Author: Vincent Diepeveen
Date: 14:09:01 07/25/03
Go up one level in this thread
On July 24, 2003 at 20:55:49, Tom Kerrigan wrote: crafty GCC x86 32 bits mode GCC 3.3: 1094 crafty GCC x86-64 64 bits mode GCC 3.3: 1562 64 bits and more registers is faster: 43% And we both know how bad GCC is for 16 registers when compared to other compilers. The big wins still have to come... see 32 bits: http://www.specbench.org/osg/cpu2000/results/res2003q2/cpu2000-20030421-02102.html see 64 bits: http://www.specbench.org/osg/cpu2000/results/res2003q2/cpu2000-20030421-02093.html >>>>On July 22, 2003 at 14:54:48, Rajen Gupta wrote: >>>> >>>>You are comparing x86 executables here. >>>>That's like running 8 bits XT executables at the P4. >>>> >>>>Do you understand that? >>>> >>>>If not then it is useless to say that things will change for chess software a >>>>lot when there is native good x86-64 compilers for opteron (and future intel >>>>x86-64 platforms which does *not* include that 3.06Ghz Xeon MP). > >Not really. > >First of all, the XT had an 8088, which was a 16 bit processor. > >Second, XT executables use a lot of the x86's more CISC-ish instructions that >run very poorly on modern processors, and they have data that isn't aligned >right for modern processors, and they make heavy use of segmentation because of >the tiny amount of flat-addressable memory, so of course they will run very >poorly on modern processors. > >Using that as an analogy for x86 vs. AMD64 is a mistake because with the >Opteron/Athlon 64, there is very little difference between how the processor >works in each mode. Instructions are fetched, decoded, issued, executed, and >retired in virtually the same way regardless of if it's 32 bit code or 64 bit >code. > >There's a small performance penalty when running 64 bit code on an AMD64 >processor vs. 32 bit code because in 64 bit mode, pointers are all twice as big, >which means a less efficient use of memory. Then again, there's a small >performance gain from having the extra registers that are available in 64 bit >mode. Overall, the performance gain from switching to 64 bit is reportedly >10-25%, although recently an AMD64 build of SETI was released and it runs slower >than the 32 bit version, by something like 10% if memory serves. > >So running regular x86 programs on an Opteron is in no way crippling it, because >the instruction mix is already good, the data is aligned right, there's almost >certainly no use of segmentation, and the only "bad" thing about it is that the >extra 8 registers aren't available, which don't make THAT big a difference. > >Of course, chess is a somewhat special case, because some programs use bitboards >and 64 bit math significantly speeds up bitboard operations. Of course, you >wouldn't see nearly a factor of 2 speedup because bitboard operations are still >a small percentage of instructions even for programs that make heavy use of >them. > >-Tom
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