Re: [AArch64] Improve TLS Descriptor pattern to release RTL loop IV opt

[Andrew Pinski <pinskia@gmail.com>]

On Fri, Sep 25, 2015 at 11:40 PM, Andrew Pinski <pinskia@gmail.com> wrote:
> On Tue, Jul 28, 2015 at 6:12 AM, Jiong Wang <jiong.wang@arm.com> wrote:
>>
>> The instruction sequences for preparing argument for TLS descriptor
>> runtime resolver and the later function call to resolver can actually be
>> hoisted out of the loop.
>>
>> Currently we can't because we have exposed the hard register X0 as
>> destination of "set".  While GCC's RTL data flow infrastructure will
>> skip or do very conservative assumption when hard register involved in
>> and thus some loop IV opportunities are missed.
>>
>> This patch add another "tlsdesc_small_pseudo_<mode>" pattern, and avoid
>> expose x0 to gcc generic code.
>>
>> Generally, we define a new register class FIXED_R0 which only contains register
>> 0, so the instruction sequences generated from the new add pattern is the same
>> as tlsdesc_small_<mode>, while the operand 0 is wrapped as pseudo register that
>> RTL IV opt can handle it.
>>
>> Ideally, we should allow operand 0 to be any pseudo register, but then
>> we can't model the override of x0 caused by the function call which is
>> hidded by the UNSPEC.
>>
>> So here, we restricting operand 0 to be x0, the override of x0 can be
>> reflected to the gcc.
>>
>> OK for trunk?
>
>
> This patch broke ILP32 because we used mode rather than ptr_mode for
> the psedu .  I have an idea on how to fix it (like tlsie_small_sidi
> case) but I still need to test it fully.
>
> This is the smallest testcase where the problem is:
> struct dtor_list
> {
>   struct dtor_list *next;
> };
> static __thread struct dtor_list *tls_dtor_list;
> __cxa_thread_atexit_impl ( struct dtor_list *new)
> {
>   new->next = tls_dtor_list;
>   tls_dtor_list = new;
> }

Actually there is another bug with respect of the output too.  Some of
the <w>0 should have been plain x0 due to only the 64bit register is
accepted in some contexts.

Thanks,
Andrew

>
>
> Thanks,
> Andrew
>
>>
>> 2015-07-28  Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>
>>             Jiong Wang  <jiong.wang@arm.com>
>>
>> gcc/
>>   * config/aarch64/aarch64.d (tlsdesc_small_pseudo_<mode>): New pattern.
>>   * config/aarch64/aarch64.h (reg_class): New enumeration FIXED_REG0.
>>   (REG_CLASS_NAMES): Likewise.
>>   (REG_CLASS_CONTENTS): Likewise.
>>   * config/aarch64/aarch64.c (aarch64_class_max_nregs): Likewise.
>>   (aarch64_register_move_cost): Likewise.
>>   (aarch64_load_symref_appropriately): Invoke the new added pattern if
>>   possible.
>>   * config/aarch64/constraints.md (Uc0): New constraint.
>>
>> gcc/testsuite.
>>   * gcc.target/aarch64/tlsdesc_hoist.c: New testcase.
>>
>> --
>> Regards,
>> Jiong
>>