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Refactor DCE internal state into a class

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Tidying up the implementation to make the next PR's changes easier to follow.
This commit is contained in:
Joe Savona
2023-03-15 13:20:26 -07:00
parent f54f653277
commit c6d1181ba3
1 changed files with 40 additions and 27 deletions
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compiler/forget/src/Optimization/DeadCodeElimination.ts
+40 -27
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@@ -27,62 +27,78 @@ import { assertExhaustive, retainWhere } from "../Utils/utils";
* Note that unreachable blocks are already pruned during HIR construction.
*/
export function deadCodeElimination(fn: HIRFunction): void {
const used = new Set<Identifier>();
const state = new State();
// If there are no back-edges the algorithm can terminate after a single iteration
// of the blocks
const hasLoop = hasBackEdge(fn);
let size = used.size;
let size = state.count;
do {
size = used.size;
size = state.count;
// Iterate blocks in postorder (successors before predecessors, excepting loops)
// to find usages before declarations
const reversedBlocks = [...fn.body.blocks.values()].reverse();
for (const block of reversedBlocks) {
for (const operand of eachTerminalOperand(block.terminal)) {
used.add(operand.identifier);
state.reference(operand.identifier);
}
for (let i = block.instructions.length - 1; i >= 0; i--) {
const instr = block.instructions[i]!;
if (
!used.has(instr.lvalue.identifier) &&
pruneableValue(instr.value, used) &&
!state.used(instr.lvalue.identifier) &&
pruneableValue(instr.value, state) &&
// Can't prune the last value of a value block, that's its value!
!(block.kind !== "block" && i === block.instructions.length - 1)
) {
continue;
}
used.add(instr.lvalue.identifier);
visitInstruction(instr, used);
state.reference(instr.lvalue.identifier);
visitInstruction(instr, state);
}
for (const phi of block.phis) {
if (used.has(phi.id)) {
if (state.used(phi.id)) {
for (const [_pred, operand] of phi.operands) {
used.add(operand);
state.reference(operand);
}
}
}
}
} while (used.size > size && hasLoop);
} while (state.count > size && hasLoop);
for (const [, block] of fn.body.blocks) {
for (const phi of block.phis) {
if (!used.has(phi.id)) {
if (!state.used(phi.id)) {
block.phis.delete(phi);
}
}
retainWhere(block.instructions, (instr) =>
used.has(instr.lvalue.identifier)
state.used(instr.lvalue.identifier)
);
}
}
function visitInstruction(instr: Instruction, used: Set<Identifier>): void {
class State {
identifiers: Set<Identifier> = new Set();
reference(identifier: Identifier): void {
this.identifiers.add(identifier);
}
used(identifier: Identifier): boolean {
return this.identifiers.has(identifier);
}
get count(): number {
return this.identifiers.size;
}
}
function visitInstruction(instr: Instruction, state: State): void {
if (instr.value.kind === "Destructure") {
// Mark the value as used, not the lvalues
used.add(instr.value.value.identifier);
state.reference(instr.value.value.identifier);
// Remove unused lvalues
switch (instr.value.lvalue.pattern.kind) {
case "ArrayPattern": {
@@ -94,12 +110,12 @@ function visitInstruction(instr: Instruction, used: Set<Identifier>): void {
for (let i = originalItems.length - 1; i >= 0; i--) {
const item = originalItems[i];
if (item.kind === "Identifier") {
if (used.has(item.identifier)) {
if (state.used(item.identifier)) {
nextItems = originalItems.slice(0, i + 1);
break;
}
} else {
if (used.has(item.place.identifier)) {
if (state.used(item.place.identifier)) {
nextItems = originalItems.slice(0, i + 1);
break;
}
@@ -119,12 +135,12 @@ function visitInstruction(instr: Instruction, used: Set<Identifier>): void {
let nextProperties: ObjectPattern["properties"] | null = null;
for (const property of instr.value.lvalue.pattern.properties) {
if (property.kind === "ObjectProperty") {
if (used.has(property.place.identifier)) {
if (state.used(property.place.identifier)) {
nextProperties ??= [];
nextProperties.push(property);
}
} else {
if (used.has(property.place.identifier)) {
if (state.used(property.place.identifier)) {
nextProperties = null;
break;
}
@@ -146,7 +162,7 @@ function visitInstruction(instr: Instruction, used: Set<Identifier>): void {
}
} else {
for (const operand of eachInstructionValueOperand(instr.value)) {
used.add(operand.identifier);
state.reference(operand.identifier);
}
}
}
@@ -155,23 +171,20 @@ function visitInstruction(instr: Instruction, used: Set<Identifier>): void {
* Returns true if it is safe to prune an instruction with the given value.
* Functions which may have side-
*/
function pruneableValue(
value: InstructionValue,
used: Set<Identifier>
): boolean {
function pruneableValue(value: InstructionValue, state: State): boolean {
switch (value.kind) {
case "DeclareLocal": {
return !used.has(value.lvalue.place.identifier);
return !state.used(value.lvalue.place.identifier);
}
case "StoreLocal": {
// Stores are pruneable only if the identifier being stored to is never read later
return !used.has(value.lvalue.place.identifier);
return !state.used(value.lvalue.place.identifier);
}
case "Destructure": {
// Destructure is pruneable only if none of the identifiers are read from later
// TODO: as an optimization, prune unused properties where safe
for (const place of eachPatternOperand(value.lvalue.pattern)) {
if (used.has(place.identifier)) {
if (state.used(place.identifier)) {
return false;
}
}