Greg Schafer wrote: > But the worst part IMHO has already been pinpointed by Bruce in that it > will encourage novice users to play with `--with-arch=my-uber-cool-cpu'. > This isn't bad in itself but it can lead to problems. For example, it has > been well known for years that you cannot compile Glibc with a GCC that > was configured as `--with-arch=i686' (unless you patch Glibc). It bombs > out due to conflicts in GCC preprocessor macros with Glibc assembler code. > This is arguably a bug in Glibc, but the fact that Glibc devs refuse to > fix it indicates rather strongly that CFLAGS is the correct way to build > Glibc. It also proves that CC="gcc -march=i?86" is wrong for Glibc. To > clarify, if you give CFLAGS to Glibc configure, it will build .c files > with those flags but it won't use them for .S files.
After doing some tests and a great deal of Googling I've come to agree with Greg's estimation, the above paragraph being part of the clincher. Setting '--with-arch=i486' for GCC results in it using '-march=i486' for *everything*, unless another -march option is specified. As the above example shows, that concept seems contrary to the intention of Glibc developers. I did indeed test building GCC using --with=arch=i686, which would be the same as using CC="gcc -march=i686" for Glibc, and as Greg stated, it causes Glibc to bomb with assembler errors. Conversely, I tested building GCC without using --with-arch and then used 'CFLAGS=-march=i686 -mtune=native -O2 -pipe' ../glibc-2.6.1/configure ...' and built successfully. It seems to prove that it's better to let the Glibc devs decide where to use -march. --with-march=i486 works, but seems prone to breakage. I think if we want to introduce optimization into LFS, then we could do better, and more correctly, than just using --with-arch on GCC. Using CFLAGS with Glibc would allow for other cpu-types for the -march statement and the possiblie introduction of -mtune=native as Greg also suggested. So, to summarize, I propose not using --with-arch, but use the following for Glibc: CFLAGS="-march=i486 -O2 -pipe" ../glibc-2.6.1/configure ...etc. Glibc requires some sort of optimization and -O2 seems safe. -pipe should also be safe on gnu systems and should speed up compile times by using pipes instead of temporary files. Anyone have any comments or suggestions? If not, and unless Matt decides to do it first, I'll be applying the patches he created plus the above change to trunk by the end of the weekend. -- JH -- http://linuxfromscratch.org/mailman/listinfo/lfs-dev FAQ: http://www.linuxfromscratch.org/faq/ Unsubscribe: See the above information page
