[Mlir-commits] [mlir] [mlir] [memref] Compile-time memref.alloc Scheduling/Merging optimization (PR #95882)

[Matthias Springer]

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+# Compile-time memref.alloc Scheduling and Merging
+
+This document describes a compile-time optimization on `memref.alloc` to reduce
+memory usage and improve memory locality.
+
+## Current status of bufferization and memref pass pipeline
+Bufferization is a process in the current MLIR of converting ops with tensor
+semantics to ops with memref semantics. One-Shot Bufferize is a new tensor
+bufferization pass designed for IR in destination-passing style, and with
+aggressive in-place bufferization. The older/partial bufferization was built
+around multiple dialects. The community is trying to gradually deprecate the
+older bufferization and replace them with one-shot bufferization. The goal of
+bufferization is to use as little memory as possible and copy as little memory
+as possible, as a result, the exsiting focus is to determine in-place or
+out-of-place among the OpOperand and OpResult of individual ops, while not
+considering much about the overall memory reuse across Operators within a
+sub-graph (or partition).
+
+The current implementation of Bufferization and memref pass pipeline focuses on
+copy-avoidance and in-place reusing of the memory. Consider a computation graph
+of 4 layers of matmul sharing the same weight:
+```mlir
+func.func @mlp(%x: tensor<128x128xf32>, %y: tensor<128x128xf32>) -> tensor<128x128xf32> {
+   %a0 = tensor.empty() : tensor<128x128xf32>
+   %a = linalg.matmul ins(%x, %y: tensor<128x128xf32>, tensor<128x128xf32>) outs(%a0: tensor<128x128xf32>) -> tensor<128x128xf32>
+   %b0 = tensor.empty() : tensor<128x128xf32>
+   %b = linalg.matmul ins(%a, %y: tensor<128x128xf32>, tensor<128x128xf32>) outs(%b0: tensor<128x128xf32>) -> tensor<128x128xf32>
+   %c0 = tensor.empty() : tensor<128x128xf32>
+   %c = linalg.matmul ins(%b, %y: tensor<128x128xf32>, tensor<128x128xf32>) outs(%c0: tensor<128x128xf32>) -> tensor<128x128xf32>
+   %d0 = tensor.empty() : tensor<128x128xf32>
+   %d = linalg.matmul ins(%c, %y: tensor<128x128xf32>, tensor<128x128xf32>) outs(%d0: tensor<128x128xf32>) -> tensor<128x128xf32>
+   return %d : tensor<128x128xf32>
+}
+```
+
+The bufferization pass will create an `memref.alloc` for each of the tensor
+`a0`, `b0` and `c0`. The bufferization result should be like:
+
+```mlir
+func.func @mlp(%x: memref<128x128xf32>, %y: memref<128x128xf32>) -> memref<128x128xf32> {
+   %a0 = memref.alloc() : memref<128x128xf32>
+   linalg.matmul ins(%x, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%a0: memref<128x128xf32>)
+   %b0 = memref.alloc() : memref<128x128xf32>
+   linalg.matmul ins(%a0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%b0: memref<128x128xf32>)
+   %c0 = memref.alloc() : memref<128x128xf32>
+   linalg.matmul ins(%b0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%c0: memref<128x128xf32>)
+   %d0 = memref.alloc() : memref<128x128xf32>
+   linalg.matmul ins(%c0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%d0: memref<128x128xf32>)
+   return %d0 : memref<128x128xf32>
+}
+```
+
+Without further optimizations, 3 temp buffers will be allocated at the runtime
+for these tensors. However, as we can see in the IR, the buffer `a0` is no
+longer used when buffer `c0` is allocated. So buffer `c0` can reuse the memory
+buffer of buffer `a0`, to reduce the memory size footprint and improve the
+locality.
+
+An observation of the current bufferization and memref passes is that they do
+not consider the memory buffer planning - to reuse the buffer/memref for less
+total size and better locality.
+
+## Merge-alloc pass
+An optimization pass has been introduced to consolidate multiple allocations
+(`memref.alloc` ops) into a single `memref.alloc` op and each static-shaped
+`memref.alloc` op will be transformed into a "slice" from the `single allocated
+buffer` with `memref.view` and some compile-time decided `offsets`. This
+optimization works on `memref` instead of `tensor` ops, so it should be executed
+after bufferization pass, and before adding buffer deallocation ops.
+
+While merging the memory allocations, the transform should consider the lifetime
+of each allocated `memref`s. By lifetime, we mean the range of time when the
+memory allocated from `memref.alloc` is actively used. The references on `view`s
+of a "base" `memref` should contribute to the lifetime of the "base". A later
+`memref.alloc` should consider to reuse the memory of a previously allocated
+memref, if the lifetime of these two does not overlap. The transform will
+perform the "reusing" of memory by setting the `offset` of the later
+`memref.view` to a position within the memory range of a previous allocation's
+`memref.alloc` from the `single allocated buffer`.
+
+Below is the expected transformation result of the example IR in the above
+section:
+
+```mlir
+func.func @mlp(%x: memref<256x128xf32>, %y: memref<128x128xf32>) -> memref<128x128xf32> {
+   %single_buffer = memref.alloc() : memref<131072xi8> // 128*128*sizeof(f32)*2
+   %a0 = memref.view %single_buffer[0][] : memref<131072xi8> to memref<128x128xf32> // a0 takes the memory from byte offset 0
+   linalg.matmul ins(%x, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%a0: memref<128x128xf32>)
+   %b0 = memref.view %single_buffer[65536][] : memref<131072xi8> to memref<128x128xf32> // b0 takes the memory from byte offset 128*128*sizeof(f32)
+   linalg.matmul ins(%a0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%b0: memref<128x128xf32>) 
+   %c0 = memref.view %single_buffer[0][] : memref<131072xi8> to memref<128x128xf32> // c0 takes the memory from byte offset 0
+   linalg.matmul ins(%b0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%c0: memref<128x128xf32>)
+   %d0 = memref.alloc() : memref<128x128xf32> // d0 is returned, do not merge
+   linalg.matmul ins(%c0, %y: memref<128x128xf32>, memref<128x128xf32>) outs(%d0: memref<128x128xf32>)
+   return %d0 : memref<128x128xf32>
+}
+```
+
+There is one single allocation `single_buffer` for all temp buffers and `alloc`
+ops for `a0`, `b0` and `c0` are removed. The returned memref `d0` is untouched.
+The memrefs `a0`, `b0` and `c0` are replaced by `memref.view` on
+`single_buffer`. Since `a0` and `b0`'s lifetime overlaps, the transformation
+will "allocate" different memory ranges on the `single_buffer` - note that `a0`
+and `b0` has different offsets `%single_buffer[0]` and `%single_buffer[65536]`
+and the memory ranges does not overlap. The memref `c0` does not overlap with
+`a0` in their lifetime, so that `c0` can reuse the memory range of `a0` by
+setting of offset to `%single_buffer[0]`, which is the same of `a0`. The final
+allocation size of temp memory buffer will be `128*128*sizeof(f32)*2` instead of
+three `memref<128x128xf32>` buffers in the original IR.
+
+
+## Other solutions besides merge-alloc
+
+Another (not yet existing) approach to resolve the memory reusing issue is to
+insert `memref.dealloc` as soon as the buffer is no longer used. For example, in
+the above "matmul" example, `memref.dealloc` can be inserted after the last use
+of `a0` at `linalg.matmul ins(%a0, %y...)`. So even without memref merging
+transformation, a common runtime memory allocator will try to reuse the memory
+free'd by `memref.dealloc(%a0)` when allocating buffer for `c0`. However, there
+are some disadvantages of this approach comparing to the compile-time memref
+merging transformation of this proposal:
+1. it depends on the implementation of the runtime memory allocator.
+2. the runtime memory allocator does not have full picture of the future
+   allocation/deallocation patterns of the program. For example, if we change
+   the above example to make buffer size `c0` greater than size of `a0`, the
+   runtime memory allocator will not likely to be able to reuse the memory of
+   `a0` for `c0`, becuase the free memory chunk size of `a0` does not fit
+   allocation of `c0`. In contrast, the proposed optimization of this document
+   has the knowledge of the allocation patterns. Thus, it can put the memory
+   chunk for `a0` in a right place of the `single allocation buffer`, so that
+   the allocation of `c0` can fit into it.
+3. calling runtime memory allocator for each buffer introduces more run time
+   overhead than a single merged allocation after allocation merging.
+
+However, utilizing runtime memory allocator can be viewed as a supplementary
+approach of the allocation merging at compile-time, for example, to handle
+memref with dynamic shapes. These two memory optimization approaches should
+coexist and cowork in the pass pipeline.
+
+## General framework for implementation of merge-alloc
+
+To make merge-alloc pass capable of handling different hardware architectures
+and runtime requirements, the pass is implemented as a general pipeline of the
+following stages:
+
+1. Collect the memory alias via `BufferViewFlowAnalysis`
+2. Collect the memory lifetime traces
+3. Schedule the buffers by an allocation algorithm to compute the offsets of
+   each allocations
+4. Rewrite the IR to replace allocations with views of merged buffers
+
+The steps 2, 3 and 4 can be implemented by the developers to customize the pass
+for their own use cases. A tick-based pipeline of the pass is provided as the
+default implementation, which will be discussed in the next section. 
+
+The following concepts should be defined by the implementation of the pass:
+ * Mergeable alloction: the memref.alloc operations that should be merged by the
----------------
matthias-springer wrote:

`allocation`

https://github.com/llvm/llvm-project/pull/95882