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Re: [PATCH 1/2] Only support ifunc in nptl/pt-vfork.c

On 05/23/2014 07:13 PM, Roland McGrath wrote:
>> 	* nptl/pt-vfork.c: Error if !HAVE_IFUNC.
>> 	[!HAVE_IFUNC] (vfork_compat): Remove.
> Yeah, but then you added it right back for aarch64.  I don't know what
> about aarch64 makes you certain that the compiler will always do a proper
> tail call there.  If it's something like that GCC is the only compiler for
> that machine and GCC support for the machine only exists in GCC versions
> that you're positive always do a proper tail call for this case at -O!=0
> (since we don't support compiling libc with -O0 anyway) even with explicit
> -fno-omit-frame-pointer (which people building libc are in fact free to
> do), then similar facts are likely to exist about some other machines too.

For aarch64, gcc must be >= 4.8, since the port is quite new.

The target does not have a got/toc/gp type register that must be preserved in
some way (unlike e.g. alpha, mips, ppc), so the compiler can (and does) emit a
normal tail-call to the plt entry at -O1.

I did initially write this as an asm(""), but then I thought...

> So we might as well keep the shared code rather than having people start
> copying and pasting your aarch64 version.

... someone could make an informed decision to share the code via

#include <sysdeps/unix/sysv/linux/aarch64/pt-vfork.c>

instead of having it in the nptl/pt-vfork.c where a port could get in trouble
by default.

>> +#if !HAVE_IFUNC
>> +# error
>> +#endif
> I suppose it doesn't much matter, but my preference is to always have some
> text in an #error or #warning.  The nearby comment can be longer and of
> course is what provides the real context.  But something like:
> 	# error must write pt-vfork for this machine or get IFUNC support!
> makes it plain even to someone who looks at the build log without
> consulting the source.

Fair enough.


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