On 11/14/14 8:56 PM, IgorStepanov wrote:
On Friday, 14 November 2014 at 23:49:00 UTC, ketmar via Digitalmars-d
wrote:
On Fri, 14 Nov 2014 23:23:17 +0000
IgorStepanov via Digitalmars-d <[email protected]> wrote:
What does the NO_INTERIOR flag?
it stops GC to acknowledge pointers inside allocated area as anchors.
i.e. if there is no pointer to the head (first address) of allocated
memory, it is assumed to be garbage.
this way we have much less "false pointers", and GC not doing
pointer->block conversions.
for buckets we certainly has "head pointer" and can't have pointers to
bucket elements without "head pointer". so it's safe to tell GC that it
shouldn't do unnecessary work.
In other words, if buckets array will contain only uint-s there is no
reason to mark buckets with NO_INTERIOR?
In case ketmar's reply doesn't drive it home...
NO_INTERIOR means that the GC should NOT consider pointers to a block as
valid references if those pointers don't point to the HEAD of the block.
In other words, they point to the interior.
An example
int * x = new int;
*x = 5;
byte *b = cast(byte *)x;
b++; // b is now an interior pointer, yet x is not.
If we had marked x's block as NO_INTERIOR, we are fine as long as x
remains. If we set x to null, there is a danger that the block is
collected, and now b is dangling.
Now, this isn't a whole lot more efficient, you still have to look up
what b points at and see that it has the flag set. BUT, where it DOES
help is if you had some size_t somewhere on a stack, that happens to be
the same value as b, the GC may think it's a pointer, but correctly not
keep the block alive just for that "false pointer". The larger the
block, the more helpful NO_INTERIOR is.
-Steve
