RE: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire semantics

2022-11-14 Thread Kyrylo Tkachov via Gcc-patches


> -Original Message-
> From: Andre Vieira (lists) 
> Sent: Monday, November 14, 2022 2:24 PM
> To: Kyrylo Tkachov ; gcc-patches@gcc.gnu.org
> Cc: Richard Earnshaw ; Richard Sandiford
> 
> Subject: Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
> semantics
> 
> 
> On 14/11/2022 14:12, Kyrylo Tkachov wrote:
> >
> >> -Original Message-
> >> From: Andre Vieira (lists) 
> >> Sent: Monday, November 14, 2022 2:09 PM
> >> To: Kyrylo Tkachov ; gcc-patches@gcc.gnu.org
> >> Cc: Richard Earnshaw ; Richard Sandiford
> >> 
> >> Subject: Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-
> acquire
> >> semantics
> >>
> >> Here is the latest version and an updated ChangeLog:
> >>
> >> 2022-11-14  Andre Vieira  
> >>      Kyrylo Tkachov 
> >>
> >> gcc/ChangeLog:
> >>
> >>       * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
> >>       (TARGET_RCPC): New Macro.
> >>       * config/aarch64/atomics.md (atomic_load): Change into an
> >> expand.
> >>       (aarch64_atomic_load_rcpc): New define_insn for ldapr.
> >>       (aarch64_atomic_load): Rename of old define_insn for ldar.
> >>       * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
> >> value.
> >>       * doc/invoke.texi (rcpc): Ammend documentation to mention the
> >> effects
> >>       on code generation.
> >>
> >> gcc/testsuite/ChangeLog:
> >>
> >>       * gcc.target/aarch64/ldapr.c: New test.
> > I don't see this test in the patch?
> > Thanks,
> > Kyrill
> >
> Oops... here it is.

Ok.
Thanks,
Kyrill



Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire semantics

2022-11-14 Thread Andre Vieira (lists) via Gcc-patches


On 14/11/2022 14:12, Kyrylo Tkachov wrote:



-Original Message-
From: Andre Vieira (lists) 
Sent: Monday, November 14, 2022 2:09 PM
To: Kyrylo Tkachov ; gcc-patches@gcc.gnu.org
Cc: Richard Earnshaw ; Richard Sandiford

Subject: Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
semantics

Here is the latest version and an updated ChangeLog:

2022-11-14  Andre Vieira  
     Kyrylo Tkachov 

gcc/ChangeLog:

      * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
      (TARGET_RCPC): New Macro.
      * config/aarch64/atomics.md (atomic_load): Change into an
expand.
      (aarch64_atomic_load_rcpc): New define_insn for ldapr.
      (aarch64_atomic_load): Rename of old define_insn for ldar.
      * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
value.
      * doc/invoke.texi (rcpc): Ammend documentation to mention the
effects
      on code generation.

gcc/testsuite/ChangeLog:

      * gcc.target/aarch64/ldapr.c: New test.

I don't see this test in the patch?
Thanks,
Kyrill


Oops... here it is.diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
index 
e60f9bce023b2cd5e7233ee9b8c61fc93c1494c2..51a8aa02a5850d5c79255dbf7e0764ffdec73ccd
 100644
--- a/gcc/config/aarch64/aarch64.h
+++ b/gcc/config/aarch64/aarch64.h
@@ -221,6 +221,7 @@ enum class aarch64_feature : unsigned char {
 #define AARCH64_ISA_V9_3A  (aarch64_isa_flags & AARCH64_FL_V9_3A)
 #define AARCH64_ISA_MOPS  (aarch64_isa_flags & AARCH64_FL_MOPS)
 #define AARCH64_ISA_LS64  (aarch64_isa_flags & AARCH64_FL_LS64)
+#define AARCH64_ISA_RCPC   (aarch64_isa_flags & AARCH64_FL_RCPC)
 
 /* Crypto is an optional extension to AdvSIMD.  */
 #define TARGET_CRYPTO (AARCH64_ISA_CRYPTO)
@@ -328,6 +329,9 @@ enum class aarch64_feature : unsigned char {
 /* SB instruction is enabled through +sb.  */
 #define TARGET_SB (AARCH64_ISA_SB)
 
+/* RCPC loads from Armv8.3-a.  */
+#define TARGET_RCPC (AARCH64_ISA_RCPC)
+
 /* Apply the workaround for Cortex-A53 erratum 835769.  */
 #define TARGET_FIX_ERR_A53_835769  \
   ((aarch64_fix_a53_err835769 == 2)\
diff --git a/gcc/config/aarch64/atomics.md b/gcc/config/aarch64/atomics.md
index 
bc95f6d9d15f190a3e33704b4def2860d5f339bd..dc5f52ee8a4b349c0d8466a16196f83604893cbb
 100644
--- a/gcc/config/aarch64/atomics.md
+++ b/gcc/config/aarch64/atomics.md
@@ -657,7 +657,38 @@
   }
 )
 
-(define_insn "atomic_load"
+(define_expand "atomic_load"
+  [(match_operand:ALLI 0 "register_operand" "=r")
+   (match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
+   (match_operand:SI   2 "const_int_operand")]
+  ""
+  {
+/* If TARGET_RCPC and this is an ACQUIRE load, then expand to a pattern
+   using UNSPECV_LDAP.  */
+enum memmodel model = memmodel_from_int (INTVAL (operands[2]));
+if (TARGET_RCPC
+   && (is_mm_acquire (model)
+   || is_mm_acq_rel (model)))
+  emit_insn (gen_aarch64_atomic_load_rcpc (operands[0], operands[1],
+operands[2]));
+else
+  emit_insn (gen_aarch64_atomic_load (operands[0], operands[1],
+   operands[2]));
+DONE;
+  }
+)
+
+(define_insn "aarch64_atomic_load_rcpc"
+  [(set (match_operand:ALLI 0 "register_operand" "=r")
+(unspec_volatile:ALLI
+  [(match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
+   (match_operand:SI 2 "const_int_operand")]   ;; model
+  UNSPECV_LDAP))]
+  "TARGET_RCPC"
+  "ldapr\t%0, %1"
+)
+
+(define_insn "aarch64_atomic_load"
   [(set (match_operand:ALLI 0 "register_operand" "=r")
 (unspec_volatile:ALLI
   [(match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
diff --git a/gcc/config/aarch64/iterators.md b/gcc/config/aarch64/iterators.md
index 
a8ad4e5ff215ade06c3ca13a24ef18d259afcb6c..d8c2f9d6c32d6f188d584c2e9d8fb36511624de6
 100644
--- a/gcc/config/aarch64/iterators.md
+++ b/gcc/config/aarch64/iterators.md
@@ -988,6 +988,7 @@
 UNSPECV_LX ; Represent a load-exclusive.
 UNSPECV_SX ; Represent a store-exclusive.
 UNSPECV_LDA; Represent an atomic load or 
load-acquire.
+UNSPECV_LDAP   ; Represent an atomic acquire load with RCpc 
semantics.
 UNSPECV_STL; Represent an atomic store or 
store-release.
 UNSPECV_ATOMIC_CMPSW   ; Represent an atomic compare swap.
 UNSPECV_ATOMIC_EXCHG   ; Represent an atomic exchange.
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 
449df59729884aa3292559fffcfbbcc99182c13a..5a32d7b6e94502c57e6438cfd2563bc5631690e1
 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -20168,9 +20168,9 @@ Enable FP16 fmla extension.  This also enables FP16 
extensions and
 floating-point instructions. This option is enabled by default for 
@option{-march=armv8.4-a}. Use of this option with architectures 

RE: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire semantics

2022-11-14 Thread Kyrylo Tkachov via Gcc-patches


> -Original Message-
> From: Andre Vieira (lists) 
> Sent: Monday, November 14, 2022 2:09 PM
> To: Kyrylo Tkachov ; gcc-patches@gcc.gnu.org
> Cc: Richard Earnshaw ; Richard Sandiford
> 
> Subject: Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
> semantics
> 
> Here is the latest version and an updated ChangeLog:
> 
> 2022-11-14  Andre Vieira  
>     Kyrylo Tkachov 
> 
> gcc/ChangeLog:
> 
>      * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
>      (TARGET_RCPC): New Macro.
>      * config/aarch64/atomics.md (atomic_load): Change into an
> expand.
>      (aarch64_atomic_load_rcpc): New define_insn for ldapr.
>      (aarch64_atomic_load): Rename of old define_insn for ldar.
>      * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
> value.
>      * doc/invoke.texi (rcpc): Ammend documentation to mention the
> effects
>      on code generation.
> 
> gcc/testsuite/ChangeLog:
> 
>      * gcc.target/aarch64/ldapr.c: New test.

I don't see this test in the patch?
Thanks,
Kyrill

> 
> On 10/11/2022 15:55, Kyrylo Tkachov wrote:
> > Hi Andre,
> >
> >> -Original Message-
> >> From: Andre Vieira (lists) 
> >> Sent: Thursday, November 10, 2022 11:17 AM
> >> To: gcc-patches@gcc.gnu.org
> >> Cc: Kyrylo Tkachov ; Richard Earnshaw
> >> ; Richard Sandiford
> >> 
> >> Subject: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
> >> semantics
> >>
> >> Hello,
> >>
> >> This patch enables the use of LDAPR for load-acquire semantics. After
> >> some internal investigation based on the work published by Podkopaev et
> >> al. (https://dl.acm.org/doi/10.1145/3290382) we can confirm that using
> >> LDAPR for the C++ load-acquire semantics is a correct relaxation.
> >>
> >> Bootstrapped and regression tested on aarch64-none-linux-gnu.
> >>
> >> OK for trunk?
> > Thanks for the patch
> >
> >> 2022-11-09  Andre Vieira  
> >>       Kyrylo Tkachov  
> >>
> >> gcc/ChangeLog:
> >>
> >>       * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
> >>       (TARGET_RCPC): New Macro.
> >>       * config/aarch64/atomics.md (atomic_load): Change into
> >>       an expand.
> >>       (aarch64_atomic_load_rcpc): New define_insn for ldapr.
> >>       (aarch64_atomic_load): Rename of old define_insn for ldar.
> >>       * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
> >> value.
> >>       *
> >> doc/gcc/gcc-command-options/machine-dependent-options/aarch64-
> >> options.rst
> >>       (rcpc): Ammend documentation to mention the effects on code
> >> generation.
> >>
> >> gcc/testsuite/ChangeLog:
> >>
> >>       * gcc.target/aarch64/ldapr.c: New test.
> >>       * lib/target-supports.exp (add_options_for_aarch64_rcpc): New
> >> options procedure.
> >>       (check_effective_target_aarch64_rcpc_ok_nocache): New
> >> check-effective-target.
> >>       (check_effective_target_aarch64_rcpc_ok): Likewise.
> > diff --git a/gcc/config/aarch64/atomics.md
> b/gcc/config/aarch64/atomics.md
> > index
> bc95f6d9d15f190a3e33704b4def2860d5f339bd..801a62bf2ba432f35ae1931b
> eb8c4405b77b36c3 100644
> > --- a/gcc/config/aarch64/atomics.md
> > +++ b/gcc/config/aarch64/atomics.md
> > @@ -657,7 +657,42 @@
> > }
> >   )
> >
> > -(define_insn "atomic_load"
> > +(define_expand "atomic_load"
> > +  [(match_operand:ALLI 0 "register_operand" "=r")
> > +   (match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
> > +   (match_operand:SI   2 "const_int_operand")]
> > +  ""
> > +  {
> > +/* If TARGET_RCPC and this is an ACQUIRE load, then expand to a
> pattern
> > +   using UNSPECV_LDAP.  */
> > +enum memmodel model = memmodel_from_int (INTVAL
> (operands[2]));
> > +if (TARGET_RCPC
> > +   && (is_mm_acquire (model)
> > +   || is_mm_acq_rel (model)))
> > +{
> > +  emit_insn (gen_aarch64_atomic_load_rcpc (operands[0],
> operands[1],
> > +operands[2]));
> > +}
> > +else
> > +{
> > +  emit_insn (gen_aarch64_atomic_load (operands[0],
> operands[1],
> > +   operands[2]));
> > +}
> >
> > No braces needed for single-statement bodies.
> >
> > diff --git a/gcc/doc/gcc/gcc-command-options/machine-dependent-
> options/aarch64-options.rst b/gcc/doc/gcc/gcc-command-options/machine-
> dependent-options/aarch64-options.rst
> > index
> c2b23a6ee97ef2b7c74119f22c1d3e3d85385f4d..25d609238db7d45845dbc44
> 6ac21d12dddcf8eac 100644
> > --- a/gcc/doc/gcc/gcc-command-options/machine-dependent-
> options/aarch64-options.rst
> > +++ b/gcc/doc/gcc/gcc-command-options/machine-dependent-
> options/aarch64-options.rst
> > @@ -437,9 +437,9 @@ the following and their inverses no
> :samp:`{feature}` :
> > floating-point instructions. This option is enabled by default for 
> > :option:`-
> march=armv8.4-a`. Use of this option with architectures prior to Armv8.2-A is
> 

Re: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire semantics

2022-11-14 Thread Andre Vieira (lists) via Gcc-patches

Here is the latest version and an updated ChangeLog:

2022-11-14  Andre Vieira  
   Kyrylo Tkachov 

gcc/ChangeLog:

    * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
    (TARGET_RCPC): New Macro.
    * config/aarch64/atomics.md (atomic_load): Change into an 
expand.

    (aarch64_atomic_load_rcpc): New define_insn for ldapr.
    (aarch64_atomic_load): Rename of old define_insn for ldar.
    * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum value.
    * doc/invoke.texi (rcpc): Ammend documentation to mention the 
effects

    on code generation.

gcc/testsuite/ChangeLog:

    * gcc.target/aarch64/ldapr.c: New test.

On 10/11/2022 15:55, Kyrylo Tkachov wrote:

Hi Andre,


-Original Message-
From: Andre Vieira (lists) 
Sent: Thursday, November 10, 2022 11:17 AM
To: gcc-patches@gcc.gnu.org
Cc: Kyrylo Tkachov ; Richard Earnshaw
; Richard Sandiford

Subject: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
semantics

Hello,

This patch enables the use of LDAPR for load-acquire semantics. After
some internal investigation based on the work published by Podkopaev et
al. (https://dl.acm.org/doi/10.1145/3290382) we can confirm that using
LDAPR for the C++ load-acquire semantics is a correct relaxation.

Bootstrapped and regression tested on aarch64-none-linux-gnu.

OK for trunk?

Thanks for the patch


2022-11-09  Andre Vieira  
      Kyrylo Tkachov  

gcc/ChangeLog:

      * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
      (TARGET_RCPC): New Macro.
      * config/aarch64/atomics.md (atomic_load): Change into
      an expand.
      (aarch64_atomic_load_rcpc): New define_insn for ldapr.
      (aarch64_atomic_load): Rename of old define_insn for ldar.
      * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
value.
      *
doc/gcc/gcc-command-options/machine-dependent-options/aarch64-
options.rst
      (rcpc): Ammend documentation to mention the effects on code
generation.

gcc/testsuite/ChangeLog:

      * gcc.target/aarch64/ldapr.c: New test.
      * lib/target-supports.exp (add_options_for_aarch64_rcpc): New
options procedure.
      (check_effective_target_aarch64_rcpc_ok_nocache): New
check-effective-target.
      (check_effective_target_aarch64_rcpc_ok): Likewise.

diff --git a/gcc/config/aarch64/atomics.md b/gcc/config/aarch64/atomics.md
index 
bc95f6d9d15f190a3e33704b4def2860d5f339bd..801a62bf2ba432f35ae1931beb8c4405b77b36c3
 100644
--- a/gcc/config/aarch64/atomics.md
+++ b/gcc/config/aarch64/atomics.md
@@ -657,7 +657,42 @@
}
  )
  
-(define_insn "atomic_load"

+(define_expand "atomic_load"
+  [(match_operand:ALLI 0 "register_operand" "=r")
+   (match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
+   (match_operand:SI   2 "const_int_operand")]
+  ""
+  {
+/* If TARGET_RCPC and this is an ACQUIRE load, then expand to a pattern
+   using UNSPECV_LDAP.  */
+enum memmodel model = memmodel_from_int (INTVAL (operands[2]));
+if (TARGET_RCPC
+   && (is_mm_acquire (model)
+   || is_mm_acq_rel (model)))
+{
+  emit_insn (gen_aarch64_atomic_load_rcpc (operands[0], operands[1],
+operands[2]));
+}
+else
+{
+  emit_insn (gen_aarch64_atomic_load (operands[0], operands[1],
+   operands[2]));
+}

No braces needed for single-statement bodies.

diff --git 
a/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst 
b/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
index 
c2b23a6ee97ef2b7c74119f22c1d3e3d85385f4d..25d609238db7d45845dbc446ac21d12dddcf8eac
 100644
--- 
a/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
+++ 
b/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
@@ -437,9 +437,9 @@ the following and their inverses no :samp:`{feature}` :
floating-point instructions. This option is enabled by default for 
:option:`-march=armv8.4-a`. Use of this option with architectures prior to 
Armv8.2-A is not supported.
  
  :samp:`rcpc`

-  Enable the RcPc extension.  This does not change code generation from GCC,
-  but is passed on to the assembler, enabling inline asm statements to use
-  instructions from the RcPc extension.
+  Enable the RcPc extension.  This enables the use of the LDAPR instructions 
for
+  load-acquire atomic semantics, and passes it on to the assembler, enabling
+  inline asm statements to use instructions from the RcPc extension.

Let's capitalize this consistently throughout the patch as "RCpc".

diff --git a/gcc/testsuite/gcc.target/aarch64/ldapr.c 
b/gcc/testsuite/gcc.target/aarch64/ldapr.c
new file mode 100644
index 
..c36edfcd79a9ee41434ab09ac47d257a692a8606
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/ldapr.c

RE: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire semantics

2022-11-10 Thread Kyrylo Tkachov via Gcc-patches
Hi Andre,

> -Original Message-
> From: Andre Vieira (lists) 
> Sent: Thursday, November 10, 2022 11:17 AM
> To: gcc-patches@gcc.gnu.org
> Cc: Kyrylo Tkachov ; Richard Earnshaw
> ; Richard Sandiford
> 
> Subject: [PATCH 1/2] aarch64: Enable the use of LDAPR for load-acquire
> semantics
> 
> Hello,
> 
> This patch enables the use of LDAPR for load-acquire semantics. After
> some internal investigation based on the work published by Podkopaev et
> al. (https://dl.acm.org/doi/10.1145/3290382) we can confirm that using
> LDAPR for the C++ load-acquire semantics is a correct relaxation.
> 
> Bootstrapped and regression tested on aarch64-none-linux-gnu.
> 
> OK for trunk?

Thanks for the patch

> 
> 2022-11-09  Andre Vieira  
>      Kyrylo Tkachov  
> 
> gcc/ChangeLog:
> 
>      * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
>      (TARGET_RCPC): New Macro.
>      * config/aarch64/atomics.md (atomic_load): Change into
>      an expand.
>      (aarch64_atomic_load_rcpc): New define_insn for ldapr.
>      (aarch64_atomic_load): Rename of old define_insn for ldar.
>      * config/aarch64/iterators.md (UNSPEC_LDAP): New unspec enum
> value.
>      *
> doc/gcc/gcc-command-options/machine-dependent-options/aarch64-
> options.rst
>      (rcpc): Ammend documentation to mention the effects on code
> generation.
> 
> gcc/testsuite/ChangeLog:
> 
>      * gcc.target/aarch64/ldapr.c: New test.
>      * lib/target-supports.exp (add_options_for_aarch64_rcpc): New
> options procedure.
>      (check_effective_target_aarch64_rcpc_ok_nocache): New
> check-effective-target.
>      (check_effective_target_aarch64_rcpc_ok): Likewise.

diff --git a/gcc/config/aarch64/atomics.md b/gcc/config/aarch64/atomics.md
index 
bc95f6d9d15f190a3e33704b4def2860d5f339bd..801a62bf2ba432f35ae1931beb8c4405b77b36c3
 100644
--- a/gcc/config/aarch64/atomics.md
+++ b/gcc/config/aarch64/atomics.md
@@ -657,7 +657,42 @@
   }
 )
 
-(define_insn "atomic_load"
+(define_expand "atomic_load"
+  [(match_operand:ALLI 0 "register_operand" "=r")
+   (match_operand:ALLI 1 "aarch64_sync_memory_operand" "Q")
+   (match_operand:SI   2 "const_int_operand")]
+  ""
+  {
+/* If TARGET_RCPC and this is an ACQUIRE load, then expand to a pattern
+   using UNSPECV_LDAP.  */
+enum memmodel model = memmodel_from_int (INTVAL (operands[2]));
+if (TARGET_RCPC
+   && (is_mm_acquire (model)
+   || is_mm_acq_rel (model)))
+{
+  emit_insn (gen_aarch64_atomic_load_rcpc (operands[0], operands[1],
+operands[2]));
+}
+else
+{
+  emit_insn (gen_aarch64_atomic_load (operands[0], operands[1],
+   operands[2]));
+}

No braces needed for single-statement bodies.

diff --git 
a/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst 
b/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
index 
c2b23a6ee97ef2b7c74119f22c1d3e3d85385f4d..25d609238db7d45845dbc446ac21d12dddcf8eac
 100644
--- 
a/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
+++ 
b/gcc/doc/gcc/gcc-command-options/machine-dependent-options/aarch64-options.rst
@@ -437,9 +437,9 @@ the following and their inverses no :samp:`{feature}` :
   floating-point instructions. This option is enabled by default for 
:option:`-march=armv8.4-a`. Use of this option with architectures prior to 
Armv8.2-A is not supported.
 
 :samp:`rcpc`
-  Enable the RcPc extension.  This does not change code generation from GCC,
-  but is passed on to the assembler, enabling inline asm statements to use
-  instructions from the RcPc extension.
+  Enable the RcPc extension.  This enables the use of the LDAPR instructions 
for
+  load-acquire atomic semantics, and passes it on to the assembler, enabling
+  inline asm statements to use instructions from the RcPc extension.

Let's capitalize this consistently throughout the patch as "RCpc".

diff --git a/gcc/testsuite/gcc.target/aarch64/ldapr.c 
b/gcc/testsuite/gcc.target/aarch64/ldapr.c
new file mode 100644
index 
..c36edfcd79a9ee41434ab09ac47d257a692a8606
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/ldapr.c
@@ -0,0 +1,35 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -std=c99" } */
+/* { dg-require-effective-target aarch64_rcpc_ok } */
+/* { dg-add-options aarch64_rcpc } */

If you're not doing an assemble here you probably don't care much about this 
target business? (it's more important on the arm side with incompatible ABIs, 
Thumb-ness).
I think in this case you can avoid introducing the effective targets and just 
add
#pragma GCC target "+rcpc"
to the body of the testcase (we use it in a few testcases for aarch64)

Otherwise looks good!
Thanks,
Kyrill