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.

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20230914174436.1597356-4-richard.henderson@linaro.org>
[PMD: Split patch in 2, extracting set_can_do_io() first]
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>

accel/tcg/translator.c

@@ -18,9 +18,12 @@
 
 static void set_can_do_io(DisasContextBase *db, bool val)
 {
-    tcg_gen_st_i32(tcg_constant_i32(val), 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)
@@ -30,10 +33,6 @@ 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;
-    }
 
     set_can_do_io(db, true);
 
@@ -144,6 +143,7 @@ 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;
 

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;