Author: Robert Hyatt
Date: 07:43:50 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 bY investigating the gcc project. In some cases they might have disjoint groups of people working on the back end, but at the "top" of the tree there is a single group of maintainers. And for many of the architectures, there is a single group working on all. This is not that uncommon. For example, Donald Becker (NASA) did almost all of the ethernet drivers for linux, even though different cards/drivers are drastically different. > >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. Sorry, but _many_ optimizations are machine / architecture independent. Any good compiler book will explain them, and new ideas are coming out every day. Of course there are also processor-specific tricks that get exposed daily as well, but that is a much smaller subset of optimizations than the overall ideas dealing with reducing operations done. That simply means that the overall optimizations are similar, and then on the _very_ "back end" of all this some processor-specific tricks are employed to further (hopefully, but not always) speed things up further. But as I said, using gcc on two processors does as much as possible to eliminate any _real_ processor-specific tinkering, but it also means you are comparing two machines with not very efficient executables. IE _every_ vendor-supplied compiler I have tested has _always_ been at least 10% faster than gcc, and that includes IBM, Sun, SGI, Intel, Cray, HP, Etc. However, the price of the gcc compiler stomps the price of any of those commercial compilers. :) > >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. You got it. Which is why my last point above is true. GCC is good, but _not_ when compared to commercial compilers. > >-Tom
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