Ömer Sinan Ağacan <omeraga...@gmail.com> writes: > Here's another improvement that fixes a very old issue in GHC's compacting GC > [1]. > > To summarize the problem: when untreading an object we update references to > the > object that we've seen so far to the object's new location. But to get the > object's new location we need to know the object's size, because depending on > the size we may need to move the object to a new block (when the current block > does not have enough space for the object). > > For this we currently walk the thread twice, once to get the info table (used > to get the size), then again to update references to the object. Ideally we > want to do just one pass when unthreading. > > The solution is explained in The GC Handbook, section 3.4. Instead of using > one > bit to mark an object, we use two bits: one for the first word of the object, > one for the last word. Using two bits is not a problem in GHC because heap > objects are at least 2 words. For example, an object with two words is marked > with `11`, 3 words is marked with `101` and so on. > > Now we can scan the bitmap to find object size, and unthread it without having > to find the info table first. > Thanks Ömer! I have plopped all of these ideas into a ticket, #20328. Hopefully someone will come along to implement.
Cheers, - Ben
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