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some thoughts on pure gc

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dcode 2019-03-20 22:40:07 +01:00
parent f44dbf2646
commit 88a595f4cb
1 changed files with 21 additions and 12 deletions
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33
std/assembly/collector/pure.ts
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@ -1,7 +1,7 @@
// A Pure Reference Counting Garbage Collector
//
// After the paper by DAVID F. BACON, CLEMENT R. ATTANASIO, V.T. RAJAN, STEPHEN E.SMITH
// D.Bacon, IBM T.J. Watson Research Center
// After the paper by DAVID F. BACON, CLEMENT R. ATTANASIO, V.T. RAJAN, STEPHEN E. SMITH
// D. Bacon, IBM T.J. Watson Research Center
// 2001 ACM 0164-0925/99/0100-0111 $00.75
import { HEADER, HEADER_SIZE } from "../runtime";
@ -10,8 +10,9 @@ ERROR("not implemented");
/* tslint:disable */
// TODO: new builtin
// TODO: new builtins
declare function ITERATECHILDREN(s: Header, fn: (t: Header) => void): void;
declare function ISACYCLIC(s: Header): bool;
/** Object Colorings for Cycle Collection */
const enum Color {
@ -42,16 +43,17 @@ var rootsBuffer: usize = 0;
var rootsOffset: usize = 0; // insertion offset
var rootsLength: usize = 0; // insertion limit
function appendRoot(header: Header): void {
function appendRoot(s: Header): void {
if (rootsOffset >= rootsLength) {
// grow for now
let newLength = rootsLength ? 2 * rootsLength : 256 * sizeof<usize>();
let newBuffer = memory.allocate(newLength);
memory.copy(newBuffer, rootsBuffer, rootsLength);
memory.copy(newBuffer, rootsBuffer, rootsOffset);
rootsBuffer = newBuffer;
rootsLength = newLength;
memory.free(rootsBuffer);
}
store<usize>(rootsBuffer + rootsOffset, header);
store<usize>(rootsBuffer + rootsOffset, s);
rootsOffset += sizeof<usize>();
}
@ -64,7 +66,7 @@ function systemFree(s: Header): void {
function increment(s: Header): void {
s.rc += 1;
s.color = Color.BLACK;
s.color = ISACYCLIC(s) ? Color.GREEN : Color.BLACK; // TODO: is this about correct?
}
// When a reference to a node S is deleted, the reference count is decremented. If the reference
@ -73,8 +75,15 @@ function increment(s: Header): void {
function decrement(s: Header): void {
s.rc -= 1;
if (!s.rc) release(s);
else possibleRoot(s);
if (s.color == Color.GREEN) { // if (ISACYCLIC<T>()) { ... }
if (!s.rc) systemFree(s);
// TODO: is this correct? here, if `decrement` was generic (propagate from UNLINK<T,TParent>)
// the green condition could be eliminated both here and in increment (just using black).
// acyclic types also don't need ITERATECHILDREN then as these really just inc/dec/free.
} else {
if (!s.rc) release(s);
else possibleRoot(s);
}
}
// When the reference count of a node reaches zero, the contained pointers are deleted, the object
@ -82,7 +91,7 @@ function decrement(s: Header): void {
// in the Roots buffer and will be freed later (in the procedure MarkRoots).
function release(s: Header): void {
ITERATECHILDREN(s, t => decrement(t));
ITERATECHILDREN(s, t => decrement(t)); // TODO: skip if acyclic ?
s.color = Color.BLACK;
if (!s.buffered) systemFree(s);
}
@ -208,7 +217,7 @@ function scanBlack(s: Header): void {
s.color = Color.BLACK;
ITERATECHILDREN(s, t => {
t.rc += 1;
if (t.color == Color.BLACK) scanBlack(t);
if (t.color != Color.BLACK) scanBlack(t);
});
}
@ -246,7 +255,7 @@ function __gc_collect(): void {
// TODO:
// A significant constant-factor improvement can be obtained for cycle collection by observice that
// A significant constant-factor improvement can be obtained for cycle collection by observing that
// some objects are inherently acyclic. We speculate that they will comprise the majorits of
// objects in many applications. Therefore, if we can avoid cycle collection for inherently acyclic
// object, we will significantly reduce the overhead of cycle collection as a whole. [...]