tcg: Clean up from 'next_tb'

The value returned from tcg_qemu_tb_exec() is the value passed to the
corresponding tcg_gen_exit_tb() at translation time of the last TB
attempted to execute. It is a little confusing to store it in a variable
named 'next_tb'. In fact, it is a combination of 4-byte aligned pointer
and additional information in its two least significant bits. Break it
down right away into two variables named 'last_tb' and 'tb_exit' which
are a pointer to the last TB attempted to execute and the TB exit
reason, correspondingly. This simplifies the code and improves its
readability.

Correct a misleading documentation comment for tcg_qemu_tb_exec() and
fix logging in cpu_tb_exec(). Also rename a misleading 'next_tb' in
another couple of places.

Signed-off-by: Sergey Fedorov <serge.fdrv@gmail.com>
Signed-off-by: Sergey Fedorov <sergey.fedorov@linaro.org>
Signed-off-by: Richard Henderson <rth@twiddle.net>

cpu-exec.c

@@ -136,7 +136,9 @@ static void init_delay_params(SyncClocks *sc, const CPUState *cpu)
 static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 {
     CPUArchState *env = cpu->env_ptr;
-    uintptr_t next_tb;
+    uintptr_t ret;
+    TranslationBlock *last_tb;
+    int tb_exit;
     uint8_t *tb_ptr = itb->tc_ptr;
 
     qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
@@ -160,36 +162,37 @@ static inline tcg_target_ulong cpu_tb_exec(CPUState *cpu, TranslationBlock *itb)
 #endif /* DEBUG_DISAS */
 
     cpu->can_do_io = !use_icount;
-    next_tb = tcg_qemu_tb_exec(env, tb_ptr);
+    ret = tcg_qemu_tb_exec(env, tb_ptr);
     cpu->can_do_io = 1;
-    trace_exec_tb_exit((void *) (next_tb & ~TB_EXIT_MASK),
-                       next_tb & TB_EXIT_MASK);
+    last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
+    tb_exit = ret & TB_EXIT_MASK;
+    trace_exec_tb_exit(last_tb, tb_exit);
 
-    if ((next_tb & TB_EXIT_MASK) > TB_EXIT_IDX1) {
+    if (tb_exit > TB_EXIT_IDX1) {
         /* We didn't start executing this TB (eg because the instruction
          * counter hit zero); we must restore the guest PC to the address
          * of the start of the TB.
          */
         CPUClass *cc = CPU_GET_CLASS(cpu);
-        TranslationBlock *tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
-        qemu_log_mask_and_addr(CPU_LOG_EXEC, itb->pc,
+        qemu_log_mask_and_addr(CPU_LOG_EXEC, last_tb->pc,
                                "Stopped execution of TB chain before %p ["
                                TARGET_FMT_lx "] %s\n",
-                               itb->tc_ptr, itb->pc, lookup_symbol(itb->pc));
+                               last_tb->tc_ptr, last_tb->pc,
+                               lookup_symbol(last_tb->pc));
         if (cc->synchronize_from_tb) {
-            cc->synchronize_from_tb(cpu, tb);
+            cc->synchronize_from_tb(cpu, last_tb);
         } else {
             assert(cc->set_pc);
-            cc->set_pc(cpu, tb->pc);
+            cc->set_pc(cpu, last_tb->pc);
         }
     }
-    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_REQUESTED) {
+    if (tb_exit == TB_EXIT_REQUESTED) {
         /* We were asked to stop executing TBs (probably a pending
          * interrupt. We've now stopped, so clear the flag.
          */
         cpu->tcg_exit_req = 0;
     }
-    return next_tb;
+    return ret;
 }
 
 #ifndef CONFIG_USER_ONLY
@@ -358,8 +361,8 @@ int cpu_exec(CPUState *cpu)
     CPUArchState *env = &x86_cpu->env;
 #endif
     int ret, interrupt_request;
-    TranslationBlock *tb;
-    uintptr_t next_tb;
+    TranslationBlock *tb, *last_tb;
+    int tb_exit = 0;
     SyncClocks sc;
 
     /* replay_interrupt may need current_cpu */
@@ -442,7 +445,7 @@ int cpu_exec(CPUState *cpu)
 #endif
             }
 
-            next_tb = 0; /* force lookup of first TB */
+            last_tb = NULL; /* forget the last executed TB after exception */
             for(;;) {
                 interrupt_request = cpu->interrupt_request;
                 if (unlikely(interrupt_request)) {
@@ -487,7 +490,7 @@ int cpu_exec(CPUState *cpu)
                     else {
                         replay_interrupt();
                         if (cc->cpu_exec_interrupt(cpu, interrupt_request)) {
-                            next_tb = 0;
+                            last_tb = NULL;
                         }
                     }
                     /* Don't use the cached interrupt_request value,
@@ -496,7 +499,7 @@ int cpu_exec(CPUState *cpu)
                         cpu->interrupt_request &= ~CPU_INTERRUPT_EXITTB;
                         /* ensure that no TB jump will be modified as
                            the program flow was changed */
-                        next_tb = 0;
+                        last_tb = NULL;
                     }
                 }
                 if (unlikely(cpu->exit_request
@@ -513,22 +516,24 @@ int cpu_exec(CPUState *cpu)
                     /* as some TB could have been invalidated because
                        of memory exceptions while generating the code, we
                        must recompute the hash index here */
-                    next_tb = 0;
+                    last_tb = NULL;
                     tcg_ctx.tb_ctx.tb_invalidated_flag = 0;
                 }
                 /* See if we can patch the calling TB. */
-                if (next_tb != 0 && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
-                    tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
-                                next_tb & TB_EXIT_MASK, tb);
+                if (last_tb && !qemu_loglevel_mask(CPU_LOG_TB_NOCHAIN)) {
+                    tb_add_jump(last_tb, tb_exit, tb);
                 }
                 tb_unlock();
                 if (likely(!cpu->exit_request)) {
+                    uintptr_t ret;
                     trace_exec_tb(tb, tb->pc);
                     /* execute the generated code */
                     cpu->current_tb = tb;
-                    next_tb = cpu_tb_exec(cpu, tb);
+                    ret = cpu_tb_exec(cpu, tb);
                     cpu->current_tb = NULL;
-                    switch (next_tb & TB_EXIT_MASK) {
+                    last_tb = (TranslationBlock *)(ret & ~TB_EXIT_MASK);
+                    tb_exit = ret & TB_EXIT_MASK;
+                    switch (tb_exit) {
                     case TB_EXIT_REQUESTED:
                         /* Something asked us to stop executing
                          * chained TBs; just continue round the main
@@ -541,7 +546,7 @@ int cpu_exec(CPUState *cpu)
                          * or cpu->interrupt_request.
                          */
                         smp_rmb();
-                        next_tb = 0;
+                        last_tb = NULL;
                         break;
                     case TB_EXIT_ICOUNT_EXPIRED:
                     {
@@ -559,12 +564,12 @@ int cpu_exec(CPUState *cpu)
                         } else {
                             if (insns_left > 0) {
                                 /* Execute remaining instructions.  */
-                                tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
-                                cpu_exec_nocache(cpu, insns_left, tb, false);
+                                cpu_exec_nocache(cpu, insns_left,
+                                                 last_tb, false);
                                 align_clocks(&sc, cpu);
                             }
                             cpu->exception_index = EXCP_INTERRUPT;
-                            next_tb = 0;
+                            last_tb = NULL;
                             cpu_loop_exit(cpu);
                         }
                         break;

tcg/tcg.h

@@ -925,7 +925,7 @@ static inline unsigned get_mmuidx(TCGMemOpIdx oi)
 
 /**
  * tcg_qemu_tb_exec:
- * @env: CPUArchState * for the CPU
+ * @env: pointer to CPUArchState for the CPU
  * @tb_ptr: address of generated code for the TB to execute
  *
  * Start executing code from a given translation block.
@@ -936,30 +936,31 @@ static inline unsigned get_mmuidx(TCGMemOpIdx oi)
  * which has not yet been directly linked, or an asynchronous
  * event such as an interrupt needs handling.
  *
- * The return value is a pointer to the next TB to execute
- * (if known; otherwise zero). This pointer is assumed to be
- * 4-aligned, and the bottom two bits are used to return further
- * information:
+ * Return: The return value is the value passed to the corresponding
+ * tcg_gen_exit_tb() at translation time of the last TB attempted to execute.
+ * The value is either zero or a 4-byte aligned pointer to that TB combined
+ * with additional information in its two least significant bits. The
+ * additional information is encoded as follows:
  *  0, 1: the link between this TB and the next is via the specified
  *        TB index (0 or 1). That is, we left the TB via (the equivalent
  *        of) "goto_tb <index>". The main loop uses this to determine
  *        how to link the TB just executed to the next.
  *  2:    we are using instruction counting code generation, and we
  *        did not start executing this TB because the instruction counter
- *        would hit zero midway through it. In this case the next-TB pointer
+ *        would hit zero midway through it. In this case the pointer
  *        returned is the TB we were about to execute, and the caller must
  *        arrange to execute the remaining count of instructions.
  *  3:    we stopped because the CPU's exit_request flag was set
  *        (usually meaning that there is an interrupt that needs to be
- *        handled). The next-TB pointer returned is the TB we were
- *        about to execute when we noticed the pending exit request.
+ *        handled). The pointer returned is the TB we were about to execute
+ *        when we noticed the pending exit request.
  *
  * If the bottom two bits indicate an exit-via-index then the CPU
  * state is correctly synchronised and ready for execution of the next
  * TB (and in particular the guest PC is the address to execute next).
  * Otherwise, we gave up on execution of this TB before it started, and
  * the caller must fix up the CPU state by calling the CPU's
- * synchronize_from_tb() method with the next-TB pointer we return (falling
+ * synchronize_from_tb() method with the TB pointer we return (falling
  * back to calling the CPU's set_pc method with tb->pb if no
  * synchronize_from_tb() method exists).
  *

tci.c

@@ -467,7 +467,7 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
 {
     long tcg_temps[CPU_TEMP_BUF_NLONGS];
     uintptr_t sp_value = (uintptr_t)(tcg_temps + CPU_TEMP_BUF_NLONGS);
-    uintptr_t next_tb = 0;
+    uintptr_t ret = 0;
 
     tci_reg[TCG_AREG0] = (tcg_target_ulong)env;
     tci_reg[TCG_REG_CALL_STACK] = sp_value;
@@ -1085,7 +1085,7 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
             /* QEMU specific operations. */
 
         case INDEX_op_exit_tb:
-            next_tb = *(uint64_t *)tb_ptr;
+            ret = *(uint64_t *)tb_ptr;
             goto exit;
             break;
         case INDEX_op_goto_tb:
@@ -1243,5 +1243,5 @@ uintptr_t tcg_qemu_tb_exec(CPUArchState *env, uint8_t *tb_ptr)
         tci_assert(tb_ptr == old_code_ptr + op_size);
     }
 exit:
-    return next_tb;
+    return ret;
 }

trace-events

@@ -1614,7 +1614,7 @@ kvm_failed_spr_get(int str, const char *msg) "Warning: Unable to retrieve SPR %d
 # cpu-exec.c
 disable exec_tb(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
 disable exec_tb_nocache(void *tb, uintptr_t pc) "tb:%p pc=0x%"PRIxPTR
-disable exec_tb_exit(void *next_tb, unsigned int flags) "tb:%p flags=%x"
+disable exec_tb_exit(void *last_tb, unsigned int flags) "tb:%p flags=%x"
 
 # translate-all.c
 translate_block(void *tb, uintptr_t pc, uint8_t *tb_code) "tb:%p, pc:0x%"PRIxPTR", tb_code:%p"