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Re: Null pointer check elimination
Mike Stump wrote:
On Nov 14, 2005, at 11:36 PM, David Daney wrote:
Perhaps not in general, but one unstated premise of this whole thread
is that for some GCC targets (most Unix like operating systems) you
*can* count on a SIGSEGV when you dereference a null pointer.
Unless that null pointer points to an object that is of the wrong size
(too large), such as an array or a structure.
The java front end ignores this case. I mean what are the chances that
someone would try to access something near the end of such an object
with out first trying to access something near the beginning of it?
IIRC, in java an object can only have 2^16 fields, so if the maximum
field size is 8 bytes (a reference), That means you only have to keep
the first 2^19 bytes unmapped. For arrays, we would generally have to
examine the 'length' field before any other access, and it would be near
the beginning also.
I think that in practice the os/runtime linker will leave a piece of the
address space much larger than 2^19 unmapped.
David Daney