Re: [Qemu-devel] [PATCH v3 14/20] tcg-arm: Cleanup goto_tb handling

[Aurelien Jarno]

On Thu, Mar 28, 2013 at 08:32:55AM -0700, Richard Henderson wrote:
> Eliminate 2 disabled code blocks.  Choose the load-to-pc method of
> jumping so that we can eliminate the 16M code_gen_buffer limitation.
> Remove a test in the indirect jump method that is always true.
> 
> Signed-off-by: Richard Henderson <address@hidden>
> ---
>  include/exec/exec-all.h | 23 +++--------------------
>  tcg/arm/tcg-target.c    | 26 ++++++--------------------
>  translate-all.c         |  2 --
>  3 files changed, 9 insertions(+), 42 deletions(-)

I already proposed such a patch, but it seems to improve things only on
some specific cases (kernel boot), while increasing the I/D cache and
TLB pressure.

See https://lists.gnu.org/archive/html/qemu-devel/2012-10/msg01684.html 

> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index e856191..190effa 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -233,26 +233,9 @@ static inline void tb_set_jmp_target1(uintptr_t 
> jmp_addr, uintptr_t addr)
>  #elif defined(__arm__)
>  static inline void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr)
>  {
> -#if !QEMU_GNUC_PREREQ(4, 1)
> -    register unsigned long _beg __asm ("a1");
> -    register unsigned long _end __asm ("a2");
> -    register unsigned long _flg __asm ("a3");
> -#endif
> -
> -    /* we could use a ldr pc, [pc, #-4] kind of branch and avoid the flush */
> -    *(uint32_t *)jmp_addr =
> -        (*(uint32_t *)jmp_addr & ~0xffffff)
> -        | (((addr - (jmp_addr + 8)) >> 2) & 0xffffff);
> -
> -#if QEMU_GNUC_PREREQ(4, 1)
> -    __builtin___clear_cache((char *) jmp_addr, (char *) jmp_addr + 4);
> -#else
> -    /* flush icache */
> -    _beg = jmp_addr;
> -    _end = jmp_addr + 4;
> -    _flg = 0;
> -    __asm __volatile__ ("swi 0x9f0002" : : "r" (_beg), "r" (_end), "r" 
> (_flg));
> -#endif
> +    /* We're using "ldr pc, [pc,#-4]", so we can just store the raw
> +       address, without caring for flushing the icache.  */
> +    *(uint32_t *)jmp_addr = addr;
>  }
>  #elif defined(__sparc__)
>  void tb_set_jmp_target1(uintptr_t jmp_addr, uintptr_t addr);
> diff --git a/tcg/arm/tcg-target.c b/tcg/arm/tcg-target.c
> index 7bcba19..6de5e90 100644
> --- a/tcg/arm/tcg-target.c
> +++ b/tcg/arm/tcg-target.c
> @@ -1595,30 +1595,16 @@ static inline void tcg_out_op(TCGContext *s, 
> TCGOpcode opc,
>          break;
>      case INDEX_op_goto_tb:
>          if (s->tb_jmp_offset) {
> -            /* Direct jump method */
> -#if defined(USE_DIRECT_JUMP)
> -            s->tb_jmp_offset[args[0]] = s->code_ptr - s->code_buf;
> -            tcg_out_b_noaddr(s, COND_AL);
> -#else
> +            /* "Direct" jump method.  Rather than limit the code gen buffer
> +               to 16M, load the destination from the next word.  */
>              tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, TCG_REG_PC, -4);
>              s->tb_jmp_offset[args[0]] = s->code_ptr - s->code_buf;
> -            tcg_out32(s, 0);
> -#endif
> +            s->code_ptr += 4;
>          } else {
> -            /* Indirect jump method */
> -#if 1
> -            c = (int) (s->tb_next + args[0]) - ((int) s->code_ptr + 8);
> -            if (c > 0xfff || c < -0xfff) {
> -                tcg_out_movi32(s, COND_AL, TCG_REG_R0,
> -                                (tcg_target_long) (s->tb_next + args[0]));
> -                tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, TCG_REG_R0, 0);
> -            } else
> -                tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, TCG_REG_PC, c);
> -#else
> -            tcg_out_ld32_12(s, COND_AL, TCG_REG_R0, TCG_REG_PC, 0);
> +            /* Indirect jump method.  */
> +            tcg_out_movi32(s, COND_AL, TCG_REG_R0,
> +                           (tcg_target_long) (s->tb_next + args[0]));
>              tcg_out_ld32_12(s, COND_AL, TCG_REG_PC, TCG_REG_R0, 0);
> -            tcg_out32(s, (tcg_target_long) (s->tb_next + args[0]));
> -#endif
>          }
>          s->tb_next_offset[args[0]] = s->code_ptr - s->code_buf;
>          break;
> diff --git a/translate-all.c b/translate-all.c
> index a98c646..3ca839f 100644
> --- a/translate-all.c
> +++ b/translate-all.c
> @@ -460,8 +460,6 @@ static inline PageDesc *page_find(tb_page_addr_t index)
>  # define MAX_CODE_GEN_BUFFER_SIZE  (2ul * 1024 * 1024 * 1024)
>  #elif defined(__sparc__)
>  # define MAX_CODE_GEN_BUFFER_SIZE  (2ul * 1024 * 1024 * 1024)
> -#elif defined(__arm__)
> -# define MAX_CODE_GEN_BUFFER_SIZE  (16u * 1024 * 1024)
>  #elif defined(__s390x__)
>    /* We have a +- 4GB range on the branches; leave some slop.  */
>  # define MAX_CODE_GEN_BUFFER_SIZE  (3ul * 1024 * 1024 * 1024)
> -- 
> 1.8.1.4
> 
> 
> 

-- 
Aurelien Jarno                          GPG: 1024D/F1BCDB73
address@hidden                 http://www.aurel32.net