1 //===- LoopCoalescing.cpp - Pass transforming loop nests into single loops-===//
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 #include "PassDetail.h"
10 #include "mlir/Dialect/SCF/SCF.h"
11 #include "mlir/Transforms/LoopUtils.h"
12 #include "mlir/Transforms/Passes.h"
13 #include "mlir/Transforms/RegionUtils.h"
14 #include "llvm/Support/Debug.h"
15
16 #define PASS_NAME "loop-coalescing"
17 #define DEBUG_TYPE PASS_NAME
18
19 using namespace mlir;
20
21 namespace {
22 struct LoopCoalescingPass : public LoopCoalescingBase<LoopCoalescingPass> {
runOnFunction__anon8324cc070111::LoopCoalescingPass23 void runOnFunction() override {
24 FuncOp func = getFunction();
25
26 func.walk([](scf::ForOp op) {
27 // Ignore nested loops.
28 if (op.getParentOfType<scf::ForOp>())
29 return;
30
31 SmallVector<scf::ForOp, 4> loops;
32 getPerfectlyNestedLoops(loops, op);
33 LLVM_DEBUG(llvm::dbgs()
34 << "found a perfect nest of depth " << loops.size() << '\n');
35
36 // Look for a band of loops that can be coalesced, i.e. perfectly nested
37 // loops with bounds defined above some loop.
38 // 1. For each loop, find above which parent loop its operands are
39 // defined.
40 SmallVector<unsigned, 4> operandsDefinedAbove(loops.size());
41 for (unsigned i = 0, e = loops.size(); i < e; ++i) {
42 operandsDefinedAbove[i] = i;
43 for (unsigned j = 0; j < i; ++j) {
44 if (areValuesDefinedAbove(loops[i].getOperands(),
45 loops[j].region())) {
46 operandsDefinedAbove[i] = j;
47 break;
48 }
49 }
50 LLVM_DEBUG(llvm::dbgs()
51 << " bounds of loop " << i << " are known above depth "
52 << operandsDefinedAbove[i] << '\n');
53 }
54
55 // 2. Identify bands of loops such that the operands of all of them are
56 // defined above the first loop in the band. Traverse the nest bottom-up
57 // so that modifications don't invalidate the inner loops.
58 for (unsigned end = loops.size(); end > 0; --end) {
59 unsigned start = 0;
60 for (; start < end - 1; ++start) {
61 auto maxPos =
62 *std::max_element(std::next(operandsDefinedAbove.begin(), start),
63 std::next(operandsDefinedAbove.begin(), end));
64 if (maxPos > start)
65 continue;
66
67 assert(maxPos == start &&
68 "expected loop bounds to be known at the start of the band");
69 LLVM_DEBUG(llvm::dbgs() << " found coalesceable band from " << start
70 << " to " << end << '\n');
71
72 auto band =
73 llvm::makeMutableArrayRef(loops.data() + start, end - start);
74 coalesceLoops(band);
75 break;
76 }
77 // If a band was found and transformed, keep looking at the loops above
78 // the outermost transformed loop.
79 if (start != end - 1)
80 end = start + 1;
81 }
82 });
83 }
84 };
85
86 } // namespace
87
createLoopCoalescingPass()88 std::unique_ptr<OperationPass<FuncOp>> mlir::createLoopCoalescingPass() {
89 return std::make_unique<LoopCoalescingPass>();
90 }
91