Re: [PATCH] ppc: Add optimization to generate endian-swapping load opcodes

[Paulo César Pereira de Andrade]

Em qua., 10 de ago. de 2022 às 13:28, Paul Cercueil
<paul@crapouillou.net> escreveu:
>
> When generating endian-swap code with jit_bswapr_us() or
> jit_bswapr_ui(), look at the last opcode generated. If it's a LHZX or
> LWZX, it can be replaced with a LHBRX or LWBRX opcode, which will
> perform the endian swap directly.
>
> If it's a LHZ or LWZ, then we can just move the immediate value into a
> temporary register, and emit a LHBRX or LWBRX opcode.
>
> The "no flag" check is needed, because we must ensure that no branch
> will jump between the load opcode that we want to modify, and the
> jit_bswapr opcode.

  Not certain if I understand the need for the no_flag. It only backtracks one
instruction, and that cannot be a jump for the pattern used. Please confirm it
is for the case:

LABEL:
  ldr_ui %r0 %r1
  bswapr_ui %r0 %r0

where the ldr_ui might be a jump target.

  Either way, basically it is implementing a load&byte_swap in a single
instruction.

> Signed-off-by: Paul Cercueil <paul@crapouillou.net>
> ---
>  lib/jit_ppc-cpu.c | 63 +++++++++++++++++++++++++++++++++++++++++------
>  lib/jit_ppc.c     | 11 +++++++--
>  2 files changed, 64 insertions(+), 10 deletions(-)
>
> diff --git a/lib/jit_ppc-cpu.c b/lib/jit_ppc-cpu.c
> index cab085f..3769a61 100644
> --- a/lib/jit_ppc-cpu.c
> +++ b/lib/jit_ppc-cpu.c
> @@ -260,7 +260,7 @@ static void 
> _FXS(jit_state_t*,int,int,int,int,int,int,int);
>  #  define LHAU(d,a,s)                  FDs(43,d,a,s)
>  #  define LHAUX(d,a,b)                 FX(31,d,a,b,375)
>  #  define LHAX(d,a,b)                  FX(31,d,a,b,343)
> -#  define LHRBX(d,a,b)                 FX(31,d,a,b,790)
> +#  define LHBRX(d,a,b)                 FX(31,d,a,b,790)
>  #  define LHZ(d,a,s)                   FDs(40,d,a,s)
>  #  define LHZU(d,a,s)                  FDs(41,d,a,s)
>  #  define LHZUX(d,a,b)                 FX(31,d,a,b,311)
> @@ -521,10 +521,12 @@ static jit_word_t 
> _movi_p(jit_state_t*,jit_int32_t,jit_word_t);
>  #    define extr_i(r0,r1)              EXTSW(r0,r1)
>  #    define extr_ui(r0,r1)             CLRLDI(r0,r1,32)
>  #  endif
> -#  define bswapr_us(r0,r1)             _bswapr_us(_jit,r0,r1)
> -static void _bswapr_us(jit_state_t*,jit_int32_t,jit_int32_t);
> -#  define bswapr_ui(r0,r1)             _bswapr_ui(_jit,r0,r1)
> -static void _bswapr_ui(jit_state_t*,jit_int32_t,jit_int32_t);
> +#  define bswapr_us_lh(r0,r1,no_flag)  _bswapr_us(_jit,r0,r1,no_flag)
> +#  define bswapr_us(r0,r1)             _bswapr_us(_jit,r0,r1,0)
> +static void _bswapr_us(jit_state_t*,jit_int32_t,jit_int32_t,jit_bool_t);
> +#  define bswapr_ui_lw(r0,r1,no_flag)  _bswapr_ui(_jit,r0,r1,no_flag)
> +#  define bswapr_ui(r0,r1)             _bswapr_ui(_jit,r0,r1,0)
> +static void _bswapr_ui(jit_state_t*,jit_int32_t,jit_int32_t,jit_bool_t);
>  #  if __WORDSIZE == 64
>  #    define bswapr_ul(r0,r1)           generic_bswapr_ul(_jit,r0,r1)
>  #  endif
> @@ -1148,8 +1150,31 @@ _movi_p(jit_state_t *_jit, jit_int32_t r0, jit_word_t 
> i0)
>  }
>
>  static void
> -_bswapr_us(jit_state_t *_jit, jit_int32_t r0, jit_int32_t r1)
> +_bswapr_us(jit_state_t *_jit, jit_int32_t r0, jit_int32_t r1, jit_bool_t 
> no_flag)
>  {
> +    jit_int32_t                reg, addr_reg;
> +
> +    if (no_flag && r0 == r1) {
> +        if ((*(_jit->pc.ui - 1) & 0xffe007ff) == (0x7c00022e | r0 << 21)) {
> +            /* Convert LHZX to LHBRX */
> +            _jit->pc.ui--;
> +            LHBRX(r0, (*_jit->pc.ui >> 16) & 0x1f, (*_jit->pc.ui >> 11) & 
> 0x1f);
> +            return;
> +        }
> +
> +        if ((*(_jit->pc.ui - 1) & 0xffe00000) == (0xa0000000 | r0 << 21)) {
> +            /* Convert LHZ to LHBRX */
> +            _jit->pc.ui--;
> +            addr_reg = (*_jit->pc.ui >> 16) & 0x1f;
> +
> +            reg = jit_get_reg(jit_class_gpr);
> +            LI(rn(reg), (short)*_jit->pc.ui);
> +            LHBRX(r0, rn(reg), addr_reg);
> +            jit_unget_reg(reg);
> +            return;
> +        }
> +    }
> +
>      if (r0 == r1) {
>          RLWIMI(r0, r0, 16, 8, 15);
>          RLWINM(r0, r0, 24, 16, 31);
> @@ -1160,9 +1185,31 @@ _bswapr_us(jit_state_t *_jit, jit_int32_t r0, 
> jit_int32_t r1)
>  }
>
>  static void
> -_bswapr_ui(jit_state_t *_jit, jit_int32_t r0, jit_int32_t r1)
> +_bswapr_ui(jit_state_t *_jit, jit_int32_t r0, jit_int32_t r1, jit_bool_t 
> no_flag)
>  {
> -    jit_int32_t                reg;
> +    jit_int32_t                reg, addr_reg;
> +
> +    if (no_flag && r0 == r1) {
> +        if ((*(_jit->pc.ui - 1) & 0xffe007ff) == (0x7c00002e | r0 << 21)) {
> +            /* Convert LWZX to LWBRX */
> +            _jit->pc.ui--;
> +            LWBRX(r0, (*_jit->pc.ui >> 16) & 0x1f, (*_jit->pc.ui >> 11) & 
> 0x1f);
> +            return;
> +        }
> +
> +        if ((*(_jit->pc.ui - 1) & 0xffe00000) == (0x80000000 | r0 << 21)) {
> +            /* Convert LWZ to LWBRX */
> +            _jit->pc.ui--;
> +            addr_reg = (*_jit->pc.ui >> 16) & 0x1f;
> +
> +            reg = jit_get_reg(jit_class_gpr);
> +            LI(rn(reg), (short)*_jit->pc.ui);
> +            LWBRX(r0, rn(reg), addr_reg);
> +            jit_unget_reg(reg);
> +            return;
> +        }
> +    }
> +
>      reg = jit_get_reg(jit_class_gpr);
>      ROTLWI(rn(reg), r1, 8);
>      RLWIMI(rn(reg), r1, 24, 0, 7);

  These could be simplified a bit, or, at least have a single visible exit
point of the function.

> diff --git a/lib/jit_ppc.c b/lib/jit_ppc.c
> index e94d1a5..28975be 100644
> --- a/lib/jit_ppc.c
> +++ b/lib/jit_ppc.c
> @@ -1148,6 +1148,7 @@ _emit_code(jit_state_t *_jit)
>      jit_word_t          word;
>      jit_int32_t                 value;
>      jit_int32_t                 offset;
> +    jit_bool_t       no_flag = 0;
>      struct {
>         jit_node_t      *node;
>         jit_word_t       word;
> @@ -1356,8 +1357,12 @@ _emit_code(jit_state_t *_jit)
>  #  if __WORDSIZE == 64
>                 case_rr(hton, _ul);
>  #  endif
> -               case_rr(bswap, _us);
> -               case_rr(bswap, _ui);
> +           case jit_code_bswapr_us:
> +               bswapr_us_lh(rn(node->u.w), rn(node->v.w), no_flag);
> +               break;
> +           case jit_code_bswapr_ui:
> +               bswapr_ui_lw(rn(node->u.w), rn(node->v.w), no_flag);
> +               break;
>  #  if __WORDSIZE == 64
>                 case_rr(bswap, _ul);
>  #  endif
> @@ -1823,6 +1828,8 @@ _emit_code(jit_state_t *_jit)
>         assert(_jitc->regarg == 0 && _jitc->synth == 0);
>         /* update register live state */
>         jit_reglive(node);
> +
> +        no_flag = !(node->flag & jit_flag_patch);
>      }
>  #undef case_brf
>  #undef case_brw
> --
> 2.35.1

  Please only clarify about the no_flag and the patch can be applied, possibly
slightly modified based on comments below.

  Should also add some comments about what it is doing.

  If some new kind of optimization pass is done at some point, adding backend
specific codes, this could be extended to work on more complex cases, where
a temporary was already used to do the previous instruction, and it was required
a spill/reload, if not required a spill, the temporary will work the
same way when
replacing the instruction.

Thanks,
Paulo