[Mlir-commits] [mlir] [mlir][gpu] Separate the barrier elimination code from transform ops (PR #71762)
llvmlistbot at llvm.org
llvmlistbot at llvm.org
Wed Nov 8 19:13:25 PST 2023
https://github.com/spaceotter created https://github.com/llvm/llvm-project/pull/71762
Allows the barrier elimination code to be run from C++ as well.
>From b23f45de22f1cab8c5eabd406b07b92437240f4c Mon Sep 17 00:00:00 2001
From: Eric Eaton <eric at nod-labs.com>
Date: Wed, 8 Nov 2023 19:03:04 -0800
Subject: [PATCH] [mlir][gpu] Separate the barrier elimination code from
transform ops
Allows the barrier elimination code to be run from C++ as well.
---
.../mlir/Dialect/GPU/Transforms/Passes.h | 4 +
.../mlir/Dialect/GPU/Transforms/Passes.td | 14 +
.../mlir/Dialect/GPU/Transforms/Utils.h | 1 +
mlir/lib/Dialect/GPU/CMakeLists.txt | 1 +
.../GPU/TransformOps/GPUTransformOps.cpp | 571 +---------------
.../GPU/Transforms/EliminateBarriers.cpp | 614 ++++++++++++++++++
.../test/Dialect/GPU/barrier-elimination.mlir | 1 +
7 files changed, 637 insertions(+), 569 deletions(-)
create mode 100644 mlir/lib/Dialect/GPU/Transforms/EliminateBarriers.cpp
diff --git a/mlir/include/mlir/Dialect/GPU/Transforms/Passes.h b/mlir/include/mlir/Dialect/GPU/Transforms/Passes.h
index 2a891a7d24f809a..992204673395cb4 100644
--- a/mlir/include/mlir/Dialect/GPU/Transforms/Passes.h
+++ b/mlir/include/mlir/Dialect/GPU/Transforms/Passes.h
@@ -165,6 +165,10 @@ void populateGpuDecomposeMemrefsPatterns(RewritePatternSet &patterns);
/// Pass decomposes memref ops inside `gpu.launch` body.
std::unique_ptr<Pass> createGpuDecomposeMemrefsPass();
+void populateGpuEliminateBarriersPatterns(RewritePatternSet &patterns);
+
+std::unique_ptr<OperationPass<func::FuncOp>> createGpuEliminateBarriersPass();
+
/// Generate the code for registering passes.
#define GEN_PASS_REGISTRATION
#include "mlir/Dialect/GPU/Transforms/Passes.h.inc"
diff --git a/mlir/include/mlir/Dialect/GPU/Transforms/Passes.td b/mlir/include/mlir/Dialect/GPU/Transforms/Passes.td
index 059893127295bf5..7326a9c2a0bf68f 100644
--- a/mlir/include/mlir/Dialect/GPU/Transforms/Passes.td
+++ b/mlir/include/mlir/Dialect/GPU/Transforms/Passes.td
@@ -37,6 +37,20 @@ def GpuMapParallelLoopsPass
let dependentDialects = ["mlir::gpu::GPUDialect"];
}
+def GpuEliminateBarriers
+ : Pass<"gpu-eliminate-barriers", "mlir::func::FuncOp"> {
+ let summary = "Erase unecessary barriers";
+ let constructor = "mlir::createGpuEliminateBarriersPass()";
+ let description = [{
+ Erase barriers that are unneeded because there are no dependent side effects
+ across the barrier.
+ }];
+ let dependentDialects = [
+ "mlir::gpu::GPUDialect", "mlir::scf::SCFDialect",
+ "mlir::memref::MemRefDialect"
+ ];
+}
+
def GpuDecomposeMemrefsPass : Pass<"gpu-decompose-memrefs"> {
let summary = "Decomposes memref index computation into explicit ops.";
let description = [{
diff --git a/mlir/include/mlir/Dialect/GPU/Transforms/Utils.h b/mlir/include/mlir/Dialect/GPU/Transforms/Utils.h
index a426bee7686dbc0..8ddff0817667d65 100644
--- a/mlir/include/mlir/Dialect/GPU/Transforms/Utils.h
+++ b/mlir/include/mlir/Dialect/GPU/Transforms/Utils.h
@@ -14,6 +14,7 @@
#define MLIR_DIALECT_GPU_TRANSFORMS_UTILS_H_
#include "mlir/Support/LLVM.h"
+#include "mlir/IR/PatternMatch.h"
#include <string>
diff --git a/mlir/lib/Dialect/GPU/CMakeLists.txt b/mlir/lib/Dialect/GPU/CMakeLists.txt
index 09a3cd06788bc5e..e26b5dacc98c365 100644
--- a/mlir/lib/Dialect/GPU/CMakeLists.txt
+++ b/mlir/lib/Dialect/GPU/CMakeLists.txt
@@ -62,6 +62,7 @@ add_mlir_dialect_library(MLIRGPUTransforms
Transforms/ShuffleRewriter.cpp
Transforms/SPIRVAttachTarget.cpp
Transforms/ROCDLAttachTarget.cpp
+ Transforms/EliminateBarriers.cpp
ADDITIONAL_HEADER_DIRS
${MLIR_MAIN_INCLUDE_DIR}/mlir/Dialect/GPU
diff --git a/mlir/lib/Dialect/GPU/TransformOps/GPUTransformOps.cpp b/mlir/lib/Dialect/GPU/TransformOps/GPUTransformOps.cpp
index d4908fa7e89e736..0d5e9ae9bbd3792 100644
--- a/mlir/lib/Dialect/GPU/TransformOps/GPUTransformOps.cpp
+++ b/mlir/lib/Dialect/GPU/TransformOps/GPUTransformOps.cpp
@@ -263,575 +263,8 @@ void transform::ApplyUnrollVectorsSubgroupMmaOp::populatePatterns(
// EliminateBarriersOp
//===----------------------------------------------------------------------===//
-// The functions below provide interface-like verification, but are too specific
-// to barrier elimination to become interfaces.
-
-/// Implement the MemoryEffectsOpInterface in the suitable way.
-static bool isKnownNoEffectsOpWithoutInterface(Operation *op) {
- // memref::AssumeAlignment is conceptually pure, but marking it as such would
- // make DCE immediately remove it.
- return isa<memref::AssumeAlignmentOp>(op);
-}
-
-/// Returns `true` if the op is defines the parallel region that is subject to
-/// barrier synchronization.
-static bool isParallelRegionBoundary(Operation *op) {
- if (op->hasAttr("__parallel_region_boundary_for_test"))
- return true;
-
- return isa<GPUFuncOp, LaunchOp>(op);
-}
-
-/// Returns `true` if the op behaves like a sequential loop, e.g., the control
-/// flow "wraps around" from the end of the body region back to its start.
-static bool isSequentialLoopLike(Operation *op) { return isa<scf::ForOp>(op); }
-
-/// Returns `true` if the regions of the op are guaranteed to be executed at
-/// most once. Thus, if an operation in one of the nested regions of `op` is
-/// executed than so are all the other operations in this region.
-static bool hasSingleExecutionBody(Operation *op) {
- return isa<scf::IfOp, memref::AllocaScopeOp>(op);
-}
-
-/// Returns `true` if the operation is known to produce a pointer-like object
-/// distinct from any other object produced by a similar operation. For example,
-/// an allocation produces such an object.
-static bool producesDistinctBase(Operation *op) {
- return isa_and_nonnull<memref::AllocOp, memref::AllocaOp>(op);
-}
-
-/// Populates `effects` with all memory effects without associating them to a
-/// specific value.
-static void addAllValuelessEffects(
- SmallVectorImpl<MemoryEffects::EffectInstance> &effects) {
- effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Read>());
- effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Write>());
- effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Allocate>());
- effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Free>());
-}
-
-/// Collect the memory effects of the given op in 'effects'. Returns 'true' if
-/// it could extract the effect information from the op, otherwise returns
-/// 'false' and conservatively populates the list with all possible effects
-/// associated with no particular value or symbol.
-static bool
-collectEffects(Operation *op,
- SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
- bool ignoreBarriers = true) {
- // Skip over barriers to avoid infinite recursion (those barriers would ask
- // this barrier again).
- if (ignoreBarriers && isa<BarrierOp>(op))
- return true;
-
- // Skip over ops that we know have no effects.
- if (isKnownNoEffectsOpWithoutInterface(op))
- return true;
-
- // Collect effect instances the operation. Note that the implementation of
- // getEffects erases all effect instances that have the type other than the
- // template parameter so we collect them first in a local buffer and then
- // copy.
- if (auto iface = dyn_cast<MemoryEffectOpInterface>(op)) {
- SmallVector<MemoryEffects::EffectInstance> localEffects;
- iface.getEffects(localEffects);
- llvm::append_range(effects, localEffects);
- return true;
- }
- if (op->hasTrait<OpTrait::HasRecursiveMemoryEffects>()) {
- for (auto ®ion : op->getRegions()) {
- for (auto &block : region) {
- for (auto &innerOp : block)
- if (!collectEffects(&innerOp, effects, ignoreBarriers))
- return false;
- }
- }
- return true;
- }
-
- // We need to be conservative here in case the op doesn't have the interface
- // and assume it can have any possible effect.
- addAllValuelessEffects(effects);
- return false;
-}
-
-/// Collects memory effects from operations that may be executed before `op` in
-/// a trivial structured control flow, e.g., without branches. Stops at the
-/// parallel region boundary or at the barrier operation if `stopAtBarrier` is
-/// set. Returns `true` if the memory effects added to `effects` are exact,
-/// `false` if they are a conservative over-approximation. The latter means that
-/// `effects` contain instances not associated with a specific value.
-static bool
-getEffectsBefore(Operation *op,
- SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
- bool stopAtBarrier) {
- if (!op->getBlock())
- return true;
-
- // If there is a non-structured control flow, bail.
- Region *region = op->getBlock()->getParent();
- if (region && !llvm::hasSingleElement(region->getBlocks())) {
- addAllValuelessEffects(effects);
- return false;
- }
-
- // Collect all effects before the op.
- if (op != &op->getBlock()->front()) {
- for (Operation *it = op->getPrevNode(); it != nullptr;
- it = it->getPrevNode()) {
- if (isa<BarrierOp>(it)) {
- if (stopAtBarrier)
- return true;
- else
- continue;
- }
- if (!collectEffects(it, effects))
- return false;
- }
- }
-
- // Stop if reached the parallel region boundary.
- if (isParallelRegionBoundary(op->getParentOp()))
- return true;
-
- // Otherwise, keep collecting above the parent operation.
- if (!getEffectsBefore(op->getParentOp(), effects, stopAtBarrier))
- return false;
-
- // If the op is loop-like, collect effects from the trailing operations until
- // we hit a barrier because they can executed before the current operation by
- // the previous iteration of this loop. For example, in the following loop
- //
- // for i = ... {
- // op1
- // ...
- // barrier
- // op2
- // }
- //
- // the operation `op2` at iteration `i` is known to be executed before the
- // operation `op1` at iteration `i+1` and the side effects must be ordered
- // appropriately.
- if (isSequentialLoopLike(op->getParentOp())) {
- // Assuming loop terminators have no side effects.
- return getEffectsBefore(op->getBlock()->getTerminator(), effects,
- /*stopAtBarrier=*/true);
- }
-
- // If the parent operation is not guaranteed to execute its (single-block)
- // region once, walk the block.
- bool conservative = false;
- if (!hasSingleExecutionBody(op->getParentOp()))
- op->getParentOp()->walk([&](Operation *in) {
- if (conservative)
- return WalkResult::interrupt();
- if (!collectEffects(in, effects)) {
- conservative = true;
- return WalkResult::interrupt();
- }
- return WalkResult::advance();
- });
-
- return !conservative;
-}
-
-/// Collects memory effects from operations that may be executed after `op` in
-/// a trivial structured control flow, e.g., without branches. Stops at the
-/// parallel region boundary or at the barrier operation if `stopAtBarrier` is
-/// set. Returns `true` if the memory effects added to `effects` are exact,
-/// `false` if they are a conservative over-approximation. The latter means that
-/// `effects` contain instances not associated with a specific value.
-static bool
-getEffectsAfter(Operation *op,
- SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
- bool stopAtBarrier) {
- if (!op->getBlock())
- return true;
-
- // If there is a non-structured control flow, bail.
- Region *region = op->getBlock()->getParent();
- if (region && !llvm::hasSingleElement(region->getBlocks())) {
- addAllValuelessEffects(effects);
- return false;
- }
-
- // Collect all effects after the op.
- if (op != &op->getBlock()->back())
- for (Operation *it = op->getNextNode(); it != nullptr;
- it = it->getNextNode()) {
- if (isa<BarrierOp>(it)) {
- if (stopAtBarrier)
- return true;
- continue;
- }
- if (!collectEffects(it, effects))
- return false;
- }
-
- // Stop if reached the parallel region boundary.
- if (isParallelRegionBoundary(op->getParentOp()))
- return true;
-
- // Otherwise, keep collecting below the parent operation.
- if (!getEffectsAfter(op->getParentOp(), effects, stopAtBarrier))
- return false;
-
- // If the op is loop-like, collect effects from the leading operations until
- // we hit a barrier because they can executed after the current operation by
- // the next iteration of this loop. For example, in the following loop
- //
- // for i = ... {
- // op1
- // ...
- // barrier
- // op2
- // }
- //
- // the operation `op1` at iteration `i` is known to be executed after the
- // operation `op2` at iteration `i-1` and the side effects must be ordered
- // appropriately.
- if (isSequentialLoopLike(op->getParentOp())) {
- if (isa<BarrierOp>(op->getBlock()->front()))
- return true;
-
- bool exact = collectEffects(&op->getBlock()->front(), effects);
- return getEffectsAfter(&op->getBlock()->front(), effects,
- /*stopAtBarrier=*/true) &&
- exact;
- }
-
- // If the parent operation is not guaranteed to execute its (single-block)
- // region once, walk the block.
- bool conservative = false;
- if (!hasSingleExecutionBody(op->getParentOp()))
- op->getParentOp()->walk([&](Operation *in) {
- if (conservative)
- return WalkResult::interrupt();
- if (!collectEffects(in, effects)) {
- conservative = true;
- return WalkResult::interrupt();
- }
- return WalkResult::advance();
- });
-
- return !conservative;
-}
-
-/// Looks through known "view-like" ops to find the base memref.
-static Value getBase(Value v) {
- while (true) {
- Operation *definingOp = v.getDefiningOp();
- if (!definingOp)
- break;
-
- bool shouldContinue =
- TypeSwitch<Operation *, bool>(v.getDefiningOp())
- .Case<memref::CastOp, memref::SubViewOp, memref::ViewOp>(
- [&](auto op) {
- v = op.getSource();
- return true;
- })
- .Case<memref::TransposeOp>([&](auto op) {
- v = op.getIn();
- return true;
- })
- .Case<memref::CollapseShapeOp, memref::ExpandShapeOp>([&](auto op) {
- v = op.getSrc();
- return true;
- })
- .Default([](Operation *) { return false; });
- if (!shouldContinue)
- break;
- }
- return v;
-}
-
-/// Returns `true` if the value is defined as a function argument.
-static bool isFunctionArgument(Value v) {
- auto arg = dyn_cast<BlockArgument>(v);
- return arg && isa<FunctionOpInterface>(arg.getOwner()->getParentOp());
-}
-
-/// Returns the operand that the operation "propagates" through it for capture
-/// purposes. That is, if the value produced by this operation is captured, then
-/// so is the returned value.
-static Value propagatesCapture(Operation *op) {
- return llvm::TypeSwitch<Operation *, Value>(op)
- .Case(
- [](ViewLikeOpInterface viewLike) { return viewLike.getViewSource(); })
- .Case([](CastOpInterface castLike) { return castLike->getOperand(0); })
- .Case([](memref::TransposeOp transpose) { return transpose.getIn(); })
- .Case<memref::ExpandShapeOp, memref::CollapseShapeOp>(
- [](auto op) { return op.getSrc(); })
- .Default([](Operation *) { return Value(); });
-}
-
-/// Returns `true` if the given operation is known to capture the given value,
-/// `false` if it is known not to capture the given value, `nullopt` if neither
-/// is known.
-static std::optional<bool> getKnownCapturingStatus(Operation *op, Value v) {
- return llvm::TypeSwitch<Operation *, std::optional<bool>>(op)
- // Store-like operations don't capture the destination, but do capture
- // the value.
- .Case<memref::StoreOp, vector::TransferWriteOp>(
- [&](auto op) { return op.getValue() == v; })
- .Case<vector::StoreOp, vector::MaskedStoreOp>(
- [&](auto op) { return op.getValueToStore() == v; })
- // These operations are known not to capture.
- .Case([](memref::DeallocOp) { return false; })
- // By default, we don't know anything.
- .Default([](Operation *) { return std::nullopt; });
-}
-
-/// Returns `true` if the value may be captured by any of its users, i.e., if
-/// the user may be storing this value into memory. This makes aliasing analysis
-/// more conservative as it cannot assume the pointer-like value is only passed
-/// around through SSA use-def.
-static bool maybeCaptured(Value v) {
- SmallVector<Value> todo = {v};
- while (!todo.empty()) {
- Value v = todo.pop_back_val();
- for (Operation *user : v.getUsers()) {
- // A user that is known to only read cannot capture.
- auto iface = dyn_cast<MemoryEffectOpInterface>(user);
- if (iface) {
- SmallVector<MemoryEffects::EffectInstance> effects;
- iface.getEffects(effects);
- if (llvm::all_of(effects,
- [](const MemoryEffects::EffectInstance &effect) {
- return isa<MemoryEffects::Read>(effect.getEffect());
- })) {
- continue;
- }
- }
-
- // When an operation is known to create an alias, consider if the
- // source is captured as well.
- if (Value v = propagatesCapture(user)) {
- todo.push_back(v);
- continue;
- }
-
- std::optional<bool> knownCaptureStatus = getKnownCapturingStatus(user, v);
- if (!knownCaptureStatus || *knownCaptureStatus)
- return true;
- }
- }
-
- return false;
-}
-
-/// Returns true if two values may be referencing aliasing memory. This is a
-/// rather naive and conservative analysis. Values defined by different
-/// allocation-like operations as well as values derived from those by casts and
-/// views cannot alias each other. Similarly, values defined by allocations
-/// inside a function cannot alias function arguments. Global values cannot
-/// alias each other or local allocations. Values that are captured, i.e.
-/// themselves potentially stored in memory, are considered as aliasing with
-/// everything. This seems sufficient to achieve barrier removal in structured
-/// control flow, more complex cases would require a proper dataflow analysis.
-static bool mayAlias(Value first, Value second) {
- DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, {
- DBGS_ALIAS() << "checking aliasing between ";
- DBGS_ALIAS() << first << "\n";
- DBGS_ALIAS() << " and ";
- DBGS_ALIAS() << second << "\n";
- });
-
- first = getBase(first);
- second = getBase(second);
-
- DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, {
- DBGS_ALIAS() << "base ";
- DBGS_ALIAS() << first << "\n";
- DBGS_ALIAS() << " and ";
- DBGS_ALIAS() << second << "\n";
- });
-
- // Values derived from the same base memref do alias (unless we do a more
- // advanced analysis to prove non-overlapping accesses).
- if (first == second) {
- DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, DBGS_ALIAS() << "-> do alias!\n");
- return true;
- }
-
- // Different globals cannot alias.
- if (auto globFirst = first.getDefiningOp<memref::GetGlobalOp>()) {
- if (auto globSecond = second.getDefiningOp<memref::GetGlobalOp>()) {
- return globFirst.getNameAttr() == globSecond.getNameAttr();
- }
- }
-
- // Two function arguments marked as noalias do not alias.
- auto isNoaliasFuncArgument = [](Value value) {
- auto bbArg = dyn_cast<BlockArgument>(value);
- if (!bbArg)
- return false;
- auto iface = dyn_cast<FunctionOpInterface>(bbArg.getOwner()->getParentOp());
- if (!iface)
- return false;
- // TODO: we need a way to not depend on the LLVM dialect here.
- return iface.getArgAttr(bbArg.getArgNumber(), "llvm.noalias") != nullptr;
- };
- if (isNoaliasFuncArgument(first) && isNoaliasFuncArgument(second))
- return false;
-
- bool isDistinct[] = {producesDistinctBase(first.getDefiningOp()),
- producesDistinctBase(second.getDefiningOp())};
- bool isGlobal[] = {first.getDefiningOp<memref::GetGlobalOp>() != nullptr,
- second.getDefiningOp<memref::GetGlobalOp>() != nullptr};
-
- // Non-equivalent distinct bases and globals cannot alias. At this point, we
- // have already filtered out based on values being equal and global name being
- // equal.
- if ((isDistinct[0] || isGlobal[0]) && (isDistinct[1] || isGlobal[1]))
- return false;
-
- bool isArg[] = {isFunctionArgument(first), isFunctionArgument(second)};
-
- // Distinct bases (allocations) cannot have been passed as an argument.
- if ((isDistinct[0] && isArg[1]) || (isDistinct[1] && isArg[0]))
- return false;
-
- // Non-captured base distinct values cannot conflict with another base value.
- if (isDistinct[0] && !maybeCaptured(first))
- return false;
- if (isDistinct[1] && !maybeCaptured(second))
- return false;
-
- // Otherwise, conservatively assume aliasing.
- DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, DBGS_ALIAS() << "-> may alias!\n");
- return true;
-}
-
-/// Returns `true` if the effect may be affecting memory aliasing the value. If
-/// the effect is not associated with any value, it is assumed to affect all
-/// memory and therefore aliases with everything.
-static bool mayAlias(MemoryEffects::EffectInstance a, Value v2) {
- if (Value v = a.getValue()) {
- return mayAlias(v, v2);
- }
- return true;
-}
-
-/// Returns `true` if the two effects may be affecting aliasing memory. If
-/// an effect is not associated with any value, it is assumed to affect all
-/// memory and therefore aliases with everything. Effects on different resources
-/// cannot alias.
-static bool mayAlias(MemoryEffects::EffectInstance a,
- MemoryEffects::EffectInstance b) {
- if (a.getResource()->getResourceID() != b.getResource()->getResourceID())
- return false;
- if (Value v2 = b.getValue()) {
- return mayAlias(a, v2);
- } else if (Value v = a.getValue()) {
- return mayAlias(b, v);
- }
- return true;
-}
-
-/// Returns `true` if any of the "before" effect instances has a conflict with
-/// any "after" instance for the purpose of barrier elimination. The effects are
-/// supposed to be limited to a barrier synchronization scope. A conflict exists
-/// if effects instances affect aliasing memory locations and at least on of
-/// then as a write. As an exception, if the non-write effect is an allocation
-/// effect, there is no conflict since we are only expected to see the
-/// allocation happening in the same thread and it cannot be accessed from
-/// another thread without capture (which we do handle in alias analysis).
-static bool
-haveConflictingEffects(ArrayRef<MemoryEffects::EffectInstance> beforeEffects,
- ArrayRef<MemoryEffects::EffectInstance> afterEffects) {
- for (const MemoryEffects::EffectInstance &before : beforeEffects) {
- for (const MemoryEffects::EffectInstance &after : afterEffects) {
- // If cannot alias, definitely no conflict.
- if (!mayAlias(before, after))
- continue;
-
- // Read/read is not a conflict.
- if (isa<MemoryEffects::Read>(before.getEffect()) &&
- isa<MemoryEffects::Read>(after.getEffect())) {
- continue;
- }
-
- // Allocate/* is not a conflict since the allocation happens within the
- // thread context.
- // TODO: This is not the case for */Free unless the allocation happened in
- // the thread context, which we could also check for.
- if (isa<MemoryEffects::Allocate>(before.getEffect()) ||
- isa<MemoryEffects::Allocate>(after.getEffect())) {
- continue;
- }
-
- // In the particular case that the before effect is a free, we only have 2
- // possibilities:
- // 1. either the program is well-formed and there must be an interleaved
- // alloc that must limit the scope of effect lookback and we can
- // safely ignore the free -> read / free -> write and free -> free
- // conflicts.
- // 2. either the program is ill-formed and we are in undefined behavior
- // territory.
- if (isa<MemoryEffects::Free>(before.getEffect()))
- continue;
-
- // Other kinds of effects create a conflict, e.g. read-after-write.
- LLVM_DEBUG(
- DBGS() << "found a conflict between (before): " << before.getValue()
- << " read:" << isa<MemoryEffects::Read>(before.getEffect())
- << " write:" << isa<MemoryEffects::Write>(before.getEffect())
- << " alloc:"
- << isa<MemoryEffects::Allocate>(before.getEffect()) << " free:"
- << isa<MemoryEffects::Free>(before.getEffect()) << "\n");
- LLVM_DEBUG(
- DBGS() << "and (after): " << after.getValue()
- << " read:" << isa<MemoryEffects::Read>(after.getEffect())
- << " write:" << isa<MemoryEffects::Write>(after.getEffect())
- << " alloc:" << isa<MemoryEffects::Allocate>(after.getEffect())
- << " free:" << isa<MemoryEffects::Free>(after.getEffect())
- << "\n");
- return true;
- }
- }
-
- return false;
-}
-
-namespace {
-/// Barrier elimination pattern. If a barrier does not enforce any conflicting
-/// pair of memory effects, including a pair that is enforced by another
-/// barrier, it is unnecessary and can be removed. Adapted from
-/// "High-Performance GPU-to-CPU Transpilation and Optimization via High-Level
-/// Parallel Constructs" by Moses, Ivanov, Domke, Endo, Doerfert, and Zinenko in
-/// PPoPP 2023 and implementation in Polygeist.
-class BarrierElimination final : public OpRewritePattern<BarrierOp> {
-public:
- using OpRewritePattern<BarrierOp>::OpRewritePattern;
-
- LogicalResult matchAndRewrite(BarrierOp barrier,
- PatternRewriter &rewriter) const override {
- LLVM_DEBUG(DBGS() << "checking the necessity of: " << barrier << " "
- << barrier.getLoc() << "\n");
-
- SmallVector<MemoryEffects::EffectInstance> beforeEffects;
- getEffectsBefore(barrier, beforeEffects, /*stopAtBarrier=*/true);
-
- SmallVector<MemoryEffects::EffectInstance> afterEffects;
- getEffectsAfter(barrier, afterEffects, /*stopAtBarrier=*/true);
-
- if (!haveConflictingEffects(beforeEffects, afterEffects)) {
- LLVM_DEBUG(DBGS() << "the surrounding barriers are sufficient, removing "
- << barrier << "\n");
- rewriter.eraseOp(barrier);
- return success();
- }
-
- LLVM_DEBUG(DBGS() << "barrier is necessary: " << barrier << " "
- << barrier.getLoc() << "\n");
- return failure();
- }
-};
-} // namespace
-
void EliminateBarriersOp::populatePatterns(RewritePatternSet &patterns) {
- patterns.insert<BarrierElimination>(getContext());
+ populateGpuEliminateBarriersPatterns(patterns);
}
//===----------------------------------------------------------------------===//
@@ -1080,7 +513,7 @@ static DiagnosedSilenceableFailure rewriteOneForallCommonImpl(
LLVM_DEBUG(
llvm::interleaveComma(
activeMappingSizes, DBGS() << "----activeMappingSizes: ");
- llvm::dbgs() << "\n";
+ llvm::dbgs() << "\n";
llvm::interleaveComma(
availableMappingSizes, DBGS() << "----availableMappingSizes: ");
llvm::dbgs() << "\n";
diff --git a/mlir/lib/Dialect/GPU/Transforms/EliminateBarriers.cpp b/mlir/lib/Dialect/GPU/Transforms/EliminateBarriers.cpp
new file mode 100644
index 000000000000000..51ed6e4dd242156
--- /dev/null
+++ b/mlir/lib/Dialect/GPU/Transforms/EliminateBarriers.cpp
@@ -0,0 +1,614 @@
+#include "mlir/Dialect/Func/IR/FuncOps.h"
+#include "mlir/Dialect/GPU/IR/GPUDialect.h"
+#include "mlir/Dialect/GPU/Transforms/Passes.h"
+#include "mlir/Dialect/MemRef/IR/MemRef.h"
+#include "mlir/Dialect/SCF/IR/SCF.h"
+#include "mlir/Dialect/Vector/IR/VectorOps.h"
+#include "mlir/IR/Operation.h"
+#include "mlir/Pass/Pass.h"
+#include "mlir/Transforms/GreedyPatternRewriteDriver.h"
+#include "llvm/ADT/TypeSwitch.h"
+#include "llvm/Support/Debug.h"
+
+namespace mlir {
+#define GEN_PASS_DEF_GPUELIMINATEBARRIERS
+#include "mlir/Dialect/GPU/Transforms/Passes.h.inc"
+} // namespace mlir
+
+using namespace mlir;
+using namespace mlir::gpu;
+
+#define DEBUG_TYPE "gpu-erase-barriers"
+#define DEBUG_TYPE_ALIAS "gpu-erase-barries-alias"
+
+#define DBGS() (llvm::dbgs() << '[' << DEBUG_TYPE << "] ")
+#define DBGS_ALIAS() (llvm::dbgs() << '[' << DEBUG_TYPE_ALIAS << "] ")
+
+// The functions below provide interface-like verification, but are too specific
+// to barrier elimination to become interfaces.
+
+/// Implement the MemoryEffectsOpInterface in the suitable way.
+static bool isKnownNoEffectsOpWithoutInterface(Operation *op) {
+ // memref::AssumeAlignment is conceptually pure, but marking it as such would
+ // make DCE immediately remove it.
+ return isa<memref::AssumeAlignmentOp>(op);
+}
+
+/// Returns `true` if the op is defines the parallel region that is subject to
+/// barrier synchronization.
+static bool isParallelRegionBoundary(Operation *op) {
+ if (op->hasAttr("__parallel_region_boundary_for_test"))
+ return true;
+
+ return isa<GPUFuncOp, LaunchOp>(op);
+}
+
+/// Returns `true` if the op behaves like a sequential loop, e.g., the control
+/// flow "wraps around" from the end of the body region back to its start.
+static bool isSequentialLoopLike(Operation *op) { return isa<scf::ForOp>(op); }
+
+/// Returns `true` if the regions of the op are guaranteed to be executed at
+/// most once. Thus, if an operation in one of the nested regions of `op` is
+/// executed than so are all the other operations in this region.
+static bool hasSingleExecutionBody(Operation *op) {
+ return isa<scf::IfOp, memref::AllocaScopeOp>(op);
+}
+
+/// Returns `true` if the operation is known to produce a pointer-like object
+/// distinct from any other object produced by a similar operation. For example,
+/// an allocation produces such an object.
+static bool producesDistinctBase(Operation *op) {
+ return isa_and_nonnull<memref::AllocOp, memref::AllocaOp>(op);
+}
+
+/// Populates `effects` with all memory effects without associating them to a
+/// specific value.
+static void addAllValuelessEffects(
+ SmallVectorImpl<MemoryEffects::EffectInstance> &effects) {
+ effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Read>());
+ effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Write>());
+ effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Allocate>());
+ effects.emplace_back(MemoryEffects::Effect::get<MemoryEffects::Free>());
+}
+
+/// Collect the memory effects of the given op in 'effects'. Returns 'true' if
+/// it could extract the effect information from the op, otherwise returns
+/// 'false' and conservatively populates the list with all possible effects
+/// associated with no particular value or symbol.
+static bool
+collectEffects(Operation *op,
+ SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
+ bool ignoreBarriers = true) {
+ // Skip over barriers to avoid infinite recursion (those barriers would ask
+ // this barrier again).
+ if (ignoreBarriers && isa<BarrierOp>(op))
+ return true;
+
+ // Skip over ops that we know have no effects.
+ if (isKnownNoEffectsOpWithoutInterface(op))
+ return true;
+
+ // Collect effect instances the operation. Note that the implementation of
+ // getEffects erases all effect instances that have the type other than the
+ // template parameter so we collect them first in a local buffer and then
+ // copy.
+ if (auto iface = dyn_cast<MemoryEffectOpInterface>(op)) {
+ SmallVector<MemoryEffects::EffectInstance> localEffects;
+ iface.getEffects(localEffects);
+ llvm::append_range(effects, localEffects);
+ return true;
+ }
+ if (op->hasTrait<OpTrait::HasRecursiveMemoryEffects>()) {
+ for (auto ®ion : op->getRegions()) {
+ for (auto &block : region) {
+ for (auto &innerOp : block)
+ if (!collectEffects(&innerOp, effects, ignoreBarriers))
+ return false;
+ }
+ }
+ return true;
+ }
+
+ // We need to be conservative here in case the op doesn't have the interface
+ // and assume it can have any possible effect.
+ addAllValuelessEffects(effects);
+ return false;
+}
+
+/// Collects memory effects from operations that may be executed before `op` in
+/// a trivial structured control flow, e.g., without branches. Stops at the
+/// parallel region boundary or at the barrier operation if `stopAtBarrier` is
+/// set. Returns `true` if the memory effects added to `effects` are exact,
+/// `false` if they are a conservative over-approximation. The latter means that
+/// `effects` contain instances not associated with a specific value.
+static bool
+getEffectsBefore(Operation *op,
+ SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
+ bool stopAtBarrier) {
+ if (!op->getBlock())
+ return true;
+
+ // If there is a non-structured control flow, bail.
+ Region *region = op->getBlock()->getParent();
+ if (region && !llvm::hasSingleElement(region->getBlocks())) {
+ addAllValuelessEffects(effects);
+ return false;
+ }
+
+ // Collect all effects before the op.
+ if (op != &op->getBlock()->front()) {
+ for (Operation *it = op->getPrevNode(); it != nullptr;
+ it = it->getPrevNode()) {
+ if (isa<BarrierOp>(it)) {
+ if (stopAtBarrier)
+ return true;
+ else
+ continue;
+ }
+ if (!collectEffects(it, effects))
+ return false;
+ }
+ }
+
+ // Stop if reached the parallel region boundary.
+ if (isParallelRegionBoundary(op->getParentOp()))
+ return true;
+
+ // Otherwise, keep collecting above the parent operation.
+ if (!getEffectsBefore(op->getParentOp(), effects, stopAtBarrier))
+ return false;
+
+ // If the op is loop-like, collect effects from the trailing operations until
+ // we hit a barrier because they can executed before the current operation by
+ // the previous iteration of this loop. For example, in the following loop
+ //
+ // for i = ... {
+ // op1
+ // ...
+ // barrier
+ // op2
+ // }
+ //
+ // the operation `op2` at iteration `i` is known to be executed before the
+ // operation `op1` at iteration `i+1` and the side effects must be ordered
+ // appropriately.
+ if (isSequentialLoopLike(op->getParentOp())) {
+ // Assuming loop terminators have no side effects.
+ return getEffectsBefore(op->getBlock()->getTerminator(), effects,
+ /*stopAtBarrier=*/true);
+ }
+
+ // If the parent operation is not guaranteed to execute its (single-block)
+ // region once, walk the block.
+ bool conservative = false;
+ if (!hasSingleExecutionBody(op->getParentOp()))
+ op->getParentOp()->walk([&](Operation *in) {
+ if (conservative)
+ return WalkResult::interrupt();
+ if (!collectEffects(in, effects)) {
+ conservative = true;
+ return WalkResult::interrupt();
+ }
+ return WalkResult::advance();
+ });
+
+ return !conservative;
+}
+
+/// Collects memory effects from operations that may be executed after `op` in
+/// a trivial structured control flow, e.g., without branches. Stops at the
+/// parallel region boundary or at the barrier operation if `stopAtBarrier` is
+/// set. Returns `true` if the memory effects added to `effects` are exact,
+/// `false` if they are a conservative over-approximation. The latter means that
+/// `effects` contain instances not associated with a specific value.
+static bool
+getEffectsAfter(Operation *op,
+ SmallVectorImpl<MemoryEffects::EffectInstance> &effects,
+ bool stopAtBarrier) {
+ if (!op->getBlock())
+ return true;
+
+ // If there is a non-structured control flow, bail.
+ Region *region = op->getBlock()->getParent();
+ if (region && !llvm::hasSingleElement(region->getBlocks())) {
+ addAllValuelessEffects(effects);
+ return false;
+ }
+
+ // Collect all effects after the op.
+ if (op != &op->getBlock()->back())
+ for (Operation *it = op->getNextNode(); it != nullptr;
+ it = it->getNextNode()) {
+ if (isa<BarrierOp>(it)) {
+ if (stopAtBarrier)
+ return true;
+ continue;
+ }
+ if (!collectEffects(it, effects))
+ return false;
+ }
+
+ // Stop if reached the parallel region boundary.
+ if (isParallelRegionBoundary(op->getParentOp()))
+ return true;
+
+ // Otherwise, keep collecting below the parent operation.
+ if (!getEffectsAfter(op->getParentOp(), effects, stopAtBarrier))
+ return false;
+
+ // If the op is loop-like, collect effects from the leading operations until
+ // we hit a barrier because they can executed after the current operation by
+ // the next iteration of this loop. For example, in the following loop
+ //
+ // for i = ... {
+ // op1
+ // ...
+ // barrier
+ // op2
+ // }
+ //
+ // the operation `op1` at iteration `i` is known to be executed after the
+ // operation `op2` at iteration `i-1` and the side effects must be ordered
+ // appropriately.
+ if (isSequentialLoopLike(op->getParentOp())) {
+ if (isa<BarrierOp>(op->getBlock()->front()))
+ return true;
+
+ bool exact = collectEffects(&op->getBlock()->front(), effects);
+ return getEffectsAfter(&op->getBlock()->front(), effects,
+ /*stopAtBarrier=*/true) &&
+ exact;
+ }
+
+ // If the parent operation is not guaranteed to execute its (single-block)
+ // region once, walk the block.
+ bool conservative = false;
+ if (!hasSingleExecutionBody(op->getParentOp()))
+ op->getParentOp()->walk([&](Operation *in) {
+ if (conservative)
+ return WalkResult::interrupt();
+ if (!collectEffects(in, effects)) {
+ conservative = true;
+ return WalkResult::interrupt();
+ }
+ return WalkResult::advance();
+ });
+
+ return !conservative;
+}
+
+/// Looks through known "view-like" ops to find the base memref.
+static Value getBase(Value v) {
+ while (true) {
+ Operation *definingOp = v.getDefiningOp();
+ if (!definingOp)
+ break;
+
+ bool shouldContinue =
+ TypeSwitch<Operation *, bool>(v.getDefiningOp())
+ .Case<memref::CastOp, memref::SubViewOp, memref::ViewOp>(
+ [&](auto op) {
+ v = op.getSource();
+ return true;
+ })
+ .Case<memref::TransposeOp>([&](auto op) {
+ v = op.getIn();
+ return true;
+ })
+ .Case<memref::CollapseShapeOp, memref::ExpandShapeOp>([&](auto op) {
+ v = op.getSrc();
+ return true;
+ })
+ .Default([](Operation *) { return false; });
+ if (!shouldContinue)
+ break;
+ }
+ return v;
+}
+
+/// Returns `true` if the value is defined as a function argument.
+static bool isFunctionArgument(Value v) {
+ auto arg = dyn_cast<BlockArgument>(v);
+ return arg && isa<FunctionOpInterface>(arg.getOwner()->getParentOp());
+}
+
+/// Returns the operand that the operation "propagates" through it for capture
+/// purposes. That is, if the value produced by this operation is captured, then
+/// so is the returned value.
+static Value propagatesCapture(Operation *op) {
+ return llvm::TypeSwitch<Operation *, Value>(op)
+ .Case(
+ [](ViewLikeOpInterface viewLike) { return viewLike.getViewSource(); })
+ .Case([](CastOpInterface castLike) { return castLike->getOperand(0); })
+ .Case([](memref::TransposeOp transpose) { return transpose.getIn(); })
+ .Case<memref::ExpandShapeOp, memref::CollapseShapeOp>(
+ [](auto op) { return op.getSrc(); })
+ .Default([](Operation *) { return Value(); });
+}
+
+/// Returns `true` if the given operation is known to capture the given value,
+/// `false` if it is known not to capture the given value, `nullopt` if neither
+/// is known.
+static std::optional<bool> getKnownCapturingStatus(Operation *op, Value v) {
+ return llvm::TypeSwitch<Operation *, std::optional<bool>>(op)
+ // Store-like operations don't capture the destination, but do capture
+ // the value.
+ .Case<memref::StoreOp, vector::TransferWriteOp>(
+ [&](auto op) { return op.getValue() == v; })
+ .Case<vector::StoreOp, vector::MaskedStoreOp>(
+ [&](auto op) { return op.getValueToStore() == v; })
+ // These operations are known not to capture.
+ .Case([](memref::DeallocOp) { return false; })
+ // By default, we don't know anything.
+ .Default([](Operation *) { return std::nullopt; });
+}
+
+/// Returns `true` if the value may be captured by any of its users, i.e., if
+/// the user may be storing this value into memory. This makes aliasing analysis
+/// more conservative as it cannot assume the pointer-like value is only passed
+/// around through SSA use-def.
+static bool maybeCaptured(Value v) {
+ SmallVector<Value> todo = {v};
+ while (!todo.empty()) {
+ Value v = todo.pop_back_val();
+ for (Operation *user : v.getUsers()) {
+ // A user that is known to only read cannot capture.
+ auto iface = dyn_cast<MemoryEffectOpInterface>(user);
+ if (iface) {
+ SmallVector<MemoryEffects::EffectInstance> effects;
+ iface.getEffects(effects);
+ if (llvm::all_of(effects,
+ [](const MemoryEffects::EffectInstance &effect) {
+ return isa<MemoryEffects::Read>(effect.getEffect());
+ })) {
+ continue;
+ }
+ }
+
+ // When an operation is known to create an alias, consider if the
+ // source is captured as well.
+ if (Value v = propagatesCapture(user)) {
+ todo.push_back(v);
+ continue;
+ }
+
+ std::optional<bool> knownCaptureStatus = getKnownCapturingStatus(user, v);
+ if (!knownCaptureStatus || *knownCaptureStatus)
+ return true;
+ }
+ }
+
+ return false;
+}
+
+/// Returns true if two values may be referencing aliasing memory. This is a
+/// rather naive and conservative analysis. Values defined by different
+/// allocation-like operations as well as values derived from those by casts and
+/// views cannot alias each other. Similarly, values defined by allocations
+/// inside a function cannot alias function arguments. Global values cannot
+/// alias each other or local allocations. Values that are captured, i.e.
+/// themselves potentially stored in memory, are considered as aliasing with
+/// everything. This seems sufficient to achieve barrier removal in structured
+/// control flow, more complex cases would require a proper dataflow analysis.
+static bool mayAlias(Value first, Value second) {
+ DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, {
+ DBGS_ALIAS() << "checking aliasing between ";
+ DBGS_ALIAS() << first << "\n";
+ DBGS_ALIAS() << " and ";
+ DBGS_ALIAS() << second << "\n";
+ });
+
+ first = getBase(first);
+ second = getBase(second);
+
+ DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, {
+ DBGS_ALIAS() << "base ";
+ DBGS_ALIAS() << first << "\n";
+ DBGS_ALIAS() << " and ";
+ DBGS_ALIAS() << second << "\n";
+ });
+
+ // Values derived from the same base memref do alias (unless we do a more
+ // advanced analysis to prove non-overlapping accesses).
+ if (first == second) {
+ DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, DBGS_ALIAS() << "-> do alias!\n");
+ return true;
+ }
+
+ // Different globals cannot alias.
+ if (auto globFirst = first.getDefiningOp<memref::GetGlobalOp>()) {
+ if (auto globSecond = second.getDefiningOp<memref::GetGlobalOp>()) {
+ return globFirst.getNameAttr() == globSecond.getNameAttr();
+ }
+ }
+
+ // Two function arguments marked as noalias do not alias.
+ auto isNoaliasFuncArgument = [](Value value) {
+ auto bbArg = dyn_cast<BlockArgument>(value);
+ if (!bbArg)
+ return false;
+ auto iface = dyn_cast<FunctionOpInterface>(bbArg.getOwner()->getParentOp());
+ if (!iface)
+ return false;
+ // TODO: we need a way to not depend on the LLVM dialect here.
+ return iface.getArgAttr(bbArg.getArgNumber(), "llvm.noalias") != nullptr;
+ };
+ if (isNoaliasFuncArgument(first) && isNoaliasFuncArgument(second))
+ return false;
+
+ bool isDistinct[] = {producesDistinctBase(first.getDefiningOp()),
+ producesDistinctBase(second.getDefiningOp())};
+ bool isGlobal[] = {first.getDefiningOp<memref::GetGlobalOp>() != nullptr,
+ second.getDefiningOp<memref::GetGlobalOp>() != nullptr};
+
+ // Non-equivalent distinct bases and globals cannot alias. At this point, we
+ // have already filtered out based on values being equal and global name being
+ // equal.
+ if ((isDistinct[0] || isGlobal[0]) && (isDistinct[1] || isGlobal[1]))
+ return false;
+
+ bool isArg[] = {isFunctionArgument(first), isFunctionArgument(second)};
+
+ // Distinct bases (allocations) cannot have been passed as an argument.
+ if ((isDistinct[0] && isArg[1]) || (isDistinct[1] && isArg[0]))
+ return false;
+
+ // Non-captured base distinct values cannot conflict with another base value.
+ if (isDistinct[0] && !maybeCaptured(first))
+ return false;
+ if (isDistinct[1] && !maybeCaptured(second))
+ return false;
+
+ // Otherwise, conservatively assume aliasing.
+ DEBUG_WITH_TYPE(DEBUG_TYPE_ALIAS, DBGS_ALIAS() << "-> may alias!\n");
+ return true;
+}
+
+/// Returns `true` if the effect may be affecting memory aliasing the value. If
+/// the effect is not associated with any value, it is assumed to affect all
+/// memory and therefore aliases with everything.
+static bool mayAlias(MemoryEffects::EffectInstance a, Value v2) {
+ if (Value v = a.getValue()) {
+ return mayAlias(v, v2);
+ }
+ return true;
+}
+
+/// Returns `true` if the two effects may be affecting aliasing memory. If
+/// an effect is not associated with any value, it is assumed to affect all
+/// memory and therefore aliases with everything. Effects on different resources
+/// cannot alias.
+static bool mayAlias(MemoryEffects::EffectInstance a,
+ MemoryEffects::EffectInstance b) {
+ if (a.getResource()->getResourceID() != b.getResource()->getResourceID())
+ return false;
+ if (Value v2 = b.getValue()) {
+ return mayAlias(a, v2);
+ } else if (Value v = a.getValue()) {
+ return mayAlias(b, v);
+ }
+ return true;
+}
+
+/// Returns `true` if any of the "before" effect instances has a conflict with
+/// any "after" instance for the purpose of barrier elimination. The effects are
+/// supposed to be limited to a barrier synchronization scope. A conflict exists
+/// if effects instances affect aliasing memory locations and at least on of
+/// then as a write. As an exception, if the non-write effect is an allocation
+/// effect, there is no conflict since we are only expected to see the
+/// allocation happening in the same thread and it cannot be accessed from
+/// another thread without capture (which we do handle in alias analysis).
+static bool
+haveConflictingEffects(ArrayRef<MemoryEffects::EffectInstance> beforeEffects,
+ ArrayRef<MemoryEffects::EffectInstance> afterEffects) {
+ for (const MemoryEffects::EffectInstance &before : beforeEffects) {
+ for (const MemoryEffects::EffectInstance &after : afterEffects) {
+ // If cannot alias, definitely no conflict.
+ if (!mayAlias(before, after))
+ continue;
+
+ // Read/read is not a conflict.
+ if (isa<MemoryEffects::Read>(before.getEffect()) &&
+ isa<MemoryEffects::Read>(after.getEffect())) {
+ continue;
+ }
+
+ // Allocate/* is not a conflict since the allocation happens within the
+ // thread context.
+ // TODO: This is not the case for */Free unless the allocation happened in
+ // the thread context, which we could also check for.
+ if (isa<MemoryEffects::Allocate>(before.getEffect()) ||
+ isa<MemoryEffects::Allocate>(after.getEffect())) {
+ continue;
+ }
+
+ // In the particular case that the before effect is a free, we only have 2
+ // possibilities:
+ // 1. either the program is well-formed and there must be an interleaved
+ // alloc that must limit the scope of effect lookback and we can
+ // safely ignore the free -> read / free -> write and free -> free
+ // conflicts.
+ // 2. either the program is ill-formed and we are in undefined behavior
+ // territory.
+ if (isa<MemoryEffects::Free>(before.getEffect()))
+ continue;
+
+ // Other kinds of effects create a conflict, e.g. read-after-write.
+ LLVM_DEBUG(
+ DBGS() << "found a conflict between (before): " << before.getValue()
+ << " read:" << isa<MemoryEffects::Read>(before.getEffect())
+ << " write:" << isa<MemoryEffects::Write>(before.getEffect())
+ << " alloc:"
+ << isa<MemoryEffects::Allocate>(before.getEffect()) << " free:"
+ << isa<MemoryEffects::Free>(before.getEffect()) << "\n");
+ LLVM_DEBUG(
+ DBGS() << "and (after): " << after.getValue()
+ << " read:" << isa<MemoryEffects::Read>(after.getEffect())
+ << " write:" << isa<MemoryEffects::Write>(after.getEffect())
+ << " alloc:" << isa<MemoryEffects::Allocate>(after.getEffect())
+ << " free:" << isa<MemoryEffects::Free>(after.getEffect())
+ << "\n");
+ return true;
+ }
+ }
+
+ return false;
+}
+
+namespace {
+/// Barrier elimination pattern. If a barrier does not enforce any conflicting
+/// pair of memory effects, including a pair that is enforced by another
+/// barrier, it is unnecessary and can be removed. Adapted from
+/// "High-Performance GPU-to-CPU Transpilation and Optimization via High-Level
+/// Parallel Constructs" by Moses, Ivanov, Domke, Endo, Doerfert, and Zinenko in
+/// PPoPP 2023 and implementation in Polygeist.
+class BarrierElimination final : public OpRewritePattern<BarrierOp> {
+public:
+ using OpRewritePattern<BarrierOp>::OpRewritePattern;
+
+ LogicalResult matchAndRewrite(BarrierOp barrier,
+ PatternRewriter &rewriter) const override {
+ LLVM_DEBUG(DBGS() << "checking the necessity of: " << barrier << " "
+ << barrier.getLoc() << "\n");
+
+ SmallVector<MemoryEffects::EffectInstance> beforeEffects;
+ getEffectsBefore(barrier, beforeEffects, /*stopAtBarrier=*/true);
+
+ SmallVector<MemoryEffects::EffectInstance> afterEffects;
+ getEffectsAfter(barrier, afterEffects, /*stopAtBarrier=*/true);
+
+ if (!haveConflictingEffects(beforeEffects, afterEffects)) {
+ LLVM_DEBUG(DBGS() << "the surrounding barriers are sufficient, removing "
+ << barrier << "\n");
+ rewriter.eraseOp(barrier);
+ return success();
+ }
+
+ LLVM_DEBUG(DBGS() << "barrier is necessary: " << barrier << " "
+ << barrier.getLoc() << "\n");
+ return failure();
+ }
+};
+
+class GpuEliminateBarriersPass
+ : public impl::GpuEliminateBarriersBase<GpuEliminateBarriersPass> {
+ void runOnOperation() override {
+ auto funcOp = getOperation();
+ RewritePatternSet patterns(&getContext());
+ mlir::populateGpuEliminateBarriersPatterns(patterns);
+ if (failed(applyPatternsAndFoldGreedily(funcOp, std::move(patterns)))) {
+ return signalPassFailure();
+ }
+ }
+};
+
+} // namespace
+
+void mlir::populateGpuEliminateBarriersPatterns(RewritePatternSet &patterns) {
+ patterns.insert<BarrierElimination>(patterns.getContext());
+}
+
+std::unique_ptr<OperationPass<func::FuncOp>>
+mlir::createGpuEliminateBarriersPass() {
+ return std::make_unique<GpuEliminateBarriersPass>();
+}
diff --git a/mlir/test/Dialect/GPU/barrier-elimination.mlir b/mlir/test/Dialect/GPU/barrier-elimination.mlir
index 03f162a92390bb0..844dc7dd6ac00da 100644
--- a/mlir/test/Dialect/GPU/barrier-elimination.mlir
+++ b/mlir/test/Dialect/GPU/barrier-elimination.mlir
@@ -1,4 +1,5 @@
// RUN: mlir-opt %s --transform-interpreter | FileCheck %s
+// RUN: mlir-opt %s --gpu-eliminate-barriers | FileCheck %s
module attributes {transform.with_named_sequence} {
transform.named_sequence @__transform_main(%arg0: !transform.any_op {transform.readonly}) {
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