Author: Robert Hyatt
Date: 07:36:51 06/28/03
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On June 28, 2003 at 04:44:10, Tom Kerrigan wrote: >On June 28, 2003 at 00:18:35, Robert Hyatt wrote: > >>On June 26, 2003 at 22:50:59, Eugene Nalimov wrote: >> >>>I didn't look at GCC sources, but I looked at sources of some other compilers, >>>and understand x86 and PPC architecture well enough, so I think I know that x86 >>>and PPC backends should be vastly different, and each should contain lot of >>>platform-specific and unique code. >>> >>>Thanks, >>>Eugene >> >>I wouldn't disagree. However, I'd suspect that both are written by the >>same core "group" of people. Which means they are probably pretty competitive >>with each other in terms of aggressive optimizations. That means that it is >>unlikely that one processor will get a huge jump on the other due to the >>optimizer gurus for one being far better. (all of that directed toward gcc >>only, of course). > >I've never looked at the gcc compiler, but I imagine that it has a pass where it >converts whatever its intermediate format is to native machine code and >optimizes that machine code, e.g., makes sure branch targets are on 16 byte >boundaries for the Athlon, makes sure to use multiplies instead of shifts in >certain situations on the P4, etc. These sorts of optimizations can make or >break the performance of an executable and they're hard enough to keep straight >for one x86 processor, much less every x86 processor AND some completely >different RISC processor (POWER4/PPC970) with rules that are probably just as >complicated, given its "bundling" setup. So unless you have information to the >contrary, I'd suspect that different sets of people work on generating this >final machine code. You can find this out b > >As for optimizations carrying over from one architecture to the other, I expect >this is very unlikely given how different the architectures are. If you order >your instructions on the PPC970 to be bundled just right for high performance, >the same ordering is obviously going to have no effect (or probably a >detremental effect) for Pentium 4 performance, because the P4 doesn't even do >bundling at all. > >If the same people are doing all of these optimizations for all platforms, what >is the likelyhood that, regardless of how "aggressive" they are, they will >overlook key optimizations for some platforms? Optimizations that would >obviously not be overlooked by somebody who spent all his time writing code for >one specific architecture. Very high, I'd imagine. > >-Tom
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