Re: hammer: MAP_32BIT

[Timothy Miller <miller@techsource.com>]

Ulrich Drepper wrote:
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> Timothy Miller wrote:
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> 
>>If your program is capable of handling an address with more than 32
>>bits, what point is there giving a hint?  Either your program can handle
>>64-bit pointers or it cannot.  Any program flexible enough to handle
>>either size dynamically would expend enough overhead checking that it
>>would be worse than if it just made a hard choice.
> 
> 
> Look at the x86-64 context switching code.  If memory addressed by the
> GDT entries has a 32-bit address it uses a different method than for
> cases where the virtual address has more than 32 bits.  This way of
> handling GDT entries is faster according to ak.  So, it's not a
> correctness thing, it's a performance thing.
> 

Alright.  Sounds great.  So my next question is this:

Why does there ever need to be an explicit HINT that you would prefer a 
<32 bit address, when it's known a priori that <32 is better?  Why 
doesn't the mapping code ALWAYS try to use 32-bit addresses before 
resorting to 64-bit?