[PATCH v1 4/5] target/riscv: progressively load the instruction during decode

["Alex Bennée" <alex.bennee@linaro.org>]

The plugin system would throw up a harmless warning when it detected
that a disassembly of an instruction didn't use all it's bytes. Fix
the riscv decoder to only load the instruction bytes it needs as it
needs them.

This drops opcode from the ctx in favour if passing the appropriately
sized opcode down a few levels of the decode.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
---
 target/riscv/translate.c | 39 ++++++++++++++++++++-------------------
 1 file changed, 20 insertions(+), 19 deletions(-)

diff --git a/target/riscv/translate.c b/target/riscv/translate.c
index 14dc71156be..99f2bcf177c 100644
--- a/target/riscv/translate.c
+++ b/target/riscv/translate.c
@@ -44,7 +44,6 @@ typedef struct DisasContext {
     /* pc_succ_insn points to the instruction following base.pc_next */
     target_ulong pc_succ_insn;
     target_ulong priv_ver;
-    uint32_t opcode;
     uint32_t mstatus_fs;
     uint32_t misa;
     uint32_t mem_idx;
@@ -492,45 +491,45 @@ static void gen_set_rm(DisasContext *ctx, int rm)
     tcg_temp_free_i32(t0);
 }
 
-static void decode_RV32_64C0(DisasContext *ctx)
+static void decode_RV32_64C0(DisasContext *ctx, uint16_t opcode)
 {
-    uint8_t funct3 = extract32(ctx->opcode, 13, 3);
-    uint8_t rd_rs2 = GET_C_RS2S(ctx->opcode);
-    uint8_t rs1s = GET_C_RS1S(ctx->opcode);
+    uint8_t funct3 = extract32(opcode, 13, 3);
+    uint8_t rd_rs2 = GET_C_RS2S(opcode);
+    uint8_t rs1s = GET_C_RS1S(opcode);
 
     switch (funct3) {
     case 3:
 #if defined(TARGET_RISCV64)
         /* C.LD(RV64/128) -> ld rd', offset[7:3](rs1')*/
         gen_load_c(ctx, OPC_RISC_LD, rd_rs2, rs1s,
-                 GET_C_LD_IMM(ctx->opcode));
+                 GET_C_LD_IMM(opcode));
 #else
         /* C.FLW (RV32) -> flw rd', offset[6:2](rs1')*/
         gen_fp_load(ctx, OPC_RISC_FLW, rd_rs2, rs1s,
-                    GET_C_LW_IMM(ctx->opcode));
+                    GET_C_LW_IMM(opcode));
 #endif
         break;
     case 7:
 #if defined(TARGET_RISCV64)
         /* C.SD (RV64/128) -> sd rs2', offset[7:3](rs1')*/
         gen_store_c(ctx, OPC_RISC_SD, rs1s, rd_rs2,
-                  GET_C_LD_IMM(ctx->opcode));
+                  GET_C_LD_IMM(opcode));
 #else
         /* C.FSW (RV32) -> fsw rs2', offset[6:2](rs1')*/
         gen_fp_store(ctx, OPC_RISC_FSW, rs1s, rd_rs2,
-                     GET_C_LW_IMM(ctx->opcode));
+                     GET_C_LW_IMM(opcode));
 #endif
         break;
     }
 }
 
-static void decode_RV32_64C(DisasContext *ctx)
+static void decode_RV32_64C(DisasContext *ctx, uint16_t opcode)
 {
-    uint8_t op = extract32(ctx->opcode, 0, 2);
+    uint8_t op = extract32(opcode, 0, 2);
 
     switch (op) {
     case 0:
-        decode_RV32_64C0(ctx);
+        decode_RV32_64C0(ctx, opcode);
         break;
     }
 }
@@ -709,22 +708,24 @@ static bool gen_shift(DisasContext *ctx, arg_r *a,
 /* Include the auto-generated decoder for 16 bit insn */
 #include "decode_insn16.inc.c"
 
-static void decode_opc(DisasContext *ctx)
+static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
 {
     /* check for compressed insn */
-    if (extract32(ctx->opcode, 0, 2) != 3) {
+    if (extract32(opcode, 0, 2) != 3) {
         if (!has_ext(ctx, RVC)) {
             gen_exception_illegal(ctx);
         } else {
             ctx->pc_succ_insn = ctx->base.pc_next + 2;
-            if (!decode_insn16(ctx, ctx->opcode)) {
+            if (!decode_insn16(ctx, opcode)) {
                 /* fall back to old decoder */
-                decode_RV32_64C(ctx);
+                decode_RV32_64C(ctx, opcode);
             }
         }
     } else {
+        uint32_t opcode32 = opcode;
+        opcode32 = deposit32(opcode32, 16, 16, translator_lduw(env, ctx->base.pc_next + 2));
         ctx->pc_succ_insn = ctx->base.pc_next + 4;
-        if (!decode_insn32(ctx, ctx->opcode)) {
+        if (!decode_insn32(ctx, opcode32)) {
             gen_exception_illegal(ctx);
         }
     }
@@ -776,9 +777,9 @@ static void riscv_tr_translate_insn(DisasContextBase *dcbase, CPUState *cpu)
 {
     DisasContext *ctx = container_of(dcbase, DisasContext, base);
     CPURISCVState *env = cpu->env_ptr;
+    uint16_t opcode16 = translator_lduw(env, ctx->base.pc_next);
 
-    ctx->opcode = translator_ldl(env, ctx->base.pc_next);
-    decode_opc(ctx);
+    decode_opc(env, ctx, opcode16);
     ctx->base.pc_next = ctx->pc_succ_insn;
 
     if (ctx->base.is_jmp == DISAS_NEXT) {
-- 
2.20.1