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/LoopOps/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(ivs, lbs, ubs, steps)([&]() {
83     auto activeIvs = llvm::makeArrayRef(ivs).take_back(rank);
84     StdIndexedValue fromHandle(from), toHandle(to);
85     toHandle(activeIvs) = fromHandle(activeIvs);
86   });
87 
88   // Map the innermost loops to threads in reverse order.
89   for (auto en :
90        llvm::enumerate(llvm::reverse(llvm::makeArrayRef(ivs).take_back(
91            GPUDialect::getNumWorkgroupDimensions())))) {
92     Value v = en.value();
93     auto loop = cast<loop::ForOp>(v.getParentRegion()->getParentOp());
94     mapLoopToProcessorIds(loop, {threadIds[en.index()]},
95                           {blockDims[en.index()]});
96   }
97 }
98 
99 /// Emits the loop nests performing the copy to the designated location in the
100 /// beginning of the region, and from the designated location immediately before
101 /// the terminator of the first block of the region. The region is expected to
102 /// have one block. This boils down to the following structure
103 ///
104 ///   ^bb(...):
105 ///     <loop-bound-computation>
106 ///     for %arg0 = ... to ... step ... {
107 ///       ...
108 ///         for %argN = <thread-id-x> to ... step <block-dim-x> {
109 ///           %0 = load %from[%arg0, ..., %argN]
110 ///           store %0, %to[%arg0, ..., %argN]
111 ///         }
112 ///       ...
113 ///     }
114 ///     gpu.barrier
115 ///     <... original body ...>
116 ///     gpu.barrier
117 ///     for %arg0 = ... to ... step ... {
118 ///       ...
119 ///         for %argN = <thread-id-x> to ... step <block-dim-x> {
120 ///           %1 = load %to[%arg0, ..., %argN]
121 ///           store %1, %from[%arg0, ..., %argN]
122 ///         }
123 ///       ...
124 ///     }
125 ///
126 /// Inserts the barriers unconditionally since different threads may be copying
127 /// values and reading them. An analysis would be required to eliminate barriers
128 /// in case where value is only used by the thread that copies it. Both copies
129 /// are inserted unconditionally, an analysis would be required to only copy
130 /// live-in and live-out values when necessary. This copies the entire memref
131 /// pointed to by "from". In case a smaller block would be sufficient, the
132 /// caller can create a subview of the memref and promote it instead.
133 static void insertCopies(Region &region, Location loc, Value from, Value to) {
134   auto fromType = from.getType().cast<MemRefType>();
135   auto toType = to.getType().cast<MemRefType>();
136   (void)fromType;
137   (void)toType;
138   assert(fromType.getShape() == toType.getShape());
139   assert(fromType.getRank() != 0);
140   assert(llvm::hasSingleElement(region) &&
141          "unstructured control flow not supported");
142 
143   OpBuilder builder(region.getContext());
144   builder.setInsertionPointToStart(&region.front());
145 
146   ScopedContext edscContext(builder, loc);
147   MemRefBoundsCapture fromBoundsCapture(from);
148   insertCopyLoops(builder, loc, fromBoundsCapture, from, to);
149   builder.create<gpu::BarrierOp>(loc);
150 
151   builder.setInsertionPoint(&region.front().back());
152   builder.create<gpu::BarrierOp>(loc);
153   insertCopyLoops(builder, loc, fromBoundsCapture, to, from);
154 }
155 
156 /// Promotes a function argument to workgroup memory in the given function. The
157 /// copies will be inserted in the beginning and in the end of the function.
158 void mlir::promoteToWorkgroupMemory(GPUFuncOp op, unsigned arg) {
159   Value value = op.getArgument(arg);
160   auto type = value.getType().dyn_cast<MemRefType>();
161   assert(type && type.hasStaticShape() && "can only promote memrefs");
162 
163   Value attribution =
164       op.addWorkgroupAttribution(type.getShape(), type.getElementType());
165 
166   // Replace the uses first since only the original uses are currently present.
167   // Then insert the copies.
168   value.replaceAllUsesWith(attribution);
169   insertCopies(op.getBody(), op.getLoc(), value, attribution);
170 }
171