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Re: [PATCH] x86-64: Align the stack in __tls_get_addr [BZ #21609]
On Tue, Jul 4, 2017 at 8:55 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Tue, Jul 4, 2017 at 8:47 AM, Florian Weimer <fweimer@redhat.com> wrote:
>> On 07/04/2017 05:16 PM, H.J. Lu wrote:
>>
>>>> Furthermore, the code GCC generates for stack realignment is really bad,
>>>
>>> GCC generates very good code for stack realignment when
>>> -maccumulate-outgoing-args is used.
>>
>> I wonder why that isn't enabled by the force attribute.
>
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81312
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81313
>
>>>> # define __tls_get_addr __tls_get_addr_default
>>>> +# include <elf/dl-tls.c>
>>>> +
>>>> +# undef __tls_get_addr_default
>>> ^^^^^^^ Shouldn't it be __tls_get_addr?
>>
>> Looks this way. I'd have to re-test and see if it makes a difference.
>>
>>>> -typedef struct dl_tls_index
>>>> -{
>>>> - uint64_t ti_module;
>>>> - uint64_t ti_offset;
>>>> -} tls_index;
>>
>>> Is this sysdeps/x86_64/dl-tlsdesc.h change related to this?
>>
>> It is because the patch includes both <dl-tls.h> and <dl-tlsdesc.h> from
>> the same file, causing a multiple definition error without the removal
>> of the conflicting definition.
>
> It is a cleanup. But I don't believe it is required here.
>
>>> __tls_get_addr_compat:
>>> + .type __tls_get_addr_compat,@function
>>> + .global __tls_get_addr_compat
>>> + strong_alias (__tls_get_addr_compat, __tls_get_addr)
>>>
>>> We can use ENTRY/END here. Why do we need __tls_get_addr_compat?
>>> Can we just have __tls_get_addr?
>>
>> That confuses the linker once we add the .symver directive.
>
> I don't think it is a case. We just define a different __tls_get_addr for
> x86-64.
>
>>> Since we are talking performance here, we should add __tls_get_addr_slow
>>> to only handle slow paths.
>>
>> I'd prefer something that adds a new symbol version for the non-aligning
>> implementation, so that we eventually move away from the aligning one.
>
> I thought about it. There is no easy way to do it without linker help. We
> can add ___tls_get_addr, similar to i386, which will take an aligned
> stack. Linker must support ___tls_get_addr. Then we can use weakref
> to redirect __tls_get_addr to ___tls_get_addr if linker supports
> ___tls_get_addr and GCC doesn't.
>
>>> Here is the patch which implements those. It is tested on x86-64
>>> and x32.
>>
>> Is this really needed on x32, too? Did GCC have the same bug?
>>
>
> Yes, it is needed for x32 since the bug is only fixed on I thiuGCC 4.9 branch
> and above.
>
> --
> H.J.
--
H.J.