accel/tcg: Track current value of can_do_io in the TB

Simplify translator_io_start by recording the current
known value of can_do_io within DisasContextBase.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
(cherry picked from commit 0ca41cc)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

accel/tcg/translator.c

@@ -16,11 +16,14 @@
 #include "tcg/tcg-op-common.h"
 #include "internal.h"
 
-static void gen_io_start(void)
+static void set_can_do_io(DisasContextBase *db, bool val)
 {
-    tcg_gen_st_i32(tcg_constant_i32(1), cpu_env,
-                   offsetof(ArchCPU, parent_obj.can_do_io) -
-                   offsetof(ArchCPU, env));
+    if (db->saved_can_do_io != val) {
+        db->saved_can_do_io = val;
+        tcg_gen_st_i32(tcg_constant_i32(val), cpu_env,
+                       offsetof(ArchCPU, parent_obj.can_do_io) -
+                       offsetof(ArchCPU, env));
+    }
 }
 
 bool translator_io_start(DisasContextBase *db)
@@ -30,12 +33,8 @@ bool translator_io_start(DisasContextBase *db)
     if (!(cflags & CF_USE_ICOUNT)) {
         return false;
     }
-    if (db->num_insns == db->max_insns && (cflags & CF_LAST_IO)) {
-        /* Already started in translator_loop. */
-        return true;
-    }
 
-    gen_io_start();
+    set_can_do_io(db, true);
 
     /*
      * Ensure that this instruction will be the last in the TB.
@@ -47,7 +46,7 @@ bool translator_io_start(DisasContextBase *db)
     return true;
 }
 
-static TCGOp *gen_tb_start(uint32_t cflags)
+static TCGOp *gen_tb_start(DisasContextBase *db, uint32_t cflags)
 {
     TCGv_i32 count = NULL;
     TCGOp *icount_start_insn = NULL;
@@ -91,12 +90,9 @@ static TCGOp *gen_tb_start(uint32_t cflags)
          * cpu->can_do_io is cleared automatically here at the beginning of
          * each translation block.  The cost is minimal and only paid for
          * -icount, plus it would be very easy to forget doing it in the
-         * translator. Doing it here means we don't need a gen_io_end() to
-         * go with gen_io_start().
+         * translator.
          */
-        tcg_gen_st_i32(tcg_constant_i32(0), cpu_env,
-                       offsetof(ArchCPU, parent_obj.can_do_io) -
-                       offsetof(ArchCPU, env));
+        set_can_do_io(db, false);
     }
 
     return icount_start_insn;
@@ -147,14 +143,15 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
     db->num_insns = 0;
     db->max_insns = *max_insns;
     db->singlestep_enabled = cflags & CF_SINGLE_STEP;
+    db->saved_can_do_io = -1;
     db->host_addr[0] = host_pc;
     db->host_addr[1] = NULL;
 
     ops->init_disas_context(db, cpu);
     tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */
 
     /* Start translating.  */
-    icount_start_insn = gen_tb_start(cflags);
+    icount_start_insn = gen_tb_start(db, cflags);
     ops->tb_start(db, cpu);
     tcg_debug_assert(db->is_jmp == DISAS_NEXT);  /* no early exit */
 
@@ -181,7 +178,7 @@ void translator_loop(CPUState *cpu, TranslationBlock *tb, int *max_insns,
            the next instruction.  */
         if (db->num_insns == db->max_insns && (cflags & CF_LAST_IO)) {
             /* Accept I/O on the last instruction.  */
-            gen_io_start();
+            set_can_do_io(db, true);
         }
         ops->translate_insn(db, cpu);
 

include/exec/translator.h

@@ -72,6 +72,7 @@ typedef enum DisasJumpType {
  * @num_insns: Number of translated instructions (including current).
  * @max_insns: Maximum number of instructions to be translated in this TB.
  * @singlestep_enabled: "Hardware" single stepping enabled.
+ * @saved_can_do_io: Known value of cpu->neg.can_do_io, or -1 for unknown.
  *
  * Architecture-agnostic disassembly context.
  */
@@ -83,6 +84,7 @@ typedef struct DisasContextBase {
     int num_insns;
     int max_insns;
     bool singlestep_enabled;
+    int8_t saved_can_do_io;
     void *host_addr[2];
 } DisasContextBase;