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December 3rd, 2006 - undocumented features

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December 3rd, 2006


08:37 pm - GCC and Intel Core
Thanks to H.J. Lu at Intel, I finally got an answer to something that's been bugging me for a while now. What CFLAGS do you use with an Intel Core or Core 2 processor? -march=pentium-m, or -march=prescott/nocona?

On Fri, Dec 01, 2006 at 06:43:46AM -0800, H. J. Lu wrote:
> On Fri, Dec 01, 2006 at 03:36:59AM -0600, Ryan Hill wrote:
> > So anyways, should -m{arch,tune}=native be setting pentium-m for Core
> > CPU's, or is prescott really the better choice in the end?
>
> It should be -march=prescott -mtune=generic. I will look into it.

I opened

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30040


H.J.


So, final answer. If you're using GCC 4.1, use -march=prescott for Intel Core Solo/Duo and -march=nocona (and an amd64 profile) for Core 2 Solo/Duo. For GCC 4.2, a Core Solo/Duo should use -march=prescott -mtune=generic, and Core 2 Solo/Duo should be set to -march=nocona -mtune=generic. GCC trunk adds -march=core2 and support for the SSSE3 instruction set, but that won't be out for quite a while yet.

If you do happen to be using GCC 4.2, check out the very cool -march=native, which will autodetect the host processor(s) and set -march and -mtune accordingly. For Core CPU's you'll also need the patch from GCC PR #30040.

dirtyepic@tycho ~ $ cat /proc/cpuinfo | grep name
model name : Genuine Intel(R) CPU T2300 @ 1.66GHz
model name : Genuine Intel(R) CPU T2300 @ 1.66GHz
dirtyepic@tycho ~ $ gcc -v -c test.c -o test -O2 -march=native
[...]
gcc version 4.2.0-pre20061203 (prerelease) (rev. 119470)
/usr/libexec/gcc/i686-pc-linux-gnu/4.2.0-pre20061203/cc1 -quiet -v test.c -march=prescott -mtune=generic -quiet -dumpbase test.c -auxbase-strip test -O2 -version -o /tmp/cclzri4i.s


You can find the backported patch here:
http://dev.gentoo.org/~dirtyepic/overlay/sys-devel/gcc/files/4.2.0/gcc-4.2-pr30040-mtune-native-core-duo.patch
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