[Qemu-devel] [PATCH v4 07/11] target/arm: optimize indirect branches

["Emilio G. Cota" <cota@braap.org>]

Speed up indirect branches by jumping to the target if it is valid.

Softmmu measurements (see later commit for user-mode results):

Note: baseline (i.e. speedup == 1x) is QEMU v2.9.0.

- Impact on Boot time

| setup  | ARM debian jessie boot+shutdown time | stddev |
|--------+--------------------------------------+--------|
| v2.9.0 |                                 8.84 |   0.07 |
| +cross |                                 8.85 |   0.03 |
| +jr    |                                 8.83 |   0.06 |

-                            NBench, arm-softmmu (debian jessie guest). Host: Intel i7-4790K @ 4.00GHz

  1.3x +-+-------------------------------------------------------------------------------------------------------------+-+
       |                                                                                                                 |
       |   cross                                                          ####                                           |
 1.25x +cross+jr..........................................................#++#.........................................+-+
       |                                                        ####      #  #                                           |
       |                                                     +++#  #      #  #                                           |
       |                                      +++            ****  #      #  #                                           |
  1.2x +-+...................................####............*..*..#......#..#.........................................+-+
       |                                  ****  #            *  *  #      #  #     ####                                  |
       |                                  *  *  #            *  *  #      #  #     #  #                                  |
 1.15x +-+................................*..*..#............*..*..#......#..#.....#..#................................+-+
       |                                  *  *  #            *  *  #      #  #     #  #                                  |
       |                                  *  *  #      ####  *  *  #      #  #     #  #                                  |
       |                                  *  *  #      #  #  *  *  #      #  #     #  #                         ####     |
  1.1x +-+................................*..*..#......#..#..*..*..#......#..#.....#..#.........................#..#...+-+
       |                                  *  *  #      #  #  *  *  #      #  #     #  #                         #  #     |
       |                                  *  *  #      #  #  *  *  #      #  #     #  #                         #  #     |
 1.05x +-+..........................####..*..*..#......#..#..*..*..#......#..#.....#..#......+++............*****..#...+-+
       |                        *****  #  *  *  #      #  #  *  *  #  *****  #     #  #   +++ |    ****###  *   *  #     |
       |                        *+++*  #  *  *  #      #  #  *  *  #  *+++*  #  ****  #  *****###  *  *  #  *   *  #     |
       |     *****###  +++####  *   *  #  *  *  #  *****  #  *  *  #  *   *  #  *  *  #  * | *++#  *  *  #  *   *  #     |
    1x +-++-+*+++*-+#++****++#++*+-+*++#+-*++*++#-+*+++*-+#++*++*++#++*+-+*++#+-*++*++#-+*+++*-+#++*++*++#++*+-+*++#+-++-+
       |     *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #     |
       |     *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #  *  *  #  *   *  #     |
 0.95x +-+---*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###--****###--*****###---+-+
       ASSIGNMENT BITFIELD   FOURFP EMULATION   HUFFMAN   LU DECOMPOSITIONEURAL NNUMERIC SOSTRING SORT     hmean
  png: http://imgur.com/eOLmZNR

NB. 'cross' represents the previous commit.

Signed-off-by: Emilio G. Cota <cota@braap.org>
---
 target/arm/translate.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/target/arm/translate.c b/target/arm/translate.c
index 02cad96..d46a576 100644
--- a/target/arm/translate.c
+++ b/target/arm/translate.c
@@ -65,6 +65,7 @@ static TCGv_i32 cpu_R[16];
 TCGv_i32 cpu_CF, cpu_NF, cpu_VF, cpu_ZF;
 TCGv_i64 cpu_exclusive_addr;
 TCGv_i64 cpu_exclusive_val;
+static bool gen_jr;
 
 /* FIXME:  These should be removed.  */
 static TCGv_i32 cpu_F0s, cpu_F1s;
@@ -221,6 +222,7 @@ static void store_reg(DisasContext *s, int reg, TCGv_i32 var)
          */
         tcg_gen_andi_i32(var, var, s->thumb ? ~1 : ~3);
         s->is_jmp = DISAS_JUMP;
+        gen_jr = true;
     }
     tcg_gen_mov_i32(cpu_R[reg], var);
     tcg_temp_free_i32(var);
@@ -893,6 +895,7 @@ static inline void gen_bx_im(DisasContext *s, uint32_t addr)
         tcg_temp_free_i32(tmp);
     }
     tcg_gen_movi_i32(cpu_R[15], addr & ~1);
+    gen_jr = true;
 }
 
 /* Set PC and Thumb state from var.  var is marked as dead.  */
@@ -902,6 +905,7 @@ static inline void gen_bx(DisasContext *s, TCGv_i32 var)
     tcg_gen_andi_i32(cpu_R[15], var, ~1);
     tcg_gen_andi_i32(var, var, 1);
     store_cpu_field(var, thumb);
+    gen_jr = true;
 }
 
 /* Variant of store_reg which uses branch&exchange logic when storing
@@ -12034,6 +12038,20 @@ void gen_intermediate_code(CPUARMState *env, TranslationBlock *tb)
             gen_set_pc_im(dc, dc->pc);
             /* fall through */
         case DISAS_JUMP:
+            /*
+             * gen_jr is not set on every DISAS_JUMP because for some of those
+             * we do want to exit to the exec loop.
+             */
+            if (gen_jr) {
+                TCGv addr = tcg_temp_new();
+
+                gen_jr = false;
+                tcg_gen_extu_i32_tl(addr, cpu_R[15]);
+                tcg_gen_lookup_and_goto_ptr(addr);
+                tcg_temp_free(addr);
+                break;
+            }
+            /* fall through */
         default:
             /* indicate that the hash table must be used to find the next TB */
             tcg_gen_exit_tb(0);
-- 
2.7.4