On Friday, 26 May 2017 at 18:19:48 UTC, H. S. Teoh wrote:
On Fri, May 26, 2017 at 06:06:42PM +0000, Mike B Johnson via
Digitalmars-d-learn wrote:
On Friday, 26 May 2017 at 14:05:34 UTC, ag0aep6g wrote:
> On 05/26/2017 10:15 AM, realhet wrote:
> > But hey, the GC knows that is should not search for any
> > pointers in those large blocks. And the buffer is full of
> > 0-s at the start, so there can't be any 'false pointers'
> > in it. And I think the GC will not search in it either.
>
> The issue is not that the block contains a false pointer,
> but that there's a false pointer elsewhere that points into
> the block. The bigger the block, the more likely it is that
> something (e.g. an int on the stack) is mistaken for a
> pointer into it.
Wow, if that is the case then the GC has some real issues. The
GC should be informed about all pointers and an int is not a
pointer.
Unfortunately, it can't, because (1) D interfaces with C code,
and you don't have this kind of information from a C object
file, and (2) you can turn a pointer into an int with a cast or
a union in @system code, and since the GC cannot assume @safe
for all code, it needs to be conservative and assume any
int-like data could potentially be a pointer.
You could improve GC performance by giving it type info from
@safe code so that it skips over blocks that *definitely* have
no pointers (it already does this to some extent, e.g., data in
an int[] will never be scanned for pointers because the GC
knows it can't contain any). But you can't make the GC fully
non-conservative because it may crash the program when it
wrongly assumes a memory block is dead when it's actually still
live. All it takes is one pointer on the stack that's wrongly
assumed to be just int, and you're screwed.
And what if one isn't interfacing to C? All pointers should be
known. You can't access memory by and int or any other
non-pointer type! Hence, when pointers are created or ints are
cast to pointers, the GC should be informed and then handle them
appropriately(then, instead of scanning a 100MB block of memory
for "pointers" it should scan the list of possible pointers(which
will generally be much much lower).
Therefor, in a true D program(no outsourcing) with no pointers
used, the GC should never have to scan anything.
It seems the GC can be smarter than it is instead of just making
blanket assumptions about the entire program(which rarely hold),
which is generally always a poor choice when it comes to
performance...
In fact, When interfacing with C or other programs, memory could
be partitioned and any memory that may escape D is treated
differently than the memory used only by D code.
After all, if we truly want to be safe, why not scan the entire
memory of the system? Who knows, some pointer externally might be
peeping in on our hello world program.
