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

["Kong, Lingling" <lingling.kong@intel.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.
> 

Ok, I will revert this patch in trunk. 
And wait until the optimization results of the actual hardware come out, and then consider to push the optimization patch.

> >
> > 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
> >