[PATCH][AArch64] Allow const0_rtx operand for atomic compare-exchange patterns

[Kyrill Tkachov]

Pinging this back into context so that I don't forget about it...

https://gcc.gnu.org/ml/gcc-patches/2017-02/msg01648.html

Thanks,
Kyrill

On 28/02/17 12:29, Kyrill Tkachov wrote:
> Hi all,
>
> For the testcase in this patch we currently generate:
> foo:
>         mov     w1, 0
>         ldaxr   w2, [x0]
>         cmp     w2, 3
>         bne     .L2
>         stxr    w3, w1, [x0]
>         cmp     w3, 0
> .L2:
>         cset    w0, eq
>         ret
>
> Note that the STXR could have been storing the WZR register instead of moving zero into w1.
> This is due to overly strict predicates and constraints in the store exclusive pattern and the
> atomic compare exchange expanders and splitters.
> This simple patch fixes that in the patterns concerned and with it we can generate:
> foo:
>         ldaxr   w1, [x0]
>         cmp     w1, 3
>         bne     .L2
>         stxr    w2, wzr, [x0]
>         cmp     w2, 0
> .L2:
>         cset    w0, eq
>         ret
>
>
> Bootstrapped and tested on aarch64-none-linux-gnu.
> Ok for GCC 8?
>
> Thanks,
> Kyrill
>
> 2017-02-28  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     * config/aarch64/atomics.md (atomic_compare_and_swap<mode> expander):
>     Use aarch64_reg_or_zero predicate for operand 4.
>     (aarch64_compare_and_swap<mode> define_insn_and_split):
>     Use aarch64_reg_or_zero predicate for operand 3.  Add 'Z' constraint.
>     (aarch64_store_exclusive<mode>): Likewise for operand 2.
>
> 2017-02-28  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>
>
>     * gcc.target/aarch64/atomic_cmp_exchange_zero_reg_1.c: New test.