Re: [PATCH 1/4] rs6000: build constant via li;rotldi

[Jiufu Guo <guojiufu@linux.ibm.com>]

Hi David,

David Edelsohn <dje.gcc@gmail.com> writes:
> On Wed, Jun 7, 2023 at 9:55 PM Jiufu Guo <guojiufu@linux.ibm.com> wrote:
>
>  Hi,
>
>  This patch checks if a constant is possible to be rotated to/from a positive
>  or negative value from "li". If so, we could use "li;rotldi" to build it.
>
>  Bootstrap and regtest pass on ppc64{,le}.
>  Is this ok for trunk?
>
>  BR,
>  Jeff (Jiufu)
>
>  gcc/ChangeLog:
>
>          * config/rs6000/rs6000.cc (can_be_rotated_to_positive_li): New function.
>          (can_be_rotated_to_negative_li): New function.
>          (can_be_built_by_li_and_rotldi): New function.
>          (rs6000_emit_set_long_const): Call can_be_built_by_li_and_rotldi.
>
>  gcc/testsuite/ChangeLog:
>
>          * gcc.target/powerpc/const-build.c: New test.
>  ---
>   gcc/config/rs6000/rs6000.cc                   | 64 +++++++++++++++++--
>   .../gcc.target/powerpc/const-build.c          | 54 ++++++++++++++++
>   2 files changed, 112 insertions(+), 6 deletions(-)
>   create mode 100644 gcc/testsuite/gcc.target/powerpc/const-build.c
>
>  diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
>  index 42f49e4a56b..1dd0072350a 100644
>  --- a/gcc/config/rs6000/rs6000.cc
>  +++ b/gcc/config/rs6000/rs6000.cc
>  @@ -10258,6 +10258,48 @@ rs6000_emit_set_const (rtx dest, rtx source)
>     return true;
>   }
>
>  +/* Check if C can be rotated to a positive value which 'li' instruction
>  +   is able to load.  If so, set *ROT to the number by which C is rotated,
>  +   and return true.  Return false otherwise.  */
>  +
>  +static bool
>  +can_be_rotated_to_positive_li (HOST_WIDE_INT c, int *rot)
>  +{
>  +  /* 49 leading zeros and 15 low bits on the positive value
>  +     generated by 'li' instruction.  */
>  +  return can_be_rotated_to_lowbits (c, 15, rot);
>  +}
>  +
>  +/* Like can_be_rotated_to_positive_li, but check the negative value of 'li'.  */
>  +
>  +static bool
>  +can_be_rotated_to_negative_li (HOST_WIDE_INT c, int *rot)
>  +{
>  +  return can_be_rotated_to_lowbits (~c, 15, rot);
>  +}
>  +
>  +/* Check if value C can be built by 2 instructions: one is 'li', another is
>  +   rotldi.
>  +
>  +   If so, *SHIFT is set to the shift operand of rotldi(rldicl), and *MASK
>  +   is set to -1, and return true.  Return false otherwise.  */
>  +
>
> I look at this feature and it's good, but I don't fully understand the benefit of this level of abstraction.  Ideally all of the above functions would
> be inlined.  They aren't reused.
>  
>  +static bool
>  +can_be_built_by_li_and_rotldi (HOST_WIDE_INT c, int *shift,
>  +                                  HOST_WIDE_INT *mask)
>  +{
>  +  int n;
>  +  if (can_be_rotated_to_positive_li (c, &n)
>  +      || can_be_rotated_to_negative_li (c, &n))
>
> Why not
>
> /* Check if C or ~C can be rotated to a positive or negative value
>     which 'li' instruction is able to load.  */
> if (can_be_rotated_to_lowbits (c, 15, &n)
>     || can_be_rotated_to_lowbits (~c, 15, &n))

 
Thanks a lot for your review!!

Your suggestions could also achieve my goal of using a new function:
Using "can_be_rotated_to_positive_li" is just trying to get a
straightforward name.  Like yours, the code's comments would also
make it easy to understand.

BR,
Jeff (Jiufu Guo)
>  
> ...
>
> This is a style of software engineering, but it seems overkill to me when the function is a single line that tail calls another function.  Am I missing
> something?
>
> The rest of this patch looks good.
>
> Thanks, David
>  
>  +    {
>  +      *mask = HOST_WIDE_INT_M1;
>  +      *shift = HOST_BITS_PER_WIDE_INT - n;
>  +      return true;
>  +    }
>  +
>  +  return false;
>  +}
>  +
>   /* Subroutine of rs6000_emit_set_const, handling PowerPC64 DImode.
>      Output insns to set DEST equal to the constant C as a series of
>      lis, ori and shl instructions.  */
>  @@ -10266,15 +10308,14 @@ static void
>   rs6000_emit_set_long_const (rtx dest, HOST_WIDE_INT c)
>   {
>     rtx temp;
>  +  int shift;
>  +  HOST_WIDE_INT mask;
>     HOST_WIDE_INT ud1, ud2, ud3, ud4;
>
>     ud1 = c & 0xffff;
>  -  c = c >> 16;
>  -  ud2 = c & 0xffff;
>  -  c = c >> 16;
>  -  ud3 = c & 0xffff;
>  -  c = c >> 16;
>  -  ud4 = c & 0xffff;
>  +  ud2 = (c >> 16) & 0xffff;
>  +  ud3 = (c >> 32) & 0xffff;
>  +  ud4 = (c >> 48) & 0xffff;
>
>     if ((ud4 == 0xffff && ud3 == 0xffff && ud2 == 0xffff && (ud1 & 0x8000))
>         || (ud4 == 0 && ud3 == 0 && ud2 == 0 && ! (ud1 & 0x8000)))
>  @@ -10305,6 +10346,17 @@ rs6000_emit_set_long_const (rtx dest, HOST_WIDE_INT c)
>         emit_move_insn (dest, gen_rtx_XOR (DImode, temp,
>                                           GEN_INT ((ud2 ^ 0xffff) << 16)));
>       }
>  +  else if (can_be_built_by_li_and_rotldi (c, &shift, &mask))
>  +    {
>  +      temp = !can_create_pseudo_p () ? dest : gen_reg_rtx (DImode);
>  +      unsigned HOST_WIDE_INT imm = (c | ~mask);
>  +      imm = (imm >> shift) | (imm << (HOST_BITS_PER_WIDE_INT - shift));
>  +
>  +      emit_move_insn (temp, GEN_INT (imm));
>  +      if (shift != 0)
>  +       temp = gen_rtx_ROTATE (DImode, temp, GEN_INT (shift));
>  +      emit_move_insn (dest, temp);
>  +    }
>     else if (ud3 == 0 && ud4 == 0)
>       {
>         temp = !can_create_pseudo_p () ? dest : gen_reg_rtx (DImode);
>  diff --git a/gcc/testsuite/gcc.target/powerpc/const-build.c b/gcc/testsuite/gcc.target/powerpc/const-build.c
>  new file mode 100644
>  index 00000000000..70f095f6bf2
>  --- /dev/null
>  +++ b/gcc/testsuite/gcc.target/powerpc/const-build.c
>  @@ -0,0 +1,54 @@
>  +/* { dg-do run } */
>  +/* { dg-options "-O2 -save-temps" } */
>  +/* { dg-require-effective-target has_arch_ppc64 } */
>  +
>  +#define NOIPA __attribute__ ((noipa))
>  +
>  +struct fun
>  +{
>  +  long long (*f) (void);
>  +  long long val;
>  +};
>  +
>  +long long NOIPA
>  +li_rotldi_1 (void)
>  +{
>  +  return 0x7531000000000LL;
>  +}
>  +
>  +long long NOIPA
>  +li_rotldi_2 (void)
>  +{
>  +  return 0x2100000000000064LL;
>  +}
>  +
>  +long long NOIPA
>  +li_rotldi_3 (void)
>  +{
>  +  return 0xffff8531ffffffffLL;
>  +}
>  +
>  +long long NOIPA
>  +li_rotldi_4 (void)
>  +{
>  +  return 0x21ffffffffffff94LL;
>  +}
>  +
>  +struct fun arr[] = {
>  +  {li_rotldi_1, 0x7531000000000LL},
>  +  {li_rotldi_2, 0x2100000000000064LL},
>  +  {li_rotldi_3, 0xffff8531ffffffffLL},
>  +  {li_rotldi_4, 0x21ffffffffffff94LL},
>  +};
>  +
>  +/* { dg-final { scan-assembler-times {\mrotldi\M} 4 } } */
>  +
>  +int
>  +main ()
>  +{
>  +  for (int i = 0; i < sizeof (arr) / sizeof (arr[0]); i++)
>  +    if ((*arr[i].f) () != arr[i].val)
>  +      __builtin_abort ();
>  +
>  +  return 0;
>  +}
>  -- 
>  2.39.1