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-
>>
>
> 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").
