1 //===- MemoryPromotion.cpp - Utilities for moving data across GPU memories ===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 //
9 // This file implements utilities that allow one to create IR moving the data
10 // across different levels of the GPU memory hierarchy.
11 //
12 //===----------------------------------------------------------------------===//
13 
14 #include "mlir/Dialect/GPU/MemoryPromotion.h"
15 #include "mlir/Dialect/GPU/GPUDialect.h"
16 #include "mlir/Dialect/SCF/EDSC/Builders.h"
17 #include "mlir/Dialect/StandardOps/EDSC/Intrinsics.h"
18 #include "mlir/Pass/Pass.h"
19 #include "mlir/Transforms/LoopUtils.h"
20 
21 using namespace mlir;
22 using namespace mlir::edsc;
23 using namespace mlir::edsc::intrinsics;
24 using namespace mlir::gpu;
25 
26 /// Returns the textual name of a GPU dimension.
27 static StringRef getDimName(unsigned dim) {
28   if (dim == 0)
29     return "x";
30   if (dim == 1)
31     return "y";
32   if (dim == 2)
33     return "z";
34 
35   llvm_unreachable("dimension ID overflow");
36 }
37 
38 /// Emits the (imperfect) loop nest performing the copy between "from" and "to"
39 /// values using the bounds derived from the "from" value. Emits at least
40 /// GPUDialect::getNumWorkgroupDimensions() loops, completing the nest with
41 /// single-iteration loops. Maps the innermost loops to thread dimensions, in
42 /// reverse order to enable access coalescing in the innermost loop.
43 static void insertCopyLoops(OpBuilder &builder, Location loc,
44                             MemRefBoundsCapture &bounds, Value from, Value to) {
45   // Create EDSC handles for bounds.
46   unsigned rank = bounds.rank();
47   SmallVector<Value, 4> lbs, ubs, steps;
48 
49   // Make sure we have enough loops to use all thread dimensions, these trivial
50   // loops should be outermost and therefore inserted first.
51   if (rank < GPUDialect::getNumWorkgroupDimensions()) {
52     unsigned extraLoops = GPUDialect::getNumWorkgroupDimensions() - rank;
53     Value zero = std_constant_index(0);
54     Value one = std_constant_index(1);
55     lbs.resize(extraLoops, zero);
56     ubs.resize(extraLoops, one);
57     steps.resize(extraLoops, one);
58   }
59 
60   // Add existing bounds.
61   lbs.append(bounds.getLbs().begin(), bounds.getLbs().end());
62   ubs.append(bounds.getUbs().begin(), bounds.getUbs().end());
63 
64   // Emit constant operations for steps.
65   steps.reserve(lbs.size());
66   llvm::transform(bounds.getSteps(), std::back_inserter(steps),
67                   [](int64_t step) { return std_constant_index(step); });
68 
69   // Obtain thread identifiers and block sizes, necessary to map to them.
70   auto indexType = builder.getIndexType();
71   SmallVector<Value, 3> threadIds, blockDims;
72   for (unsigned i = 0; i < 3; ++i) {
73     auto dimName = builder.getStringAttr(getDimName(i));
74     threadIds.push_back(
75         builder.create<gpu::ThreadIdOp>(loc, indexType, dimName));
76     blockDims.push_back(
77         builder.create<gpu::BlockDimOp>(loc, indexType, dimName));
78   }
79 
80   // Produce the loop nest with copies.
81   SmallVector<Value, 8> ivs(lbs.size());
82   loopNestBuilder(lbs, ubs, steps, [&](ValueRange loopIvs) {
83     ivs.assign(loopIvs.begin(), loopIvs.end());
84     auto activeIvs = llvm::makeArrayRef(ivs).take_back(rank);
85     StdIndexedValue fromHandle(from), toHandle(to);
86     toHandle(activeIvs) = fromHandle(activeIvs);
87   });
88 
89   // Map the innermost loops to threads in reverse order.
90   for (auto en :
91        llvm::enumerate(llvm::reverse(llvm::makeArrayRef(ivs).take_back(
92            GPUDialect::getNumWorkgroupDimensions())))) {
93     Value v = en.value();
94     auto loop = cast<scf::ForOp>(v.getParentRegion()->getParentOp());
95     mapLoopToProcessorIds(loop, {threadIds[en.index()]},
96                           {blockDims[en.index()]});
97   }
98 }
99 
100 /// Emits the loop nests performing the copy to the designated location in the
101 /// beginning of the region, and from the designated location immediately before
102 /// the terminator of the first block of the region. The region is expected to
103 /// have one block. This boils down to the following structure
104 ///
105 ///   ^bb(...):
106 ///     <loop-bound-computation>
107 ///     for %arg0 = ... to ... step ... {
108 ///       ...
109 ///         for %argN = <thread-id-x> to ... step <block-dim-x> {
110 ///           %0 = load %from[%arg0, ..., %argN]
111 ///           store %0, %to[%arg0, ..., %argN]
112 ///         }
113 ///       ...
114 ///     }
115 ///     gpu.barrier
116 ///     <... original body ...>
117 ///     gpu.barrier
118 ///     for %arg0 = ... to ... step ... {
119 ///       ...
120 ///         for %argN = <thread-id-x> to ... step <block-dim-x> {
121 ///           %1 = load %to[%arg0, ..., %argN]
122 ///           store %1, %from[%arg0, ..., %argN]
123 ///         }
124 ///       ...
125 ///     }
126 ///
127 /// Inserts the barriers unconditionally since different threads may be copying
128 /// values and reading them. An analysis would be required to eliminate barriers
129 /// in case where value is only used by the thread that copies it. Both copies
130 /// are inserted unconditionally, an analysis would be required to only copy
131 /// live-in and live-out values when necessary. This copies the entire memref
132 /// pointed to by "from". In case a smaller block would be sufficient, the
133 /// caller can create a subview of the memref and promote it instead.
134 static void insertCopies(Region &region, Location loc, Value from, Value to) {
135   auto fromType = from.getType().cast<MemRefType>();
136   auto toType = to.getType().cast<MemRefType>();
137   (void)fromType;
138   (void)toType;
139   assert(fromType.getShape() == toType.getShape());
140   assert(fromType.getRank() != 0);
141   assert(llvm::hasSingleElement(region) &&
142          "unstructured control flow not supported");
143 
144   OpBuilder builder(region.getContext());
145   builder.setInsertionPointToStart(&region.front());
146 
147   ScopedContext edscContext(builder, loc);
148   MemRefBoundsCapture fromBoundsCapture(from);
149   insertCopyLoops(builder, loc, fromBoundsCapture, from, to);
150   builder.create<gpu::BarrierOp>(loc);
151 
152   builder.setInsertionPoint(&region.front().back());
153   builder.create<gpu::BarrierOp>(loc);
154   insertCopyLoops(builder, loc, fromBoundsCapture, to, from);
155 }
156 
157 /// Promotes a function argument to workgroup memory in the given function. The
158 /// copies will be inserted in the beginning and in the end of the function.
159 void mlir::promoteToWorkgroupMemory(GPUFuncOp op, unsigned arg) {
160   Value value = op.getArgument(arg);
161   auto type = value.getType().dyn_cast<MemRefType>();
162   assert(type && type.hasStaticShape() && "can only promote memrefs");
163 
164   // Get the type of the buffer in the workgroup memory.
165   int workgroupMemoryAddressSpace = gpu::GPUDialect::getWorkgroupAddressSpace();
166   auto bufferType = MemRefType::get(type.getShape(), type.getElementType(), {},
167                                     workgroupMemoryAddressSpace);
168 
169   Value attribution = op.addWorkgroupAttribution(bufferType);
170 
171   // Replace the uses first since only the original uses are currently present.
172   // Then insert the copies.
173   value.replaceAllUsesWith(attribution);
174   insertCopies(op.getBody(), op.getLoc(), value, attribution);
175 }
176