https://github.com/joker-eph updated https://github.com/llvm/llvm-project/pull/70283
>From 0bdf7a0bc1c1e3b5fc3280e9ba5f5cacfeeb5f7f Mon Sep 17 00:00:00 2001 From: Mehdi Amini <joker....@gmail.com> Date: Wed, 25 Oct 2023 19:17:32 -0700 Subject: [PATCH 1/3] Update Bufferization.md --- mlir/docs/Bufferization.md | 39 ++++++++++++++++++++++++++++---------- 1 file changed, 29 insertions(+), 10 deletions(-) diff --git a/mlir/docs/Bufferization.md b/mlir/docs/Bufferization.md index d9d0751cae8c9dd..ea3593549ca1563 100644 --- a/mlir/docs/Bufferization.md +++ b/mlir/docs/Bufferization.md @@ -101,10 +101,28 @@ bufferization strategy would be unacceptable for high-performance codegen. When choosing an already existing buffer, we must be careful not to accidentally overwrite data that is still needed later in the program. -To simplify this problem, One-Shot Bufferize was designed for ops that are in -*destination-passing style*. For every tensor result, such ops have a tensor -operand, whose buffer could be utilized for storing the result of the op in the -absence of other conflicts. We call such tensor operands the *destination*. +To simplify this problem, One-Shot Bufferize was designed to take advantage of +*destination-passing style*. This form exists in itself independently of +bufferization and is tied to SSA semantics: many ops are “updating” part of +their input SSA variable. For example the LLVM instruction +[`insertelement`](https://llvm.org/docs/LangRef.html#insertelement-instruction) +is inserting an element inside a vector. Since SSA values are immutable, the +operation returns a copy of the input vector with the element inserted. +Another example in MLIR is `linalg.generic`, which always has an extra `outs` +operand which provides the initial values to update (for example when the +operation is doing a reduction). + +This input is referred to as "destination" in the following (quotes are +important are this operand isn't modified in place but copied) and come into +place in the context of bufferization as a possible "anchor" for the +bufferization algorithm. This allows the user to shape the input in a form that +guarantees close to optimal bufferization result when carefully choosing the +SSA value used as "destination". + +For every tensor result, a "destination-passing" style op has a corresponding +tensor operand. If there aren't any other uses of this tensor, the bufferization +can alias it with the op result and perform the operation "in-place" by reusing +the buffer allocated for this "destination" input. As an example, consider the following op: `%0 = tensor.insert %cst into %t[%idx] : tensor<?xf32>` @@ -112,15 +130,16 @@ As an example, consider the following op: `%0 = tensor.insert %cst into `%t` is the destination in this example. When choosing a buffer for the result `%0`, One-Shot Bufferize considers only two options: -1. buffer(`%0`) = buffer(`%t`). -2. buffer(`%0`) is a newly allocated buffer. +1. buffer(`%0`) = buffer(`%t`): alias the destination tensor with the + result and perform the operation in-place. +3. buffer(`%0`) is a newly allocated buffer. There may be other buffers in the same function that could potentially be used for buffer(`%0`), but those are not considered by One-Shot Bufferize to keep the bufferization simple. One-Shot Bufferize could be extended to consider such buffers in the future to achieve a better quality of bufferization. -Tensor ops that are not in destination-passing style always bufferize to a +Tensor ops that are not in destination-passing style always bufferized to a memory allocation. E.g.: ```mlir @@ -159,9 +178,9 @@ slice of a tensor: ``` The above example bufferizes to a `memref.subview`, followed by a -"`linalg.generic` on memrefs" that overwrites the memory of the subview. The -`tensor.insert_slice` bufferizes to a no-op (in the absence of RaW conflicts -such as a subsequent read of `%s`). +"`linalg.generic` on memrefs" that overwrites the memory of the subview, assuming +that the slice `%t` has no other user. The `tensor.insert_slice` then bufferizes +to a no-op (in the absence of RaW conflicts such as a subsequent read of `%s`). RaW conflicts are detected with an analysis of SSA use-def chains (details later). One-Shot Bufferize works best if there is a single SSA use-def chain, >From a06bcdde0c75dbab260d7d31d4dcaf0b169d8811 Mon Sep 17 00:00:00 2001 From: Mehdi Amini <joker....@gmail.com> Date: Wed, 25 Oct 2023 20:42:41 -0700 Subject: [PATCH 2/3] Update Bufferization.md --- mlir/docs/Bufferization.md | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/mlir/docs/Bufferization.md b/mlir/docs/Bufferization.md index ea3593549ca1563..88a2e50e85d938d 100644 --- a/mlir/docs/Bufferization.md +++ b/mlir/docs/Bufferization.md @@ -113,7 +113,7 @@ operand which provides the initial values to update (for example when the operation is doing a reduction). This input is referred to as "destination" in the following (quotes are -important are this operand isn't modified in place but copied) and come into +important as this operand isn't modified in place but copied) and comes into place in the context of bufferization as a possible "anchor" for the bufferization algorithm. This allows the user to shape the input in a form that guarantees close to optimal bufferization result when carefully choosing the >From 2ff038576a0c505da1601d614d27110bc00624c9 Mon Sep 17 00:00:00 2001 From: Mehdi Amini <joker....@gmail.com> Date: Wed, 25 Oct 2023 20:44:48 -0700 Subject: [PATCH 3/3] Update Bufferization.md --- mlir/docs/Bufferization.md | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/mlir/docs/Bufferization.md b/mlir/docs/Bufferization.md index 64b10d49bd11cbf..8329999162fb5aa 100644 --- a/mlir/docs/Bufferization.md +++ b/mlir/docs/Bufferization.md @@ -184,9 +184,7 @@ to a no-op (in the absence of RaW conflicts such as a subsequent read of `%s`). RaW conflicts are detected with an analysis of SSA use-def chains (details later). One-Shot Bufferize works best if there is a single SSA use-def chain, -where the result of a tensor op is the -operand of the next tensor -ops, e.g.: +where the result of a tensor op is the operand of the next tensor ops, e.g.: ```mlir %0 = "my_dialect.some_op"(%t) : (tensor<?xf32>) -> (tensor<?xf32>) _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
