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
