[ARM] PR ld/20608

Christophe Lyon christophe.lyon@linaro.org
Tue Sep 27 23:57:00 GMT 2016


Sorry for the delay, I've just committed the patch as approved by Nick.

Tristan, is it OK to backport this patch to 2.27 ? 2.26 ?

On 23 September 2016 at 01:25, Alan Modra <amodra@gmail.com> wrote:
> On Thu, Sep 22, 2016 at 04:39:30PM +0100, Richard Earnshaw (lists) wrote:
>> So this patch got me wondering.  If the PLT slot is so far away that we
>> need an indirect jump to get there, why don't we clone the PLT code
>> sequence at the veneer location?  As I recall there's nothing
>> architectural about placing all the PLT slots together, it's just more
>> convenient to do that.  We could even have the clone in Thumb-2 code if
>> that's appropriate, so that it's compatible with tail calls.

If there are multiple PLT slots for the same function,
doesn't this break the uniqueness of a function pointer?

> Yeah, that's what ppc64 does.  It does have one downside in that
> synthetic plt symbols are not so easy to add.
>           /* FIXME: It would be very much nicer to put sym@plt on the
>              stub rather than on the glink branch table entry.  The
>              objdump disassembler would then use a sensible symbol
>              name on plt calls.  The difficulty in doing so is
>              a) finding the stubs, and,
>              b) matching stubs against plt entries, and,
>              c) there can be multiple stubs for a given plt entry.
>              Solving (a) could be done by code scanning, but older
>              ppc64 binaries used different stubs to current code.
>              (b) is the tricky one since you need to known the toc
>              pointer for at least one function that uses a pic stub to
>              be able to calculate the plt address referenced.
>              (c) means gdb would need to set multiple breakpoints (or
>              find the glink branch itself) when setting breakpoints
>              for pending shared library loads.  */
> We avoid the problem by enabling --emit-stub-syms by default.
> --
> Alan Modra
> Australia Development Lab, IBM

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