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

From: Paul Cercueil
Subject: Re: [PATCH] ppc: Add optimization to generate endian-swapping load opcodes
Date: Thu, 11 Aug 2022 19:18:17 +0200 
Hi Paulo,
Le jeu., août 11 2022 at 13:59:47 -0300, Paulo César Pereira de Andrade <paulo.cesar.pereira.de.andrade@gmail.com> a écrit : 
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.
Actually that case would be fine. It's for the case where the LABEL is between the ldr_ui and the bswapr_ui. I do check the previous opcode, because that's where the jit_flag_patch seems to be.
Either way, basically it is implementing a load&byte_swap in a single 
instruction.


Correct.


 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.
I couldn't find a way to only use if/else blocks without having the default path duplicated... 


 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.
You mean for no_flag? Because I did write the comment "Convert LWZ to LWBRX" :)
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.
Honestly I'm fine with the current handling. Trying to do better might end making it too complex.
Similarly for my delay-slot-filling patch on MIPS, I only swap the jump opcode with the previous instruction if possible, I don't backtrack further to find an instruction that can be used. 

Cheers,
-Paul
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