions may be lifted in the future.

Output IR

Functions with affine.for and affine.if operations eliminated. These functions may contain operations from the Standard dialect in addition to those already present before the pass.

Invariants

  • Functions without a body are not modified.
  • The semantics of the other functions is preserved.
  • Individual operations other than those mentioned above are not modified if they do not depend on the loop iterator value or on the result of affine.apply.

Conversion from Standard to LLVM IR dialect (-convert-std-to-llvm)

Convert standard operations into the LLVM IR dialect operations.

Input invariant

  • operations including: arithmetic on integers and floats, constants, direct calls, returns and branches;
  • no tensor types;
  • all vector are one-dimensional;
  • all blocks are reachable by following the successors of the first basic block;

If other operations are present and their results are required by the LLVM IR dialect operations, the pass will fail. Any LLVM IR operations or types already present in the IR will be kept as is.

Output IR

Functions converted to LLVM IR. Function arguments types are converted one-to-one. Function results are converted one-to-one and, in case more than 1 value is returned, packed into an LLVM IR struct type. Function calls and returns are updated accordingly. Block argument types are updated to use LLVM IR types.

Data Copy DMA generation (-affine-data-copy-generate)

Replaces all loads and stores on memref’s living in ‘slowMemorySpace’ by introducing DMA operations (strided DMA if necessary) to transfer data to/from fastMemorySpace and rewriting the original load’s/store’s to instead load/store from the allocated fast memory buffers. Additional options specify the identifier corresponding to the fast memory space and the amount of fast memory space available. The pass traverses through the nesting structure, recursing to inner levels if necessary to determine at what depth DMA transfers need to be placed so that the allocated buffers fit within the memory capacity provided. If this is not possible (for example, when the elemental type itself is of size larger than the DMA capacity), an error with location information is emitted. The DMA transfers are also hoisted up past all loops with respect to which the transfers are invariant.

Input

func @loop_nest_tiled() -> memref<256x1024xf32> {
  %0 = alloc() : memref<256x1024xf32>
  affine.for %i0 = 0 to 256 step 32 {
    affine.for %i1 = 0 to 1024 step 32 {
      affine.for %i2 = (d0) -> (d0)(%i0) to (d0) -> (d0 + 32)(%i0) {
        affine.for %i3 = (d0) -> (d0)(%i1) to (d0) -> (d0 + 32)(%i1) {
          %1 = affine.load %0[%i2, %i3] : memref<256x1024xf32>
        }
      }
    }
  }
  return %0 : memref<256x1024xf32>
}

Output (with flags: -affine-data-copy-generate -affine-data-copy-generate-fast-mem-space=2)

module {
  func @loop_nest_tiled() -> memref<256x1024xf32> {
    %c262144 = constant 262144 : index
    %c0 = constant 0 : index
    %0 = alloc() : memref<256x1024xf32>
    %1 = alloc() : memref<256x1024xf32, 2>
    %2 = alloc() : memref<1xi32>
    affine.dma_start %0[%c0, %c0], %1[%c0, %c0], %2[%c0], %c262144 : memref<256x1024xf32>, memref<256x1024xf32, 2>, memref<1xi32>
    affine.dma_wait %2[%c0], %c262144 : memref<1xi32>
    affine.for %arg0 = 0 to 256 step 32 {
      affine.for %arg1 = 0 to 1024 step 32 {
        affine.for %arg2 = #map1(%arg0) to #map2(%arg0) {
          affine.for %arg3 = #map1(%arg1) to #map2(%arg1) {
            %3 = affine.load %1[%arg2, %arg3] : memref<256x1024xf32, 2>
          }
        }
      }
    }
    dealloc %2 : memref<1xi32>
    dealloc %1 : memref<256x1024xf32, 2>
    return %0 : memref<256x1024xf32>
  }
}

Loop tiling (-affine-loop-tile)

Performs tiling or blocking of loop nests. It currently works on perfect loop nests.

Loop unroll (-affine-loop-unroll)

This pass implements loop unrolling. It is able to unroll loops with arbitrary bounds, and generate a cleanup loop when necessary.

Loop unroll and jam (-affine-loop-unroll-jam)

This pass implements unroll and jam for loops. It works on both perfect or imperfect loop nests.

Loop fusion (-affine-loop-fusion)

Performs fusion of loop nests using a slicing-based approach. The fused loop nests, when possible, are rewritten to access significantly smaller local buffers instead of the original memref’s, and the latter are often either completely optimized away or contracted. This transformation leads to enhanced locality and lower memory footprint through the elimination or contraction of temporaries / intermediate memref’s. These benefits are sometimes achieved at the expense of redundant computation through a cost model that evaluates available choices such as the depth at which a source slice should be materialized in the designation slice.

Memref bound checking (-memref-bound-check)

Checks all load’s and store’s on memref’s for out of bound accesses, and reports any out of bound accesses (both overrun and underrun) with location information.

test/Transforms/memref-bound-check.mlir:19:13: error: 'load' op memref out of upper bound access along dimension #2
      %x  = load %A[%idx0, %idx1] : memref<9 x 9 x i32>
            ^
test/Transforms/memref-bound-check.mlir:19:13: error: 'load' op memref out of lower bound access along dimension #2
      %x  = load %A[%idx0, %idx1] : memref<9 x 9 x i32>
            ^

Memref dataflow optimization (-memref-dataflow-opt)

This pass performs store to load forwarding for memref’s to eliminate memory accesses and potentially the entire memref if all its accesses are forwarded.

Input

func @store_load_affine_apply() -> memref<10x10xf32> {
  %cf7 = constant 7.0 : f32
  %m = alloc() : memref<10x10xf32>
  affine.for %i0 = 0 to 10 {
    affine.for %i1 = 0 to 10 {
      affine.store %cf7, %m[%i0, %i1] : memref<10x10xf32>
      %v0 = affine.load %m[%i0, %i1] : memref<10x10xf32>
      %v1 = addf %v0, %v0 : f32
    }
  }
  return %m : memref<10x10xf32>
}

Output

module {
  func @store_load_affine_apply() -> memref<10x10xf32> {
    %cst = constant 7.000000e+00 : f32
    %0 = alloc() : memref<10x10xf32>
    affine.for %arg0 = 0 to 10 {
      affine.for %arg1 = 0 to 10 {
        affine.store %cst, %0[%arg0, %arg1] : memref<10x10xf32>
        %1 = addf %cst, %cst : f32
      }
    }
    return %0 : memref<10x10xf32>
  }
}

Memref dependence analysis (-memref-dependence-check)

This pass performs dependence analysis to determine dependences between pairs of memory operations (load’s and store’s) on memref’s. Dependence analysis exploits polyhedral information available (affine maps, expressions, and affine.apply operations) to precisely represent dependences using affine constraints, while also computing dependence vectors from them, where each component of the dependence vector provides a lower and an upper bound on the dependence distance along the corresponding dimension.

test/Transforms/memref-dataflow-opt.mlir:232:7: note: dependence from 2 to 1 at depth 1 = ([1, 1], [-inf, +inf])
      store %cf9, %m[%idx] : memref<10xf32>

Pipeline data transfer (-affine-pipeline-data-transfer)

This pass performs a transformation to overlap non-blocking DMA operations in a loop with computations through double buffering. This is achieved by advancing dma_start operations with respect to other operations.

Input

func @pipelinedatatransfer() {
  %0 = alloc() : memref<256xf32>
  %1 = alloc() : memref<32xf32, 1>
  %2 = alloc() : memref<1xf32>
  %c0 = constant 0 : index
  %c128 = constant 128 : index
  affine.for %i0 = 0 to 8 {
    affine.dma_start %0[%i0], %1[%i0], %2[%c0], %c128 : memref<256xf32>, memref<32xf32, 1>, memref<1xf32>
    affine.dma_wait %2[%c0], %c128 : memref<1xf32>
    %3 = affine.load %1[%i0] : memref<32xf32, 1>
    %4 = "compute"(%3) : (f32) -> f32
    affine.store %4, %1[%i0] : memref<32xf32, 1>
  }
  return
}

Output

module {
  func @pipelinedatatransfer() {
    %c8 = constant 8 : index
    %c0 = constant 0 : index
    %0 = alloc() : memref<256xf32>
    %c0_0 = constant 0 : index
    %c128 = constant 128 : index
    %1 = alloc() : memref<2x32xf32, 1>
    %2 = alloc() : memref<2x1xf32>
    affine.dma_start %0[%c0], %1[%c0 mod 2, %c0], %2[%c0 mod 2, symbol(%c0_0)], %c128 : memref<256xf32>, memref<2x32xf32, 1>, memref<2x1xf32>
    affine.for %arg0 = 1 to 8 {
      affine.dma_start %0[%arg0], %1[%arg0 mod 2, %arg0], %2[%arg0 mod 2, symbol(%c0_0)], %c128 : memref<256xf32>, memref<2x32xf32, 1>, memref<2x1xf32>
      %8 = affine.apply #map3(%arg0)
      %9 = affine.apply #map4(%8)
      %10 = affine.apply #map4(%8)
      affine.dma_wait %2[%8 mod 2, symbol(%c0_0)], %c128 : memref<2x1xf32>
      %11 = affine.load %1[%8 mod 2, %8] : memref<2x32xf32, 1>
      %12 = "compute"(%11) : (f32) -> f32
      affine.store %12, %1[%8 mod 2, %8] : memref<2x32xf32, 1>
    }
    %3 = affine.apply #map3(%c8)
    %4 = affine.apply #map4(%3)
    %5 = affine.apply #map4(%3)
    affine.dma_wait %2[%3 mod 2, symbol(%c0_0)], %c128 : memref<2x1xf32>
    %6 = affine.load %1[%3 mod 2, %3] : memref<2x32xf32, 1>
    %7 = "compute"(%6) : (f32) -> f32
    affine.store %7, %1[%3 mod 2, %3] : memref<2x32xf32, 1>
    dealloc %2 : memref<2x1xf32>
    dealloc %1 : memref<2x32xf32, 1>
    return
  }
}