Add RV32I memory ordering instruction FENCE

The RV32I memory ordering instruction FENCE is used to order device
I/O and memory accesses as viewed by other RISC-V harts and external
devices or coprocessors. Linux kernel leverage this instruction for
synchronization, support emulating this instruction is needed to boot
Linux kernel. The implemention of this instruction is straight
forward since there is only single hart at present.

Related: sysprog21#310

src/decode.c

@@ -908,25 +908,35 @@ static inline bool op_system(rv_insn_t *ir, const uint32_t insn)
     return true;
 }
 
-#if RV32_HAS(Zifencei)
 /* MISC-MEM: I-type
  *  31       20 19   15 14    12 11   7 6      0
  * | imm[11:0] |  rs1  | funct3 |  rd  | opcode |
  */
-static inline bool op_misc_mem(rv_insn_t *ir, const uint32_t insn UNUSED)
+static inline bool op_misc_mem(rv_insn_t *ir, const uint32_t insn)
 {
-    /* inst   imm[11:0] rs1 funct3 rd opcode
-     * ------+---------+---+------+--+-------
-     * FENCEI imm[11:0] rs1 001    rd 0001111
+    /* inst      fm       pred      succ       rs1   funct3  rd   opcode
+     * ------+---------+----------+-----------+-----+-------+----+-------
+     * FENCE   FM[3:0]   pred[3:0]  succ[3:0]  rs1   000     rd   0001111
+     * FENCEI            imm[11:0]             rs1   001     rd   0001111
      */
 
-    /* FIXME: fill real implementations */
-    ir->opcode = rv_insn_fencei;
-    return true;
+    const uint32_t funct3 = decode_funct3(insn);
+
+    switch (funct3) {
+    case 0b000:
+        ir->opcode = rv_insn_fence;
+        return true;
+#if RV32_HAS(Zifencei)
+    case 0b001:
+        ir->opcode = rv_insn_fencei;
+        return true;
+#endif       /* RV32_HAS(Zifencei) */
+    default: /* illegal instruction */
+        return false;
+    }
+
+    return false;
 }
-#else
-#define op_misc_mem OP_UNIMP
-#endif /* RV32_HAS(Zifencei) */
 
 #if RV32_HAS(EXT_A)
 /* AMO: R-type

src/decode.h

@@ -72,6 +72,7 @@ enum op_field {
     _(sra, 0, 4, 1, ENC(rs1, rs2, rd))                 \
     _(or, 0, 4, 1, ENC(rs1, rs2, rd))                  \
     _(and, 0, 4, 1, ENC(rs1, rs2, rd))                 \
+    _(fence, 1, 4, 0, ENC(rs1, rd))                    \
     _(ecall, 1, 4, 1, ENC(rs1, rd))                    \
     _(ebreak, 1, 4, 1, ENC(rs1, rd))                   \
     /* RISC-V Privileged Instruction */                \

src/rv32_constopt.c

@@ -356,6 +356,12 @@ CONSTOPT(and, {
         info->is_constant[ir->rd] = false;
 })
 
+/*
+ * FENCE: order device I/O and memory accesses as viewed by other
+ * RISC-V harts and external devices or coprocessors
+ */
+CONSTOPT(fence, {})
+
 /* ECALL: Environment Call */
 CONSTOPT(ecall, {})
 

src/rv32_template.c

@@ -927,6 +927,21 @@ RVOP(
      }))
 /* clang-format on */
 
+/*
+ * FENCE: order device I/O and memory accesses as viewed by other
+ * RISC-V harts and external devices or coprocessors
+ */
+RVOP(
+    fence,
+    {
+        PC += 4;
+        /* FIXME: fill real implementations */
+        goto end_op;
+    },
+    GEN({
+        assert; /* FIXME: Implement */
+    }))
+
 /* ECALL: Environment Call */
 RVOP(
     ecall,

src/t2c_template.c

@@ -281,6 +281,8 @@ T2C_OP(and, {
     LLVMBuildStore(*builder, res, t2c_gen_rd_addr(start, builder, ir));
 })
 
+T2C_OP(fence, { __UNREACHABLE; })
+
 T2C_OP(ecall, {
     T2C_LLVM_GEN_STORE_IMM32(*builder, ir->pc,
                              t2c_gen_PC_addr(start, builder, ir));