RISC-V: Strengthen atomic stores

This change makes atomic stores strictly stronger than table A.6 of the
ISA manual. This mapping makes the overall patchset compatible with
table A.7 as well.

2023-04-27 Patrick O'Neill <patrick@rivosinc.com>

	PR target/89835

gcc/ChangeLog:

	* config/riscv/sync.md (atomic_store<mode>): Use simple store
	instruction in combination with fence(s).

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/pr89835.c: New test.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>

2 files changed, 27 insertions, 3 deletions

diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
index 5620d6f..1acb78a 100644
--- a/gcc/config/riscv/sync.md
+++ b/gcc/config/riscv/sync.md
@@ -56,7 +56,9 @@
 
 ;; Atomic memory operations.
 
-;; Implement atomic stores with amoswap.  Fall back to fences for atomic loads.
+;; Implement atomic stores with conservative fences.  Fall back to fences for
+;; atomic loads.
+;; This allows us to be compatible with the ISA manual Table A.6 and Table A.7.
 (define_insn "atomic_store<mode>"
   [(set (match_operand:GPR 0 "memory_operand" "=A")
     (unspec_volatile:GPR
@@ -64,9 +66,22 @@
        (match_operand:SI 2 "const_int_operand")]      ;; model
       UNSPEC_ATOMIC_STORE))]
   "TARGET_ATOMIC"
-  "%F2amoswap.<amo>%A2 zero,%z1,%0"
+  {
+    enum memmodel model = (enum memmodel) INTVAL (operands[2]);
+    model = memmodel_base (model);
+
+    if (model == MEMMODEL_SEQ_CST)
+      return "fence\trw,w\;"
+	     "s<amo>\t%z1,%0\;"
+	     "fence\trw,rw";
+    if (model == MEMMODEL_RELEASE)
+      return "fence\trw,w\;"
+	     "s<amo>\t%z1,%0";
+    else
+      return "s<amo>\t%z1,%0";
+  }
   [(set_attr "type" "atomic")
-   (set (attr "length") (const_int 8))])
+   (set (attr "length") (const_int 12))])
 
 (define_insn "atomic_<atomic_optab><mode>"
   [(set (match_operand:GPR 0 "memory_operand" "+A")
diff --git a/gcc/testsuite/gcc.target/riscv/pr89835.c b/gcc/testsuite/gcc.target/riscv/pr89835.c
new file mode 100644
index 0000000..ab190e1
--- /dev/null
+++ b/gcc/testsuite/gcc.target/riscv/pr89835.c
@@ -0,0 +1,9 @@
+/* { dg-do compile } */
+/* Verify that relaxed atomic stores use simple store instuctions.  */
+/* { dg-final { scan-assembler-not "amoswap" } } */
+
+void
+foo(int bar, int baz)
+{
+  __atomic_store_n(&bar, baz, __ATOMIC_RELAXED);
+}