[libdispatch-dev] Question about "better assembly generation" in _dispatch_continuation_pop

[Dave Zarzycki]

Normally, any "use after free" behavior by code is a bug. But in this case, we know that doing so is safe because the object being freed cannot be reused until the callback happens on this line:

        dc->dc_func(dc->dc_ctxt);

The code is faster now because the continuations is immediately made available for reuse (as opposed to freeing it after dc_func returns). The better assembly is a side effect of this optimization. If you want to see the difference, just apply the following gratuitous "use after free" fix and compare the resulting assembly.

Index: src/queue.c
===================================================================
--- src/queue.c	(revision 55311)
+++ src/queue.c	(working copy)
@@ -358,15 +358,15 @@
 	// The ccache version is per-thread.
 	// Therefore, the object has not been reused yet.
 	// This generates better assembly.
-	if ((long)dou._do->do_vtable & DISPATCH_OBJ_ASYNC_BIT) {
-		_dispatch_continuation_free(dc);
-	}
 	if ((long)dou._do->do_vtable & DISPATCH_OBJ_GROUP_BIT) {
 		dg = dc->dc_group;
 	} else {
 		dg = NULL;
 	}
 	dc->dc_func(dc->dc_ctxt);
+	if ((long)dou._do->do_vtable & DISPATCH_OBJ_ASYNC_BIT) {
+		_dispatch_continuation_free(dc);
+	}
 	if (dg) {
 		dispatch_group_leave(dg);
 		_dispatch_release(dg);


davez



On Aug 1, 2010, at 10:33 PM, cee1 wrote:

> Hi all,
> 
> I've read "_dispatch_continuation_pop" in src/queue.c(line 338). For async  dispatch_continuation_t, it will first free, then execute it. The comment says this will generate better assembly, why?
> 
> 
> Regards,
> -- cee1
> _______________________________________________
> libdispatch-dev mailing list
> libdispatch-dev at lists.macosforge.org
> http://lists.macosforge.org/mailman/listinfo.cgi/libdispatch-dev

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