Re: [PATCH] i386: Prefer remote atomic insn for atomic_fetch{add, and, or, xor}

[Uros Bizjak <ubizjak@gmail.com>]

On Sun, Nov 6, 2022 at 2:00 PM Kong, Lingling via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi
>
> The patch is to add flag -mprefer-remote-atomic to control whether to generate raoint insn for atomic operations.
> Ok for trunk?

Please note TARGET_AVOID_MFENCE tuning flag, introduced a while ago
due to the fact that several targets perform LOCK OR faster than
MFENCE.

It was determined that MFENCE/SFENCE/LFENCE are much more complex
instructions compared to LOCK OR, since they have to handle cases that
C memory model never describes (some MMIO, or such). Considering that
ordinary LOCKed operations adequately cover C memory model, and are
probably faster than new instructions that have to cover all special
cases, I wonder if there is really benefit to emit these insns instead
of existing LOCKed operations. These should IMO be used only via
relevant builtins.

Uros.

>
> BRs,
> Lingling
>
> gcc/ChangeLog:
>
>         * config/i386/i386.opt:Add -mprefer-remote-atomic.
>         * config/i386/sync.md (atomic_<plus_logic><mode>):
>         New define_expand.
>         (atomic_add<mode>): Rename to below one.
>         (atomic_add<mode>_1): To this.
>         (atomic_<logic><mode>): Ditto.
>         (atomic_<logic><mode>_1): Ditto.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.target/i386/raoint-atomic-fetch.c: New test.
> ---
>  gcc/config/i386/i386.opt                      |  4 +++
>  gcc/config/i386/sync.md                       | 29 ++++++++++++++++---
>  .../gcc.target/i386/raoint-atomic-fetch.c     | 29 +++++++++++++++++++
>  3 files changed, 58 insertions(+), 4 deletions(-)  create mode 100644 gcc/testsuite/gcc.target/i386/raoint-atomic-fetch.c
>
> diff --git a/gcc/config/i386/i386.opt b/gcc/config/i386/i386.opt index 415c52e1bb4..abb1e5ecbdc 100644
> --- a/gcc/config/i386/i386.opt
> +++ b/gcc/config/i386/i386.opt
> @@ -1246,3 +1246,7 @@ Support PREFETCHI built-in functions and code generation.
>  mraoint
>  Target Mask(ISA2_RAOINT) Var(ix86_isa_flags2) Save  Support RAOINT built-in functions and code generation.
> +
> +mprefer-remote-atomic
> +Target Var(flag_prefer_remote_atomic) Init(0) Prefer use remote atomic
> +insn for atomic operations.
> diff --git a/gcc/config/i386/sync.md b/gcc/config/i386/sync.md index e6543a5efb0..08e944fc9b7 100644
> --- a/gcc/config/i386/sync.md
> +++ b/gcc/config/i386/sync.md
> @@ -37,7 +37,7 @@
>    UNSPECV_CMPXCHG
>    UNSPECV_XCHG
>    UNSPECV_LOCK
> -
> +
>    ;; For CMPccXADD support
>    UNSPECV_CMPCCXADD
>
> @@ -791,7 +791,28 @@
>  (define_code_iterator any_plus_logic [and ior xor plus])  (define_code_attr plus_logic [(and "and") (ior "or") (xor "xor") (plus "add")])
>
> -(define_insn "rao_a<plus_logic><mode>"
> +(define_expand "atomic_<plus_logic><mode>"
> +  [(match_operand:SWI 0 "memory_operand")
> +   (any_plus_logic:SWI (match_dup 0)
> +                      (match_operand:SWI 1 "nonmemory_operand"))
> +   (match_operand:SI 2 "const_int_operand")]
> +  ""
> +{
> +  if (flag_prefer_remote_atomic
> +      && TARGET_RAOINT && operands[2] == const0_rtx
> +      && (<MODE>mode == SImode || <MODE>mode == DImode))
> +  {
> +    if (CONST_INT_P (operands[1]))
> +      operands[1] = force_reg (<MODE>mode, operands[1]);
> +    emit_insn (maybe_gen_rao_a (<CODE>, <MODE>mode, operands[0],
> +operands[1]));
> +  }
> +  else
> +    emit_insn (gen_atomic_<plus_logic><mode>_1 (operands[0], operands[1],
> +                                               operands[2]));
> +  DONE;
> +})
> +
> +(define_insn "@rao_a<plus_logic><mode>"
>    [(set (match_operand:SWI48 0 "memory_operand" "+m")
>         (unspec_volatile:SWI48
>           [(any_plus_logic:SWI48 (match_dup 0) @@ -801,7 +822,7 @@
>    "TARGET_RAOINT"
>    "a<plus_logic>\t{%1, %0|%0, %1}")
>
> -(define_insn "atomic_add<mode>"
> +(define_insn "atomic_add<mode>_1"
>    [(set (match_operand:SWI 0 "memory_operand" "+m")
>         (unspec_volatile:SWI
>           [(plus:SWI (match_dup 0)
> @@ -855,7 +876,7 @@
>    return "lock{%;} %K2sub{<imodesuffix>}\t{%1, %0|%0, %1}";
>  })
>
> -(define_insn "atomic_<logic><mode>"
> +(define_insn "atomic_<logic><mode>_1"
>    [(set (match_operand:SWI 0 "memory_operand" "+m")
>         (unspec_volatile:SWI
>           [(any_logic:SWI (match_dup 0)
> diff --git a/gcc/testsuite/gcc.target/i386/raoint-atomic-fetch.c b/gcc/testsuite/gcc.target/i386/raoint-atomic-fetch.c
> new file mode 100644
> index 00000000000..ac4099d888e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/raoint-atomic-fetch.c
> @@ -0,0 +1,29 @@
> +/* { dg-do compile } */
> +/* { dg-options "-mraoint -O2 -mprefer-remote-atomic" } */
> +/* { dg-final { scan-assembler-times "aadd" 2 { target {! ia32 } } } }
> +*/
> +/* { dg-final { scan-assembler-times "aand" 2 { target {! ia32 } } } }
> +*/
> +/* { dg-final { scan-assembler-times "aor" 2 { target {! ia32 } } } }
> +*/
> +/* { dg-final { scan-assembler-times "axor" 2 { target {! ia32 } } } }
> +*/
> +/* { dg-final { scan-assembler-times "aadd" 1 { target ia32 } } } */
> +/* { dg-final { scan-assembler-times "aand" 1 { target ia32 } } } */
> +/* { dg-final { scan-assembler-times "aor" 1 { target ia32 } } } */
> +/* { dg-final { scan-assembler-times "axor" 1 { target ia32 } } } */
> +volatile int x; volatile long long y; int *a; long long *b;
> +
> +void extern
> +rao_int_test (void)
> +{
> +  __atomic_add_fetch (a, x, __ATOMIC_RELAXED);
> +  __atomic_and_fetch (a, x, __ATOMIC_RELAXED);
> +  __atomic_or_fetch (a, x, __ATOMIC_RELAXED);
> +  __atomic_xor_fetch (a, x, __ATOMIC_RELAXED); #ifdef __x86_64__
> +  __atomic_add_fetch (b, y, __ATOMIC_RELAXED);
> +  __atomic_and_fetch (b, y, __ATOMIC_RELAXED);
> +  __atomic_or_fetch (b, y, __ATOMIC_RELAXED);
> +  __atomic_xor_fetch (b, y, __ATOMIC_RELAXED); #endif }
> --
> 2.27.0
>