Re: [Qemu-devel] [PATCH for-next 6/8] tcg-i386: Use new return-argument ld/st helpers

[Aurelien Jarno <aurelien@aurel32.net>]

On Mon, Aug 05, 2013 at 08:07:23AM -1000, Richard Henderson wrote:
> Discontinue the jump-around-jump-to-jump scheme, trading it for a single
> immediate move instruction.  The two extra jumps always consume 7 bytes,
> whereas the immediate move is either 5 or 7 bytes depending on where the
> code_gen_buffer gets located.
> 
> Signed-off-by: Richard Henderson <rth@twiddle.net>
> ---
>  include/exec/exec-all.h |  13 +------
>  tcg/i386/tcg-target.c   | 100 +++++++++++++++++++++---------------------------
>  2 files changed, 46 insertions(+), 67 deletions(-)
> 
> diff --git a/include/exec/exec-all.h b/include/exec/exec-all.h
> index 5920f73..b70028a 100644
> --- a/include/exec/exec-all.h
> +++ b/include/exec/exec-all.h
> @@ -326,18 +326,9 @@ extern uintptr_t tci_tb_ptr;
>     (6) jump to corresponding code of the next of fast path
>   */
>  # if defined(__i386__) || defined(__x86_64__)
> -/* To avoid broken disassembling, long jmp is used for embedding fast path pc,
> -   so that the destination is the next code of fast path, though this jmp is
> -   never executed.
> -
> -   call MMU helper
> -   jmp POST_PROC (2byte)    <- GETRA()
> -   jmp NEXT_CODE (5byte)
> -   POST_PROCESS ...         <- GETRA() + 7
> - */
>  #  define GETRA() ((uintptr_t)__builtin_return_address(0))
> -#  define GETPC_LDST() ((uintptr_t)(GETRA() + 7 + \
> -                                    *(int32_t *)((void *)GETRA() + 3) - 1))
> +/* The return address argument for ldst is passed directly.  */
> +#  define GETPC_LDST()  (abort(), 0)

Why an abort here, while in the arm version, you adds support for
not defining GETPC_LDST?

>  # elif defined (_ARCH_PPC) && !defined (_ARCH_PPC64)
>  #  define GETRA() ((uintptr_t)__builtin_return_address(0))
>  #  define GETPC_LDST() ((uintptr_t) ((*(int32_t *)(GETRA() - 4)) - 1))
> diff --git a/tcg/i386/tcg-target.c b/tcg/i386/tcg-target.c
> index 8addfa1..c7a02a3 100644
> --- a/tcg/i386/tcg-target.c
> +++ b/tcg/i386/tcg-target.c
> @@ -190,11 +190,11 @@ static int target_parse_constraint(TCGArgConstraint *ct, const char **pct_str)
>          /* qemu_ld/st address constraint */
>      case 'L':
>          ct->ct |= TCG_CT_REG;
> -#if TCG_TARGET_REG_BITS == 64
> +        if (TCG_TARGET_REG_BITS == 64) {
>              tcg_regset_set32(ct->u.regs, 0, 0xffff);
> -#else
> +        } else {
>              tcg_regset_set32(ct->u.regs, 0, 0xff);
> -#endif
> +        }
>          tcg_regset_reset_reg(ct->u.regs, TCG_REG_L0);
>          tcg_regset_reset_reg(ct->u.regs, TCG_REG_L1);
>          break;
> @@ -1015,22 +1015,24 @@ static void tcg_out_jmp(TCGContext *s, tcg_target_long dest)
>  
>  #include "exec/softmmu_defs.h"
>  
> -/* helper signature: helper_ld_mmu(CPUState *env, target_ulong addr,
> -   int mmu_idx) */
> -static const void *qemu_ld_helpers[4] = {
> -    helper_ldb_mmu,
> -    helper_ldw_mmu,
> -    helper_ldl_mmu,
> -    helper_ldq_mmu,
> +/* helper signature: helper_ret_ld_mmu(CPUState *env, target_ulong addr,
> + *                                     int mmu_idx, uintptr_t ra)
> + */
> +static const void * const qemu_ld_helpers[4] = {
> +    helper_ret_ldb_mmu,
> +    helper_ret_ldw_mmu,
> +    helper_ret_ldl_mmu,
> +    helper_ret_ldq_mmu,
>  };
>  
> -/* helper signature: helper_st_mmu(CPUState *env, target_ulong addr,
> -   uintxx_t val, int mmu_idx) */
> -static const void *qemu_st_helpers[4] = {
> -    helper_stb_mmu,
> -    helper_stw_mmu,
> -    helper_stl_mmu,
> -    helper_stq_mmu,
> +/* helper signature: helper_ret_st_mmu(CPUState *env, target_ulong addr,
> + *                                     uintxx_t val, int mmu_idx, uintptr_t ra)
> + */
> +static const void * const qemu_st_helpers[4] = {
> +    helper_ret_stb_mmu,
> +    helper_ret_stw_mmu,
> +    helper_ret_stl_mmu,
> +    helper_ret_stq_mmu,
>  };
>  
>  static void add_qemu_ldst_label(TCGContext *s,
> @@ -1458,6 +1460,12 @@ static void add_qemu_ldst_label(TCGContext *s,
>      }
>  }
>  
> +/* See the GETPC definition in include/exec/exec-all.h.  */
> +static inline uintptr_t do_getpc(uint8_t *raddr)
> +{
> +    return (uintptr_t)raddr - 1;
> +}
> +
>  /*
>   * Generate code for the slow path for a load at the end of block
>   */
> @@ -1490,33 +1498,21 @@ static void tcg_out_qemu_ld_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
>  
>          tcg_out_movi(s, TCG_TYPE_I32, TCG_REG_EAX, l->mem_index);
>          tcg_out_st(s, TCG_TYPE_I32, TCG_REG_EAX, TCG_REG_ESP, ofs);
> +        ofs += 4;
> +
> +        tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_EAX, do_getpc(l->raddr));
> +        tcg_out_st(s, TCG_TYPE_I32, TCG_REG_EAX, TCG_REG_ESP, ofs);

Same as the other patch, this can be done in one instruction.

>      } else {
> -        tcg_out_mov(s, TCG_TYPE_I64, tcg_target_call_iarg_regs[0], TCG_AREG0);
> +        tcg_out_mov(s, TCG_TYPE_PTR, tcg_target_call_iarg_regs[0], TCG_AREG0);
>          /* The second argument is already loaded with addrlo.  */
>          tcg_out_movi(s, TCG_TYPE_I32, tcg_target_call_iarg_regs[2],
>                       l->mem_index);
> +        tcg_out_movi(s, TCG_TYPE_PTR, tcg_target_call_iarg_regs[3],
> +                     do_getpc(l->raddr));
>      }
>  
> -    /* Code generation of qemu_ld/st's slow path calling MMU helper
> -
> -       PRE_PROC ...
> -       call MMU helper
> -       jmp POST_PROC (2b) : short forward jump <- GETRA()
> -       jmp next_code (5b) : dummy long backward jump which is never executed
> -       POST_PROC ... : do post-processing <- GETRA() + 7
> -       jmp next_code : jump to the code corresponding to next IR of qemu_ld/st
> -    */
> -
>      tcg_out_calli(s, (tcg_target_long)qemu_ld_helpers[s_bits]);
>  
> -    /* Jump to post-processing code */
> -    tcg_out8(s, OPC_JMP_short);
> -    tcg_out8(s, 5);
> -    /* Dummy backward jump having information of fast path'pc for MMU helpers */
> -    tcg_out8(s, OPC_JMP_long);
> -    *(int32_t *)s->code_ptr = (int32_t)(l->raddr - s->code_ptr - 4);
> -    s->code_ptr += 4;
> -
>      data_reg = l->datalo_reg;
>      switch(opc) {
>      case 0 | 4:
> @@ -1598,36 +1594,28 @@ static void tcg_out_qemu_st_slow_path(TCGContext *s, TCGLabelQemuLdst *l)
>  
>          tcg_out_movi(s, TCG_TYPE_I32, TCG_REG_EAX, l->mem_index);
>          tcg_out_st(s, TCG_TYPE_I32, TCG_REG_EAX, TCG_REG_ESP, ofs);
> +        ofs += 4;
> +
> +        tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_EAX, do_getpc(l->raddr));
> +        tcg_out_st(s, TCG_TYPE_I32, TCG_REG_EAX, TCG_REG_ESP, ofs);
>      } else {
> -        tcg_out_mov(s, TCG_TYPE_I64, tcg_target_call_iarg_regs[0], TCG_AREG0);
> +        tcg_out_mov(s, TCG_TYPE_PTR, tcg_target_call_iarg_regs[0], TCG_AREG0);
>          /* The second argument is already loaded with addrlo.  */
>          tcg_out_mov(s, (opc == 3 ? TCG_TYPE_I64 : TCG_TYPE_I32),
>                      tcg_target_call_iarg_regs[2], l->datalo_reg);
>          tcg_out_movi(s, TCG_TYPE_I32, tcg_target_call_iarg_regs[3],
>                       l->mem_index);
> +        if (ARRAY_SIZE(tcg_target_call_iarg_regs) > 4) {
> +            tcg_out_movi(s, TCG_TYPE_PTR, tcg_target_call_iarg_regs[4],
> +                         do_getpc(l->raddr));
> +        } else {
> +            tcg_out_movi(s, TCG_TYPE_PTR, TCG_REG_RAX, do_getpc(l->raddr));
> +            tcg_out_st(s, TCG_TYPE_PTR, TCG_REG_RAX, TCG_REG_ESP, 0);
> +        }
>      }
>  
> -    /* Code generation of qemu_ld/st's slow path calling MMU helper
> -
> -       PRE_PROC ...
> -       call MMU helper
> -       jmp POST_PROC (2b) : short forward jump <- GETRA()
> -       jmp next_code (5b) : dummy long backward jump which is never executed
> -       POST_PROC ... : do post-processing <- GETRA() + 7
> -       jmp next_code : jump to the code corresponding to next IR of qemu_ld/st
> -    */
> -
>      tcg_out_calli(s, (tcg_target_long)qemu_st_helpers[s_bits]);
>  
> -    /* Jump to post-processing code */
> -    tcg_out8(s, OPC_JMP_short);
> -    tcg_out8(s, 5);
> -    /* Dummy backward jump having information of fast path'pc for MMU helpers */
> -    tcg_out8(s, OPC_JMP_long);
> -    *(int32_t *)s->code_ptr = (int32_t)(l->raddr - s->code_ptr - 4);
> -    s->code_ptr += 4;
> -
> -    /* Jump to the code corresponding to next IR of qemu_st */
>      tcg_out_jmp(s, (tcg_target_long)l->raddr);
>  }
>  

Beside the small nitpicking above, it looks fine to me.

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