[1/6] tcg: Document tcg_qemu_tb_exec() and provide constants for low bit uses

diff --git a/cpu-exec.c b/cpu-exec.c
index 9fcfe9e0..ea63e7d 100644
--- a/cpu-exec.c
+++ b/cpu-exec.c
@@ -72,7 +72,7 @@  static void cpu_exec_nocache(CPUArchState *env, int max_cycles,
     next_tb = tcg_qemu_tb_exec(env, tb->tc_ptr);
     cpu->current_tb = NULL;
 
-    if ((next_tb & 3) == 2) {
+    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_ICOUNT_EXPIRED) {
         /* Restore PC.  This may happen if async event occurs before
            the TB starts executing.  */
         cpu_pc_from_tb(env, tb);
@@ -584,7 +584,8 @@  int cpu_exec(CPUArchState *env)
                    spans two pages, we cannot safely do a direct
                    jump. */
                 if (next_tb != 0 && tb->page_addr[1] == -1) {
-                    tb_add_jump((TranslationBlock *)(next_tb & ~3), next_tb & 3, tb);
+                    tb_add_jump((TranslationBlock *)(next_tb & ~TB_EXIT_MASK),
+                                next_tb & TB_EXIT_MASK, tb);
                 }
                 spin_unlock(&tcg_ctx.tb_ctx.tb_lock);
 
@@ -598,10 +599,10 @@  int cpu_exec(CPUArchState *env)
                     tc_ptr = tb->tc_ptr;
                     /* execute the generated code */
                     next_tb = tcg_qemu_tb_exec(env, tc_ptr);
-                    if ((next_tb & 3) == 2) {
+                    if ((next_tb & TB_EXIT_MASK) == TB_EXIT_ICOUNT_EXPIRED) {
                         /* Instruction counter expired.  */
                         int insns_left;
-                        tb = (TranslationBlock *)(next_tb & ~3);
+                        tb = (TranslationBlock *)(next_tb & ~TB_EXIT_MASK);
                         /* Restore PC.  */
                         cpu_pc_from_tb(env, tb);
                         insns_left = env->icount_decr.u32;
diff --git a/include/exec/gen-icount.h b/include/exec/gen-icount.h
index 8043b3b..c858a73 100644
--- a/include/exec/gen-icount.h
+++ b/include/exec/gen-icount.h
@@ -32,7 +32,7 @@  static void gen_icount_end(TranslationBlock *tb, int num_insns)
     if (use_icount) {
         *icount_arg = num_insns;
         gen_set_label(icount_label);
-        tcg_gen_exit_tb((tcg_target_long)tb + 2);
+        tcg_gen_exit_tb((tcg_target_long)tb + TB_EXIT_ICOUNT_EXPIRED);
     }
 }
 
diff --git a/tcg/tcg.h b/tcg/tcg.h
index 51c8176..2ebde07 100644
--- a/tcg/tcg.h
+++ b/tcg/tcg.h
@@ -669,7 +669,49 @@  TCGv_i64 tcg_const_i64(int64_t val);
 TCGv_i32 tcg_const_local_i32(int32_t val);
 TCGv_i64 tcg_const_local_i64(int64_t val);
 
-/* TCG targets may use a different definition of tcg_qemu_tb_exec. */
+/**
+ * tcg_qemu_tb_exec:
+ * @env: CPUArchState * for the CPU
+ * @tb_ptr: address of generated code for the TB to execute
+ *
+ * Start executing code from a given translation block.
+ * Where translation blocks have been linked, execution
+ * may proceed from the given TB into successive ones.
+ * Control eventually returns only when some action is needed
+ * from the top-level loop: either control must pass to a TB
+ * 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:
+ *  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
+ *        returned is the TB we were about to execute, and the caller must
+ *        arrange to execute the remaining count of instructions.
+ *
+ * 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 cpu_pc_from_tb()
+ * with the next-TB pointer we return.
+ *
+ * Note that TCG targets may use a different definition of tcg_qemu_tb_exec
+ * to this default (which just calls the prologue.code emitted by
+ * tcg_target_qemu_prologue()).
+ */
+#define TB_EXIT_MASK 3
+#define TB_EXIT_IDX0 0
+#define TB_EXIT_IDX1 1
+#define TB_EXIT_ICOUNT_EXPIRED 2
+
 #if !defined(tcg_qemu_tb_exec)
 # define tcg_qemu_tb_exec(env, tb_ptr) \
     ((tcg_target_ulong (*)(void *, void *))tcg_ctx.code_gen_prologue)(env, \