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

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

Hi,

David Edelsohn <dje.gcc@gmail.com> writes:

> On Mon, Jun 12, 2023 at 11:30 PM Jiufu Guo <guojiufu@linux.ibm.com> wrote:
>>
>>
>> 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.
>
> I recognize that you are trying to be consistent with the other
> functions that you add in later patches, but it feels like overkill in
Yes :)
> abstraction to me.  Or maybe combine postive_li and negative_li into a
> single function so that the abstraction serves a purpose other than a
> tail call and creating an alias for a specific invocation of
> can_be_rotated_to_lowbits.
Get it.

Thanks for your valuable suggestion!

BR,
Jeff (Jiufu Guo)

>
> Thanks, David
>
>>
>> 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