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

["Andre Vieira (lists)" <andre.simoesdiasvieira@arm.com>]

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On 14/11/2022 14:12, Kyrylo Tkachov wrote:
>
>> -----Original Message-----
>> From: Andre Vieira (lists) <andre.simoesdiasvieira@arm.com>
>> Sent: Monday, November 14, 2022 2:09 PM
>> To: Kyrylo Tkachov <Kyrylo.Tkachov@arm.com>; gcc-patches@gcc.gnu.org
>> Cc: Richard Earnshaw <Richard.Earnshaw@arm.com>; Richard Sandiford
>> <Richard.Sandiford@arm.com>
>> 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  <andre.simoesdiasvieira@arm.com>
>>                          Kyrylo Tkachov <kyrylo.tkachov@arm.com>
>>
>> gcc/ChangeLog:
>>
>>           * config/aarch64/aarch64.h (AARCH64_ISA_RCPC): New Macro.
>>           (TARGET_RCPC): New Macro.
>>           * config/aarch64/atomics.md (atomic_load<mode>): Change into an
>> expand.
>>           (aarch64_atomic_load<mode>_rcpc): New define_insn for ldapr.
>>           (aarch64_atomic_load<mode>): 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.

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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<mode>"
+(define_expand "atomic_load<mode>"
+  [(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<mode>_rcpc (operands[0], operands[1],
+						     operands[2]));
+    else
+      emit_insn (gen_aarch64_atomic_load<mode> (operands[0], operands[1],
+						operands[2]));
+    DONE;
+  }
+)
+
+(define_insn "aarch64_atomic_load<mode>_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<atomic_sfx>\t%<w>0, %1"
+)
+
+(define_insn "aarch64_atomic_load<mode>"
   [(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 prior to Armv8.2-A is not supported.
 
 @item 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.
 @item dotprod
 Enable the Dot Product extension.  This also enables Advanced SIMD instructions.
 @item aes
diff --git a/gcc/testsuite/gcc.target/aarch64/ldapr.c b/gcc/testsuite/gcc.target/aarch64/ldapr.c
new file mode 100644
index 0000000000000000000000000000000000000000..21f6464c60e0c1239385fb3a366f9f9f84b3cadf
--- /dev/null
+++ b/gcc/testsuite/gcc.target/aarch64/ldapr.c
@@ -0,0 +1,34 @@
+/* { dg-do compile } */
+/* { dg-options "-O1 -std=c99" } */
+#include <stdatomic.h>
+
+#pragma GCC target "+rcpc"
+atomic_ullong u64;
+atomic_llong s64;
+atomic_uint u32;
+atomic_int s32;
+atomic_ushort u16;
+atomic_short s16;
+atomic_uchar u8;
+atomic_schar s8;
+
+#define TEST(size, rettype)					\
+rettype								\
+test_##size (void)						\
+{								\
+  return atomic_load_explicit (&size, memory_order_acquire);	\
+}								\
+
+TEST(u64, unsigned long long)
+TEST(s64, long long)
+TEST(u32, unsigned int)
+TEST(s32, int)
+TEST(u16, unsigned short)
+TEST(s16, short)
+TEST(u8, unsigned char)
+TEST(s8, signed char)
+
+/* { dg-final { scan-assembler-times "ldapr\tx" 2 } } */
+/* { dg-final { scan-assembler-times "ldapr\tw" 2 } } */
+/* { dg-final { scan-assembler-times "ldaprh\tw" 2 } } */
+/* { dg-final { scan-assembler-times "ldaprb\tw" 2 } } */