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I'm pretty sure though that there will be a bigger difference when compiling x86 binaries as x86-64 binaries because there will be more register available to the compiler. I'm guessing the reason that the 32vs64 bit difference is so small in nitrile's case is because the SPARC was RISC already so the UltraSPARC just added 32 bits... the x86-64 IIRC adds new registers. So as interesting as this is I'd like to see the smae tests performed on Solaris x86 and x86-64 when it comes out.
DISCLAIMER: I didn't RTFA but scanned it. I've never looked at the UltraSPARC ISA but have programmed in both x86 and SPARC assembly.