[PATCH v2 1/2] target/riscv: iterate over a table of decoders

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

To split up the decoder into multiple functions (both to support
vendor-specific opcodes in separate files and to simplify maintenance
of orthogonal extensions), this changes decode_op to iterate over a
table of decoders predicated on guard functions.

This commit only adds the new structure and the table, allowing for
the easy addition of additional decoders in the future.

Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
---

Changes in v2:
- (new patch) iterate over a table of guarded decoder functions

 target/riscv/translate.c | 38 ++++++++++++++++++++++++++++++++------
 1 file changed, 32 insertions(+), 6 deletions(-)

diff --git a/target/riscv/translate.c b/target/riscv/translate.c
index 615048ec87..2cbf9cbb6f 100644
--- a/target/riscv/translate.c
+++ b/target/riscv/translate.c
@@ -116,6 +116,12 @@ static inline bool has_ext(DisasContext *ctx, uint32_t ext)
     return ctx->misa_ext & ext;
 }
 
+static inline bool always_true_p(CPURISCVState *env  __attribute__((__unused__)),
+                                 DisasContext *ctx  __attribute__((__unused__)))
+{
+    return true;
+}
+
 #ifdef TARGET_RISCV32
 #define get_xl(ctx)    MXL_RV32
 #elif defined(CONFIG_USER_ONLY)
@@ -844,16 +850,28 @@ static uint32_t opcode_at(DisasContextBase *dcbase, target_ulong pc)
 
 static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
 {
-    /* check for compressed insn */
+    /* If not handled, we'll raise an illegal instruction exception */
+    bool handled = false;
+
+    /*
+     * A table with predicate (i.e., guard) functions and decoder functions
+     * that are tested in-order until a decoder matches onto the opcode.
+     */
+    const struct {
+        bool (*guard_func)(CPURISCVState *, DisasContext *);
+        bool (*decode_func)(DisasContext *, uint32_t);
+    } decoders[] = {
+        { always_true_p,  decode_insn32 },
+    };
+
+    /* Check for compressed insn */
     if (extract16(opcode, 0, 2) != 3) {
         if (!has_ext(ctx, RVC)) {
             gen_exception_illegal(ctx);
         } else {
             ctx->opcode = opcode;
             ctx->pc_succ_insn = ctx->base.pc_next + 2;
-            if (!decode_insn16(ctx, opcode)) {
-                gen_exception_illegal(ctx);
-            }
+            handled = decode_insn16(ctx, opcode);
         }
     } else {
         uint32_t opcode32 = opcode;
@@ -862,10 +880,18 @@ static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
                                              ctx->base.pc_next + 2));
         ctx->opcode = opcode32;
         ctx->pc_succ_insn = ctx->base.pc_next + 4;
-        if (!decode_insn32(ctx, opcode32)) {
-            gen_exception_illegal(ctx);
+
+        for (size_t i = 0; i < ARRAY_SIZE(decoders); ++i) {
+            if (!decoders[i].guard_func(env, ctx))
+                continue;
+
+            if ((handled = decoders[i].decode_func(ctx, opcode32)))
+                break;
         }
     }
+
+    if (!handled)
+        gen_exception_illegal(ctx);
 }
 
 static void riscv_tr_init_disas_context(DisasContextBase *dcbase, CPUState *cs)
-- 
2.33.1