target/riscv: Minimize the calls to decode_save_opc

The set of instructions that require decode_save_opc for
unwinding is really fairly small -- only insns that can
raise ILLEGAL_INSN at runtime.  This includes CSR, anything
that uses a *new* fp rounding mode, and many privileged insns.

Since unwind info is stored as the difference from the
previous insn, storing a 0 for most insns minimizes the
size of the unwind info.

Booting a debian kernel image to the missing rootfs panic yields

- gen code size       22226819/1026886656
+ gen code size       21601907/1026886656

on 41k TranslationBlocks, a savings of 610kB or a bit less than 3%.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220604231004.49990-4-richard.henderson@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/insn_trans/trans_privileged.c.inc

@@ -75,6 +75,7 @@ static bool trans_sret(DisasContext *ctx, arg_sret *a)
 {
 #ifndef CONFIG_USER_ONLY
     if (has_ext(ctx, RVS)) {
+        decode_save_opc(ctx);
         gen_helper_sret(cpu_pc, cpu_env);
         tcg_gen_exit_tb(NULL, 0); /* no chaining */
         ctx->base.is_jmp = DISAS_NORETURN;
@@ -90,6 +91,7 @@ static bool trans_sret(DisasContext *ctx, arg_sret *a)
 static bool trans_mret(DisasContext *ctx, arg_mret *a)
 {
 #ifndef CONFIG_USER_ONLY
+    decode_save_opc(ctx);
     gen_helper_mret(cpu_pc, cpu_env);
     tcg_gen_exit_tb(NULL, 0); /* no chaining */
     ctx->base.is_jmp = DISAS_NORETURN;
@@ -102,6 +104,7 @@ static bool trans_mret(DisasContext *ctx, arg_mret *a)
 static bool trans_wfi(DisasContext *ctx, arg_wfi *a)
 {
 #ifndef CONFIG_USER_ONLY
+    decode_save_opc(ctx);
     gen_set_pc_imm(ctx, ctx->pc_succ_insn);
     gen_helper_wfi(cpu_env);
     return true;
@@ -113,6 +116,7 @@ static bool trans_wfi(DisasContext *ctx, arg_wfi *a)
 static bool trans_sfence_vma(DisasContext *ctx, arg_sfence_vma *a)
 {
 #ifndef CONFIG_USER_ONLY
+    decode_save_opc(ctx);
     gen_helper_tlb_flush(cpu_env);
     return true;
 #endif

target/riscv/insn_trans/trans_rvh.c.inc

@@ -169,6 +169,7 @@ static bool trans_hfence_gvma(DisasContext *ctx, arg_sfence_vma *a)
 {
     REQUIRE_EXT(ctx, RVH);
 #ifndef CONFIG_USER_ONLY
+    decode_save_opc(ctx);
     gen_helper_hyp_gvma_tlb_flush(cpu_env);
     return true;
 #endif
@@ -179,6 +180,7 @@ static bool trans_hfence_vvma(DisasContext *ctx, arg_sfence_vma *a)
 {
     REQUIRE_EXT(ctx, RVH);
 #ifndef CONFIG_USER_ONLY
+    decode_save_opc(ctx);
     gen_helper_hyp_tlb_flush(cpu_env);
     return true;
 #endif

target/riscv/insn_trans/trans_rvi.c.inc

@@ -818,6 +818,8 @@ static bool trans_fence_i(DisasContext *ctx, arg_fence_i *a)
 
 static bool do_csr_post(DisasContext *ctx)
 {
+    /* The helper may raise ILLEGAL_INSN -- record binv for unwind. */
+    decode_save_opc(ctx);
     /* We may have changed important cpu state -- exit to main loop. */
     gen_set_pc_imm(ctx, ctx->pc_succ_insn);
     tcg_gen_exit_tb(NULL, 0);

target/riscv/translate.c

@@ -206,6 +206,13 @@ static void gen_check_nanbox_s(TCGv_i64 out, TCGv_i64 in)
     tcg_gen_movcond_i64(TCG_COND_GEU, out, in, t_max, in, t_nan);
 }
 
+static void decode_save_opc(DisasContext *ctx)
+{
+    assert(ctx->insn_start != NULL);
+    tcg_set_insn_start_param(ctx->insn_start, 1, ctx->opcode);
+    ctx->insn_start = NULL;
+}
+
 static void gen_set_pc_imm(DisasContext *ctx, target_ulong dest)
 {
     if (get_xl(ctx) == MXL_RV32) {
@@ -635,6 +642,8 @@ static void gen_set_rm(DisasContext *ctx, int rm)
         return;
     }
 
+    /* The helper may raise ILLEGAL_INSN -- record binv for unwind. */
+    decode_save_opc(ctx);
     gen_helper_set_rounding_mode(cpu_env, tcg_constant_i32(rm));
 }
 
@@ -1013,13 +1022,6 @@ static uint32_t opcode_at(DisasContextBase *dcbase, target_ulong pc)
 /* Include decoders for factored-out extensions */
 #include "decode-XVentanaCondOps.c.inc"
 
-static inline void decode_save_opc(DisasContext *ctx, target_ulong opc)
-{
-    assert(ctx->insn_start != NULL);
-    tcg_set_insn_start_param(ctx->insn_start, 1, opc);
-    ctx->insn_start = NULL;
-}
-
 static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
 {
     /*
@@ -1036,7 +1038,6 @@ static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
 
     /* Check for compressed insn */
     if (extract16(opcode, 0, 2) != 3) {
-        decode_save_opc(ctx, opcode);
         if (!has_ext(ctx, RVC)) {
             gen_exception_illegal(ctx);
         } else {
@@ -1051,7 +1052,6 @@ static void decode_opc(CPURISCVState *env, DisasContext *ctx, uint16_t opcode)
         opcode32 = deposit32(opcode32, 16, 16,
                              translator_lduw(env, &ctx->base,
                                              ctx->base.pc_next + 2));
-        decode_save_opc(ctx, opcode32);
         ctx->opcode = opcode32;
         ctx->pc_succ_insn = ctx->base.pc_next + 4;