Re: CFP (Call for Participation) Debian PPC64
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. On powerpc-linux, it takes only 2.
This makes for a better icache footprint. The powerpc64-linux
ABI also defines a TOC which also adds a little overhead. This is
the reason 32-bit executables are the default on ppc64 systems.
On Opteron, 32-bit executables have the same icache footprint benefit
compared 64-bit executables. However, on Opteron, unlike ppc64, 64-bit
executables have access to twice the number of registers that 32-bit
executables have access too. On a register starved arch like x86,
access to twice the number of regs is a huge win.
Peter
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