Re: __builtin_* expansion to outcalls vs name space issues

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

On 02/05/2013 08:41 PM, H.J. Lu wrote:
> On Tue, Feb 5, 2013 at 12:35 PM, Roland McGrath <roland@hack.frob.com> wrote:
>> I happened to be looking at disassembly of elf/dl-hwcaps.o and noticed
>> that it had calls to mempcpy.  That set off alarm bells since this is
>> deep internals code that should never be using symbols outside the C89
>> implementation name space, which mempcpy is.  Looking deeper, I found
>> that all that source code is in fact properly calling __mempcpy as it
>> should.
>>
>> The issue arises because <bits/string2.h> defines __mempcpy as a macro
>> using __builtin_mempcpy.  Then GCC decides not to inline these cases,
>> and generates calls to mempcpy.  (Looking at the compiler, it seems like
>> it might well treat mempcpy as __builtin_mempcpy too in -std=gnu99 mode.
>> But it would never treat __mempcpy that way AFAIK.)
>>
>> What should be the expectation of responsibility here?  On the one hand,
>> it seems vaguely reasonable that __builtin_foobar degenerates to calling
>> foobar.  On the other hand, it is surprising to me that the compiler
>> should ever generate an implicit call to a symbol outside the most
>> constrained implementation name space.  If my source code uses only
>> symbols in a limited name space, then the compiler should not expand the
>> scope of name space my code interacts with.
>>
>> So, are we wrong for using __builtin_mempcpy in a place where a call to
>> mempcpy is not kosher?  Or is GCC wrong for having __builtin_* ever
>> generate a call to something outside the C89 implementation name space?
>>
>> In either case, what do we do to fix it?  In the former, we'd have to
>> give up some compiler-managed optimization opportunities and enforce a
>> nonobvious discipline about referring to __builtin_*.  In the latter, we
>> need to teach GCC to call __mempcpy instead, but is that going to cause
>> any problems?
>>
> 
> If you compile glibc with -Os, there will be more unexpected entries
> in elf/check-localplt.out.

Please file bugs for these, we want to be able to build reliably at -Os.

Cheers,
Carlos.