| 1 | /** |
| 2 | * Copyright (c) Meta Platforms, Inc. and affiliates. |
| 3 | * |
| 4 | * This source code is licensed under the MIT license found in the |
| 5 | * LICENSE file in the root directory of this source tree. |
| 6 | */ |
| 7 | |
| 8 | import {CompilerError} from '../CompilerError'; |
| 9 | import { |
| 10 | BlockId, |
| 11 | Effect, |
| 12 | GeneratedSource, |
| 13 | HIRFunction, |
| 14 | Instruction, |
| 15 | Place, |
| 16 | } from './HIR'; |
| 17 | import {markPredecessors} from './HIRBuilder'; |
| 18 | import {terminalFallthrough, terminalHasFallthrough} from './visitors'; |
| 19 | |
| 20 | /* |
| 21 | * Merges sequences of blocks that will always execute consecutively — |
| 22 | * ie where the predecessor always transfers control to the successor |
| 23 | * (ie ends in a goto) and where the predecessor is the only predecessor |
| 24 | * for that successor (ie, there is no other way to reach the successor). |
| 25 | * |
| 26 | * Note that this pass leaves value/loop blocks alone because they cannot |
| 27 | * be merged without breaking the structure of the high-level terminals |
| 28 | * that reference them. |
| 29 | */ |
| 30 | export function mergeConsecutiveBlocks(fn: HIRFunction): void { |
| 31 | const merged = new MergedBlocks(); |
| 32 | const fallthroughBlocks = new Set<BlockId>(); |
| 33 | for (const [, block] of fn.body.blocks) { |
| 34 | const fallthrough = terminalFallthrough(block.terminal); |
| 35 | if (fallthrough !== null) { |
| 36 | fallthroughBlocks.add(fallthrough); |
| 37 | } |
| 38 | |
| 39 | for (const instr of block.instructions) { |
| 40 | if ( |
| 41 | instr.value.kind === 'FunctionExpression' || |
| 42 | instr.value.kind === 'ObjectMethod' |
| 43 | ) { |
| 44 | mergeConsecutiveBlocks(instr.value.loweredFunc.func); |
| 45 | } |
| 46 | } |
| 47 | |
| 48 | if ( |
| 49 | // Can only merge blocks with a single predecessor |
| 50 | block.preds.size !== 1 || |
| 51 | // Value blocks cannot merge |
| 52 | block.kind !== 'block' || |
| 53 | // Merging across fallthroughs could move the predecessor out of its block scope |
| 54 | fallthroughBlocks.has(block.id) |
| 55 | ) { |
| 56 | continue; |
| 57 | } |
| 58 | const originalPredecessorId = Array.from(block.preds)[0]!; |
| 59 | const predecessorId = merged.get(originalPredecessorId); |
| 60 | const predecessor = fn.body.blocks.get(predecessorId); |
| 61 | CompilerError.invariant(predecessor !== undefined, { |
| 62 | reason: `Expected predecessor ${predecessorId} to exist`, |
| 63 | loc: GeneratedSource, |
| 64 | }); |
| 65 | if (predecessor.terminal.kind !== 'goto' || predecessor.kind !== 'block') { |
| 66 | /* |
| 67 | * The predecessor is not guaranteed to transfer control to this block, |
| 68 | * they aren't consecutive. |
| 69 | */ |
| 70 | continue; |
| 71 | } |
| 72 | |
| 73 | // Replace phis in the merged block with canonical assignments to the single operand value |
| 74 | for (const phi of block.phis) { |
| 75 | CompilerError.invariant(phi.operands.size === 1, { |
| 76 | reason: `Found a block with a single predecessor but where a phi has multiple (${phi.operands.size}) operands`, |
| 77 | loc: GeneratedSource, |
| 78 | }); |
| 79 | const operand = Array.from(phi.operands.values())[0]!; |
| 80 | const lvalue: Place = { |
| 81 | kind: 'Identifier', |
| 82 | identifier: phi.place.identifier, |
| 83 | effect: Effect.ConditionallyMutate, |
| 84 | reactive: false, |
| 85 | loc: GeneratedSource, |
| 86 | }; |
| 87 | const instr: Instruction = { |
| 88 | id: predecessor.terminal.id, |
| 89 | lvalue: {...lvalue}, |
| 90 | value: { |
| 91 | kind: 'LoadLocal', |
| 92 | place: {...operand}, |
| 93 | loc: GeneratedSource, |
| 94 | }, |
| 95 | effects: [{kind: 'Alias', from: {...operand}, into: {...lvalue}}], |
| 96 | loc: GeneratedSource, |
| 97 | }; |
| 98 | predecessor.instructions.push(instr); |
| 99 | } |
| 100 | |
| 101 | predecessor.instructions.push(...block.instructions); |
| 102 | predecessor.terminal = block.terminal; |
| 103 | merged.merge(block.id, predecessorId); |
| 104 | fn.body.blocks.delete(block.id); |
| 105 | } |
| 106 | for (const [, block] of fn.body.blocks) { |
| 107 | for (const phi of block.phis) { |
| 108 | for (const [predecessorId, operand] of phi.operands) { |
| 109 | const mapped = merged.get(predecessorId); |
| 110 | if (mapped !== predecessorId) { |
| 111 | phi.operands.delete(predecessorId); |
| 112 | phi.operands.set(mapped, operand); |
| 113 | } |
| 114 | } |
| 115 | } |
| 116 | } |
| 117 | markPredecessors(fn.body); |
| 118 | for (const [, {terminal}] of fn.body.blocks) { |
| 119 | if (terminalHasFallthrough(terminal)) { |
| 120 | terminal.fallthrough = merged.get(terminal.fallthrough); |
| 121 | } |
| 122 | } |
| 123 | } |
| 124 | |
| 125 | class MergedBlocks { |
| 126 | #map: Map<BlockId, BlockId> = new Map(); |
| 127 | |
| 128 | // Record that @param block was merged into @param into. |
| 129 | merge(block: BlockId, into: BlockId): void { |
| 130 | const target = this.get(into); |
| 131 | this.#map.set(block, target); |
| 132 | } |
| 133 | |
| 134 | /* |
| 135 | * Get the id of the block that @param block has been merged into. |
| 136 | * This is transitive, in the case that eg @param block was merged |
| 137 | * into a block which later merged into another block. |
| 138 | */ |
| 139 | get(block: BlockId): BlockId { |
| 140 | let current = block; |
| 141 | while (this.#map.has(current)) { |
| 142 | current = this.#map.get(current) ?? current; |
| 143 | } |
| 144 | return current; |
| 145 | } |
| 146 | } |