Re: glibc compiled with mcmodel=large

["Carlos O'Donell" <carlos at systemhalted dot org>]

On Mon, Feb 20, 2012 at 9:53 AM, Zeng Bin <ezengbin@gmail.com> wrote:
> Thanks for the suggestion! Definitely, I can track down the bug with
> -E and -save-temps. The question is whether glibc-2.14 which is the
> latest release supports being compiled with -mcmodel=large? If it
> does, there might be only a few bugs in it when compiled with ubuntu
> system. If it does not (when it was designed, it was not taken into
> careful consideration because few people would use this feature), it
> might take too much efforts to fix all of them. It might save time and
> even increase the performance by going the other direction and trying
> hard to optimize the size of code so that no glibc compiled with
> mcmodel=large is needed. For example, ?in the following code snippet
> from start.S from gcc-4.6.2:
> ? ? ?/* Pass address of our own entry points to .fini and .init. ?*/
> ? ? ? ?/* movq $__libc_csu_fini, %r8 ?by bzeng */
> ? ? ? ?movabs $__libc_csu_fini, %r8
> ? ? ? ?/* movq $__libc_csu_init, %rcx by bzeng */
> ? ? ? ?movabs $__libc_csu_init, %rcx
>
> ? ? ? ?/* movq $BP_SYM (main), %rdi */
> ? ? ? ?movabs $BP_SYM (main), %rdi
>
> ? ? ? ?/* Call the user's main function, and exit with its value.
> ? ? ? ? ? But let the libc call main. ? ?*/
> ? ? ? ?call BP_SYM (__libc_start_main)
> The movq instruction needs to be replaced with movabs to support mcmodel=large.
>
> Can anyone confirm that glibc was coded to support mcmodel=large? Is
> there any configuration flag that I can use to compile only 64-bit
> libraries instead of both 32-bit and 64-bit? Definitely, 32-bit
> libraries do not need to support mcmodel=large due to the address
> space restriction.

I can confirm that in the past 10+ years of working on the project
we've never talked about mcmodel=large.

Therefore it was likely never written to support that option.

You only compile glibc once (distros and tool vendors compile it many
times for many options) for either 64-bit or 32-bit. The code doesn't
compile both at the same time.

Speaking for upstream, we'd be happy to support mcmodel=large, but
you'd have to make a case for it and present the patches to enable it.
That discussion would happen on libc-alpha@sourceware.org.

Cheers,
Carlos.