On Thu, Dec 22, 2016 at 12:33 PM, Joerg Sonnenberger <jo...@bec.de> wrote:
> On Thu, Dec 22, 2016 at 05:47:05PM +0100, Christian Weisgerber wrote:
>> Building ld.so with clang on amd64 fails with undefined references
>> to memset and memcpy.  That is odd, since neither function appears
>> in the source.  Apparently clang optimizes the _dl_memset and
>> _dl_bcopy functions into calls to memset and memcpy, respectively.
>> I tentatively propose to add -fno-builtin to fix this.
>> (Builds, runs, and passes regress with gcc on amd64 and i386.)
> You are lucky if it fixes the problem, infact, -fno-builtin has quite a
> chance to make the problem worse. Clang assumes the same set of basic
> functions to be present even for -ffreestanding environments and can
> lower e.g. initialisation of local variables to memset/memcpy.

Yeah.  I suspect the better answer is to
a) convert _dl_bcopy() to _dl_memcpy()   (it's misnamed anyway, as it
doesn't behave like bcopy), and
b) in a header that's #included everywhere in ld.so, use the gcc asm
names extension to redirect memcpy and memset
   to _dl_memcpy and _dl_memset (and as long as you're doing that,
mark them hidden...)

I'm assuming clang handles asm names like gcc, such that declaring
   void  *memcpy(void *__restrict, const void *__restrict, __size_t)
__dso_hidden __asm("_dl_memcpy");

will make even internally generated calls go to _dl_memcpy instead.

Philip Guenther

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