This commit generates the mb operation. In Wasm, it uses the atomic.fence
instruction as the fence operator [1]. TCI instruction is also generated in
the same way as the original TCI backend using smp_mb().

[1] https://webassembly.github.io/threads/core/syntax/instructions.html#atomic-memory-instructions

Signed-off-by: Kohei Tokunaga <ktokunaga.mail@gmail.com>
---
 tcg/wasm.c                |  4 ++++
 tcg/wasm/tcg-target.c.inc | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+)

diff --git a/tcg/wasm.c b/tcg/wasm.c
index 793c1807c2..1cc2e45e77 100644
--- a/tcg/wasm.c
+++ b/tcg/wasm.c
@@ -566,6 +566,10 @@ static uintptr_t tcg_qemu_tb_exec_tci(CPUArchState *env, const void *v_tb_ptr)
             taddr = regs[r1];
             tci_qemu_st(env, taddr, regs[r0], oi, tb_ptr);
             break;
+        case INDEX_op_mb:
+            /* Ensure ordering for all kinds */
+            smp_mb();
+            break;
         default:
             g_assert_not_reached();
         }
diff --git a/tcg/wasm/tcg-target.c.inc b/tcg/wasm/tcg-target.c.inc
index e1ee2f6485..1d639561db 100644
--- a/tcg/wasm/tcg-target.c.inc
+++ b/tcg/wasm/tcg-target.c.inc
@@ -1116,6 +1116,11 @@ static void tcg_out_op_r(TCGContext *s, TCGOpcode op, TCGReg r0)
     tcg_out32(s, insn);
 }
 
+static void tcg_out_op_v(TCGContext *s, TCGOpcode op)
+{
+    tcg_out32(s, (uint8_t)op);
+}
+
 static void tcg_out_op_ri(TCGContext *s, TCGOpcode op, TCGReg r0, int32_t i1)
 {
     tcg_insn_unit_tci insn = 0;
@@ -1931,6 +1936,20 @@ static bool tcg_out_qemu_st_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
     g_assert_not_reached();
 }
 
+static void tcg_out_mb(TCGContext *s, unsigned a0)
+{
+    tcg_out_op_v(s, INDEX_op_mb);
+
+    /*
+     * Wasm's threading proposal provides atomic.fence instruction as the fence
+     * operator.
+     * https://webassembly.github.io/threads/core/syntax/instructions.html#atomic-memory-instructions
+     */
+    tcg_wasm_out8(s, 0xfe);
+    tcg_wasm_out8(s, 0x03);
+    tcg_wasm_out8(s, 0x00);
+}
+
 static void tcg_out_tb_start(TCGContext *s)
 {
     init_sub_buf();
-- 
2.43.0