On Wed, 2004-04-28 at 13:38, Peter Bergner wrote: > Getting back to what Ben said, yes, PPC64 apps do tend to need more > instructions for some things. I will say that the compiler is smart > enough to use fewer than 5 instructions for loading constants when > it can. It's cases like code with RELOCS as well as the use of the > TOC where we tend to get more instructions relative to 32-bit apps > and that doesn't help your icache.
The more you wait, the more the ABI gets set in stone. BTW, it would only have taken 1/512 of the instruction space to allow for generation of arbitrary 64-bit constants in 3 instructions. (sign-extended 22-bit load immediate, xor top 21 bits immediate, and xor next-to-top 21 bits immediate) Perhaps even now this is worth adding. > Yes, all x86_64 distros are making 64-bit binaries the default. > The question is why are they doing that, not "hmmm, if they're > doing that for x64_64, that must be the correct answer for ppc64 > too". The reason 64-bit is the default for x86_64 is as Ben > mentioned above, they have access to more registers which leads > to vastly less spill code versus 32-bit apps. Sixty-four bit > apps on x86_64 have the same problem with extra code for some > RELOCS, but that is overwhelmed by the reduction in spill code. > Therefore, the correct solution for x86_64 is 64-bit apps are > the default. Following that logic leads to a different conclusion. 1. put the CPU in "long mode" (64-bit, more registers) 2. use an ILP32 programming model That gets you the best of both worlds, hmmm? It even lets you have a "fast" OpenOffice. This is not done though, for obvious reasons that apply to ppc64 as well. > There are 2 reasons why we choose 32-bit apps as the default: > 1) It was easier during the initial port of the ppc64 kernel! :-) > 2) It's the right choice ignoring 1). ... > Now that we have a 64-bit userspace, I think it's still the right choice > given the performance benefit of 32-bit apps over 64-bit apps discussed > above. People complain about having multiple glibc's laying around, but > disk is cheap and if you don't use 64-bit apps that often, it won't even > eat up your memory. However, I'll agree to disagree with anyone who > wants 64-bit apps to be the default. In that case, you can just run a 32-bit kernel and so on. If you don't want 64-bit, don't have it. If it's just a few odd apps you need, modify your 32-bit kernel to save the full 64-bit registers. This way, any 32-bit performance boost will apply to your kernel as well.
