Null pointer check elimination

Tue Nov 15 19:44:00 GMT 2005

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