Performance Issues and processor arquitecture!

Jonathan Wakely jwakely.gcc@gmail.com
Fri Jan 14 08:56:00 GMT 2011


On 14 January 2011 04:06, Marcelo Pugliesi wrote:
> Hello everybody, I will try to explain as best as possible to make my question!
>
> Well, I'm compiling the kernel 2.6.34 or 2.6.36 for Ubuntu 10.10 64bits. I have
> a core i7 860 processor, accessing the menuconfig, right at processor family, if
> I check the family Core2/Xeon I have less performance from my kernel than if I
> use the option Pentium 4! No matter if I use the GCC version 4.4.4 or 4.5.1!
> My question then is simple, Why I have a superior performance when using Pentium
> 4 architecture if the GCC documentation tells me I should use the Core2/Xeon for
> a better result?
>
> I found this topic : [GCC] Compiling for a generic x86 architecture
> (http://cboard.cprogramming.com/c-programming/127502-%5Bgcc%5D-compiling-generic-x86-architecture-2.html)
>
>
> Where cyberfish said:
>
> "mtune=... does NOT affect the instruction sets used, or machines the executable
> is run on.
>
> For that (eg, enabling SSE), you'll need march=....
>
> If you do march=core2 for example (on a new GCC), it will use all the
> instruction sets available to Core 2 CPUs. march=x also sets mtune=x. The
> executable won't run on older CPUs.
>
> If you ONLY use mtune=core2, it will generate code that runs the best on a Core
> 2, but will still only use instructions available to all x86 CPUs (eg, no SSE),
> hence it will still run on old CPUs, just a little slower.
>
> As a real world example, I think a few years ago some Linux distribution decides
> to use -march=pentium3 -mtune=pentium4, or something like that. That means, the
> code is guaranteed to run on a P3, but optimized for a P4, since they predict
> most people will be running for a P4.
>
> If you don't use any flag, GCC will assume -march=i386 (lowest x86).
>
> If you want GCC to use all instruction sets on your CPU, and optimize for your
> CPU (because, for example, the code will only be run on your machine), you can
> do -march=native (which also sets mtune=native). Only available in newer GCC (it
> was introduced in 4.3 or 4.4 I THINK).
>
> -m32 and -m64 are only for generating 32-bit code on a 64-bit machine, or
> generating 64-bit code on a 32-bit machine, respectively. GCC defaults to 32-bit
> on 32-bit, and 64-bit on 64-bit."
>
> So, the menuconfig processor family just change de mtune flag, and doesn't
> affect the march flag?
>
> Can someone please give me an answer about this, please?

Someone already did
http://gcc.gnu.org/ml/gcc-help/2011-01/msg00125.html



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