Re: [PATCH 4/4] rs6000: build constant via li/lis;rldic

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

Hi David,

Thanks for your valuable comments!

David Edelsohn <dje.gcc@gmail.com> writes:
>  
> On Wed, Jun 7, 2023 at 9:56 PM Jiufu Guo <guojiufu@linux.ibm.com> wrote:
>
>  Hi,
>
>  This patch checks if a constant is possible to be built by "li;rldic".
>  We only need to take care of "negative li", other forms do not need to check.
>  For example, "negative lis" is just a "negative li" with an additional shift.
>
>  Bootstrap and regtest pass on ppc64{,le}.
>  Is this ok for trunk?
>
>  BR,
>  Jeff (Jiufu)
>
>  gcc/ChangeLog:
>
>          * config/rs6000/rs6000.cc (can_be_built_by_li_and_rldic): New function.
>          (rs6000_emit_set_long_const): Call can_be_built_by_li_and_rldic.
>
> This is okay.
>
> Do you have any measurement of how expensive it is to test all of these additional methods to generate a constant?  How much does this affect the
> compile time?

Yeap, Thanks for this very good question!
This patch is mostly using bitwise operations and if-conditions,
it would be expected not expensive.

Testcases were checked.  For example:
A case with ~1000 constants: most of them hit this feature.
With this feature, the compiling time is slightly faster.

0m1.985s(without patch) vs. 0m1.874s(with patch)
(note:D rs6000_emit_set_long_const does not occur in hot perf
functions.  So, the tricky time saving would not directly cause
by this feature.)

A case with ~1000 constants:(most are not hit by this feature)
0m2.493s(without patch) vs. 0m2.558s(with patch).

For runtime, actually, with the patch, it seems there is no visible
improvement in SPEC2017.  While I still feel this patch is
doing the right thing: use fewer instructions to build the constant.

BR,
Jeff (Jiufu Guo)

>
> Thanks, David
>
>  
>  
>  gcc/testsuite/ChangeLog:
>
>          * gcc.target/powerpc/const-build.c: Add more tests.
>  ---
>   gcc/config/rs6000/rs6000.cc                   | 61 ++++++++++++++++++-
>   .../gcc.target/powerpc/const-build.c          | 28 +++++++++
>   2 files changed, 88 insertions(+), 1 deletion(-)
>
>  diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
>  index 2a3fa733b45..cd04b6b5c82 100644
>  --- a/gcc/config/rs6000/rs6000.cc
>  +++ b/gcc/config/rs6000/rs6000.cc
>  @@ -10387,6 +10387,64 @@ can_be_built_by_li_lis_and_rldicr (HOST_WIDE_INT c, int *shift,
>     return false;
>   }
>
>  +/* Check if value C can be built by 2 instructions: one is 'li', another is
>  +   rldic.
>  +
>  +   If so, *SHIFT is set to the 'shift' operand of rldic; and *MASK is set
>  +   to the mask value about the 'mb' operand of rldic; and return true.
>  +   Return false otherwise.  */
>  +
>  +static bool
>  +can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask)
>  +{
>  +  /* There are 49 successive ones in the negative value of 'li'.  */
>  +  int ones = 49;
>  +
>  +  /* 1..1xx1..1: negative value of li --> 0..01..1xx0..0:
>  +     right bits are shifted as 0's, and left 1's(and x's) are cleaned.  */
>  +  int tz = ctz_hwi (c);
>  +  int lz = clz_hwi (c);
>  +  int middle_ones = clz_hwi (~(c << lz));
>  +  if (tz + lz + middle_ones >= ones)
>  +    {
>  +      *mask = ((1LL << (HOST_BITS_PER_WIDE_INT - tz - lz)) - 1LL) << tz;
>  +      *shift = tz;
>  +      return true;
>  +    }
>  +
>  +  /* 1..1xx1..1 --> 1..1xx0..01..1: some 1's(following x's) are cleaned. */
>  +  int leading_ones = clz_hwi (~c);
>  +  int tailing_ones = ctz_hwi (~c);
>  +  int middle_zeros = ctz_hwi (c >> tailing_ones);
>  +  if (leading_ones + tailing_ones + middle_zeros >= ones)
>  +    {
>  +      *mask = ~(((1ULL << middle_zeros) - 1ULL) << tailing_ones);
>  +      *shift = tailing_ones + middle_zeros;
>  +      return true;
>  +    }
>  +
>  +  /* xx1..1xx: --> xx0..01..1xx: some 1's(following x's) are cleaned. */
>  +  /* Get the position for the first bit of successive 1.
>  +     The 24th bit would be in successive 0 or 1.  */
>  +  HOST_WIDE_INT low_mask = (1LL << 24) - 1LL;
>  +  int pos_first_1 = ((c & (low_mask + 1)) == 0)
>  +                     ? clz_hwi (c & low_mask)
>  +                     : HOST_BITS_PER_WIDE_INT - ctz_hwi (~(c | low_mask));
>  +  middle_ones = clz_hwi (~c << pos_first_1);
>  +  middle_zeros = ctz_hwi (c >> (HOST_BITS_PER_WIDE_INT - pos_first_1));
>  +  if (pos_first_1 < HOST_BITS_PER_WIDE_INT
>  +      && middle_ones + middle_zeros < HOST_BITS_PER_WIDE_INT
>  +      && middle_ones + middle_zeros >= ones)
>  +    {
>  +      *mask = ~(((1ULL << middle_zeros) - 1LL)
>  +               << (HOST_BITS_PER_WIDE_INT - pos_first_1));
>  +      *shift = HOST_BITS_PER_WIDE_INT - pos_first_1 + middle_zeros;
>  +      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.  */
>  @@ -10435,7 +10493,8 @@ rs6000_emit_set_long_const (rtx dest, HOST_WIDE_INT c)
>       }
>     else if (can_be_built_by_li_lis_and_rotldi (c, &shift, &mask)
>             || can_be_built_by_li_lis_and_rldicl (c, &shift, &mask)
>  -          || can_be_built_by_li_lis_and_rldicr (c, &shift, &mask))
>  +          || can_be_built_by_li_lis_and_rldicr (c, &shift, &mask)
>  +          || can_be_built_by_li_and_rldic (c, &shift, &mask))
>       {
>         temp = !can_create_pseudo_p () ? dest : gen_reg_rtx (DImode);
>         unsigned HOST_WIDE_INT imm = (c | ~mask);
>  diff --git a/gcc/testsuite/gcc.target/powerpc/const-build.c b/gcc/testsuite/gcc.target/powerpc/const-build.c
>  index 8c209921d41..b503ee31c7c 100644
>  --- a/gcc/testsuite/gcc.target/powerpc/const-build.c
>  +++ b/gcc/testsuite/gcc.target/powerpc/const-build.c
>  @@ -82,6 +82,29 @@ lis_rldicr_12 (void)
>     return 0x5310000ffffffff0LL;
>   }
>
>  +long long NOIPA
>  +li_rldic_13 (void)
>  +{
>  +  return 0x000f853100000000LL;
>  +}
>  +long long NOIPA
>  +li_rldic_14 (void)
>  +{
>  +  return 0xffff853100ffffffLL;
>  +}
>  +
>  +long long NOIPA
>  +li_rldic_15 (void)
>  +{
>  +  return 0x800000ffffffff31LL;
>  +}
>  +
>  +long long NOIPA
>  +li_rldic_16 (void)
>  +{
>  +  return 0x800000000fffff31LL;
>  +}
>  +
>   struct fun arr[] = {
>     {li_rotldi_1, 0x7531000000000LL},
>     {li_rotldi_2, 0x2100000000000064LL},
>  @@ -95,11 +118,16 @@ struct fun arr[] = {
>     {li_rldicr_10, 0xffff8531fff00000LL},
>     {li_rldicr_11, 0x21fffffffff00000LL},
>     {lis_rldicr_12, 0x5310000ffffffff0LL},
>  +  {li_rldic_13, 0x000f853100000000LL},
>  +  {li_rldic_14, 0xffff853100ffffffLL},
>  +  {li_rldic_15, 0x800000ffffffff31LL},
>  +  {li_rldic_16, 0x800000000fffff31LL}
>   };
>
>   /* { dg-final { scan-assembler-times {\mrotldi\M} 6 } } */
>   /* { dg-final { scan-assembler-times {\mrldicl\M} 3 } } */
>   /* { dg-final { scan-assembler-times {\mrldicr\M} 3 } } */
>  +/* { dg-final { scan-assembler-times {\mrldic\M} 4 } } */
>
>   int
>   main ()
>  -- 
>  2.39.1