From: Szabolcs Nagy <szabolcs.nagy@arm.com>
To: "gcc-patches@gcc.gnu.org" <gcc-patches@gcc.gnu.org>
Cc: Andreas Krebbel <krebbel@linux.vnet.ibm.com>,
Bernd Schmidt <bschmidt@redhat.com>,
Jakub Jelinek <jakub@redhat.com>,
Richard Henderson <rth@redhat.com>
Subject: Re: [PATCH] Fix default_binds_local_p_2 for extern protected data
Date: Mon, 19 Oct 2015 10:12:00 -0000 [thread overview]
Message-ID: <5624B7B3.6020005@arm.com> (raw)
In-Reply-To: <561E26A8.5030800@arm.com>
On 14/10/15 10:55, Szabolcs Nagy wrote:
> On 30/09/15 20:23, Andreas Krebbel wrote:
>> On 09/30/2015 06:21 PM, Szabolcs Nagy wrote:
>>> On 30/09/15 14:47, Bernd Schmidt wrote:
>>>> On 09/17/2015 11:15 AM, Szabolcs Nagy wrote:
>>>>> ping 2.
>>>>>
>>>>> this patch is needed for working visibility ("protected")
>>>>> attribute for extern data on targets using default_binds_local_p_2.
>>>>> https://gcc.gnu.org/ml/gcc-patches/2015-07/msg01871.html
>>>>
>>>> I hesitate to review this one since I don't think I understand the
>>>> issues on the various affected arches well enough. It looks like Jakub
>>>> had some input on the earlier changes, maybe he could take a look? Or
>>>> maybe rth knows best. Adding Ccs.
>>>>
>>>> It would help to have examples of code generation demonstrating the
>>>> problem and how you would solve it. Input from the s390 maintainers
>>>> whether this is correct for their port would also be appreciated.
>>
>> We are having the same problem on S/390. I think the GCC change is correct for S/390 as well.
>>
>> -Andreas-
>>
>
> i think the approvals of arm and aarch64 maintainers
> are needed to apply this fix for pr target/66912.
>
> (only s390, arm and aarch64 use this predicate.)
>
i was told this needs global maintainer approval.
adding jakub and rth back to cc.
>>>>
>>>
>>> consider the TU
>>>
>>> __attribute__((visibility("protected"))) int n;
>>>
>>> int f () { return n; }
>>>
>>> if n "binds_local" then gcc -O -fpic -S is like
>>>
>>> .text
>>> .align 2
>>> .global f
>>> .arch armv8-a+fp+simd
>>> .type f, %function
>>> f:
>>> adrp x0, n
>>> ldr w0, [x0, #:lo12:n]
>>> ret
>>> .size f, .-f
>>> .protected n
>>> .comm n,4,4
>>>
>>> so 'n' is a direct reference, not accessed through
>>> the GOT ('n' will be in the .bss of the dso).
>>> this is the current behavior.
>>>
>>> if i remove the protected visibility attribute
>>> then the access goes through GOT:
>>>
>>> .text
>>> .align 2
>>> .global f
>>> .arch armv8-a+fp+simd
>>> .type f, %function
>>> f:
>>> adrp x0, _GLOBAL_OFFSET_TABLE_
>>> ldr x0, [x0, #:gotpage_lo15:n]
>>> ldr w0, [x0]
>>> ret
>>> .size f, .-f
>>> .comm n,4,4
>>>
>>> protected visibility means the definition cannot
>>> be overridden by another module, but it should
>>> still allow extern references.
>>>
>>> if the main module references such an object then
>>> (as an implementation detail) it may use copy
>>> relocation against it, which places 'n' in the
>>> main module and the dynamic linker should make
>>> sure that references to 'n' point there.
>>>
>>> this is only possible if references to 'n' go
>>> through the GOT (i.e. it should not be "binds_local").
>>
>>
>>
>>
>>
>
next prev parent reply other threads:[~2015-10-19 9:28 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-07-22 17:06 Szabolcs Nagy
2015-08-10 11:04 ` Szabolcs Nagy
2015-09-17 9:32 ` Szabolcs Nagy
2015-09-30 15:02 ` Bernd Schmidt
2015-09-30 16:43 ` Szabolcs Nagy
2015-09-30 20:28 ` Andreas Krebbel
2015-10-14 9:56 ` Szabolcs Nagy
2015-10-19 10:12 ` Szabolcs Nagy [this message]
2015-10-19 18:53 ` Richard Henderson
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