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

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

Ping?

On 3 January 2017 at 16:45, Christophe Lyon <christophe.lyon@linaro.org> wrote:
> Ping?
>
> The patch is at https://gcc.gnu.org/ml/gcc-patches/2016-12/msg00078.html
>
>
> On 14 December 2016 at 16:29, Christophe Lyon
> <christophe.lyon@linaro.org> wrote:
>> Ping^2 ?
>>
>> As a reminder, this patch mimics what aarch64 does wrt to references to weak
>> symbols such that they are not resolved by the assembler, in case a strong
>> definition overrides the local one at link time.
>>
>> Christophe
>>
>>
>> On 8 December 2016 at 09:17, Christophe Lyon <christophe.lyon@linaro.org> wrote:
>>> 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
>>>>>>>
>>>>>>