Re: [ARM] PR 78253 do not resolve weak ref locally

[Christophe Lyon <christophe dot lyon at linaro dot org>]

Ping?

On 1 December 2016 at 15:27, Christophe Lyon <christophe.lyon@linaro.org> wrote:
> Hi,
>
>
> On 10 November 2016 at 15:10, Christophe Lyon
> <christophe.lyon@linaro.org> wrote:
>> On 10 November 2016 at 11:05, Richard Earnshaw
>> <Richard.Earnshaw@foss.arm.com> wrote:
>>> On 09/11/16 21:29, Christophe Lyon wrote:
>>>> Hi,
>>>>
>>>> PR 78253 shows that the handling of weak references has changed for
>>>> ARM with gcc-5.
>>>>
>>>> When r220674 was committed, default_binds_local_p_2 gained a new
>>>> parameter (weak_dominate), which, when true, implies that a reference
>>>> to a weak symbol defined locally will be resolved locally, even though
>>>> it could be overridden by a strong definition in another object file.
>>>>
>>>> With r220674, default_binds_local_p forces weak_dominate=true,
>>>> effectively changing the previous behavior.
>>>>
>>>> The attached patch introduces default_binds_local_p_4 which is a copy
>>>> of default_binds_local_p_2, but using weak_dominate=false, and updates
>>>> the ARM target to call default_binds_local_p_4 instead of
>>>> default_binds_local_p_2.
>>>>
>>>> I ran cross-tests on various arm* configurations with no regression,
>>>> and checked that the test attached to the original bugzilla now works
>>>> as expected.
>>>>
>>>> I am not sure why weak_dominate defaults to true, and I couldn't
>>>> really understand why by reading the threads related to r220674 and
>>>> following updates to default_binds_local_p_* which all deal with other
>>>> corner cases and do not discuss the weak_dominate parameter.
>>>>
>>>> Or should this patch be made more generic?
>>>>
>>>
>>> I certainly don't think it should be ARM specific.
>> That was my feeling too.
>>
>>>
>>> The questions I have are:
>>>
>>> 1) What do other targets do today.  Are they the same, or different?
>>
>> arm, aarch64, s390 use default_binds_local_p_2 since PR 65780, and
>> default_binds_local_p before that. Both have weak_dominate=true
>> i386 has its own version, calling default_binds_local_p_3 with true
>> for weak_dominate
>>
>> But the behaviour of default_binds_local_p changed with r220674 as I said above.
>> See https://gcc.gnu.org/viewcvs/gcc?view=revision&revision=220674 and
>> notice how weak_dominate was introduced
>>
>> The original bug report is about a different case:
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32219
>>
>> The original patch submission is
>> https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00410.html
>> and the 1st version with weak_dominate is in:
>> https://gcc.gnu.org/ml/gcc-patches/2015-02/msg00469.html
>> but it's not clear to me why this was introduced
>>
>>> 2) If different why?
>> on aarch64, although binds_local_p returns true, the relocations used when
>> building the function pointer is still the same (still via the GOT).
>>
>> aarch64 has different logic than arm when accessing a symbol
>> (eg aarch64_classify_symbol)
>>
>>> 3) Is the current behaviour really what was intended by the patch?  ie.
>>> Was the old behaviour actually wrong?
>>>
>> That's what I was wondering.
>> Before r220674, calling a weak function directly or via a function
>> pointer had the same effect (in other words, the function pointer
>> points to the actual implementation: the strong one if any, the weak
>> one otherwise).
>>
>> After r220674, on arm the function pointer points to the weak
>> definition, which seems wrong to me, it should leave the actual
>> resolution to the linker.
>>
>>
>
> After looking at the aarch64 port, I think that references to weak symbols
> have to be handled carefully, to make sure they cannot be resolved
> by the assembler, since the weak symbol can be overridden by a strong
> definition at link-time.
>
> Here is a new patch which does that.
> Validated on arm* targets with no regression, and I checked that the
> original testcase now executes as expected.
>
> Christophe
>
>
>>> R.
>>>> Thanks,
>>>>
>>>> Christophe
>>>>
>>>