target/arm: [tcg] Port to tb_start

Incrementally paves the way towards using the generic instruction translation
loop.

Backports commit b14768544fd715a3f1742c10fc36ae81c703cbc1 from qemu

qemu/target/arm/translate.c

@@ -12102,6 +12102,50 @@ static int arm_tr_init_disas_context(DisasContextBase *dcbase,
     return max_insns;
 }
 
+static void arm_tr_tb_start(DisasContextBase *dcbase, CPUState *cpu)
+{
+    DisasContext *dc = container_of(dcbase, DisasContext, base);
+    TCGContext *tcg_ctx = cpu->uc->tcg_ctx;
+
+    /* A note on handling of the condexec (IT) bits:
+     *
+     * We want to avoid the overhead of having to write the updated condexec
+     * bits back to the CPUARMState for every instruction in an IT block. So:
+     * (1) if the condexec bits are not already zero then we write
+     * zero back into the CPUARMState now. This avoids complications trying
+     * to do it at the end of the block. (For example if we don't do this
+     * it's hard to identify whether we can safely skip writing condexec
+     * at the end of the TB, which we definitely want to do for the case
+     * where a TB doesn't do anything with the IT state at all.)
+     * (2) if we are going to leave the TB then we call gen_set_condexec()
+     * which will write the correct value into CPUARMState if zero is wrong.
+     * This is done both for leaving the TB at the end, and for leaving
+     * it because of an exception we know will happen, which is done in
+     * gen_exception_insn(). The latter is necessary because we need to
+     * leave the TB with the PC/IT state just prior to execution of the
+     * instruction which caused the exception.
+     * (3) if we leave the TB unexpectedly (eg a data abort on a load)
+     * then the CPUARMState will be wrong and we need to reset it.
+     * This is handled in the same way as restoration of the
+     * PC in these situations; we save the value of the condexec bits
+     * for each PC via tcg_gen_insn_start(), and restore_state_to_opc()
+     * then uses this to restore them after an exception.
+     *
+     * Note that there are no instructions which can read the condexec
+     * bits, and none which can write non-static values to them, so
+     * we don't need to care about whether CPUARMState is correct in the
+     * middle of a TB.
+     */
+
+    /* Reset the conditional execution bits immediately. This avoids
+       complications trying to do it at the end of the block.  */
+    if (dc->condexec_mask || dc->condexec_cond) {
+        TCGv_i32 tmp = tcg_temp_new_i32(tcg_ctx);
+        tcg_gen_movi_i32(tcg_ctx, tmp, 0);
+        store_cpu_field(tcg_ctx, tmp, condexec_bits);
+    }
+}
+
 /* generate intermediate code for basic block 'tb'.  */
 void gen_intermediate_code(CPUState *cs, TranslationBlock *tb)
 {
@@ -12163,45 +12207,8 @@ void gen_intermediate_code(CPUState *cs, TranslationBlock *tb)
     }
 
     gen_tb_start(tcg_ctx, tb);
+    arm_tr_tb_start(&dc->base, cs);
 
-    /* A note on handling of the condexec (IT) bits:
-     *
-     * We want to avoid the overhead of having to write the updated condexec
-     * bits back to the CPUARMState for every instruction in an IT block. So:
-     * (1) if the condexec bits are not already zero then we write
-     * zero back into the CPUARMState now. This avoids complications trying
-     * to do it at the end of the block. (For example if we don't do this
-     * it's hard to identify whether we can safely skip writing condexec
-     * at the end of the TB, which we definitely want to do for the case
-     * where a TB doesn't do anything with the IT state at all.)
-     * (2) if we are going to leave the TB then we call gen_set_condexec()
-     * which will write the correct value into CPUARMState if zero is wrong.
-     * This is done both for leaving the TB at the end, and for leaving
-     * it because of an exception we know will happen, which is done in
-     * gen_exception_insn(). The latter is necessary because we need to
-     * leave the TB with the PC/IT state just prior to execution of the
-     * instruction which caused the exception.
-     * (3) if we leave the TB unexpectedly (eg a data abort on a load)
-     * then the CPUARMState will be wrong and we need to reset it.
-     * This is handled in the same way as restoration of the
-     * PC in these situations; we save the value of the condexec bits
-     * for each PC via tcg_gen_insn_start(), and restore_state_to_opc()
-     * then uses this to restore them after an exception.
-     *
-     * Note that there are no instructions which can read the condexec
-     * bits, and none which can write non-static values to them, so
-     * we don't need to care about whether CPUARMState is correct in the
-     * middle of a TB.
-     */
-
-    /* Reset the conditional execution bits immediately. This avoids
-       complications trying to do it at the end of the block.  */
-    if (dc->condexec_mask || dc->condexec_cond)
-      {
-        TCGv_i32 tmp = tcg_temp_new_i32(tcg_ctx);
-        tcg_gen_movi_i32(tcg_ctx, tmp, 0);
-        store_cpu_field(tcg_ctx, tmp, condexec_bits);
-      }
     do {
         dc->base.num_insns++;
         dc->insn_start_idx = tcg_op_buf_count(tcg_ctx);