Re: CFP (Call for Participation) Debian PPC64
On Tue, 2003-11-11 at 19:16, Albert Cahalan wrote:
> On Tue, 2003-11-11 at 15:37, Peter Bergner wrote:
> > Kevin B.Hendricks wrote:
> > > One question, why do 32 bit apps run faster than 64 bit apps, is it all
> > > of the memory accesses needed to load 64 bit immediate and other data
> > > when restricted to 32 bit instruction lengths?
> >
> > Yes, on powerpc64-linux, it takes 5 instructions to load a 64-bit
> > immediate and/or address.
>
> That's in the fully general case, assuming you
> don't do it with ONE instruction via a load
> involving the GOT or some such.
>
> Typically sign-extension or zero-extension will
> cut that down.
BTW, it's no big loss to require all .text symbols
to fall in the low (or high) 4 GB. The same goes
for the other symbols almost as well; that would
only pose a problem for FORTRAN with massive arrays.
Note that I'm not saying to restrict malloc() and
mmap() at all. It's just the things compiled into
the executable that would be restricted.
Mapped below 4 GB: code, globals, static stuff
Mapped anywhere: stack, malloc(), mmap()
I recall this being the normal 64-bit model on
the Opteron.
So that gets you back to two-opcode immediate
generation for all addresses. (the ones returned
by mmap and malloc have to be loaded already)
Hmmm. This isn't already the ABI? What a shame.
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