Re: [PATCH] optabs: Implement double-word ctz and ffs expansion

[Richard Biener <rguenther@suse.de>]

> Am 07.06.2023 um 18:59 schrieb Jakub Jelinek via Gcc-patches <gcc-patches@gcc.gnu.org>:
> 
> Hi!
> 
> We have expand_doubleword_clz for a couple of years, where we emit
> double-word CLZ as if (high_word == 0) return CLZ (low_word) + word_size;
> else return CLZ (high_word);
> We can do something similar for CTZ and FFS IMHO, just with the 2
> words swapped.  So if (low_word == 0) return CTZ (high_word) + word_size;
> else return CTZ (low_word); for CTZ and
> if (low_word == 0) { return high_word ? FFS (high_word) + word_size : 0;
> else return FFS (low_word);
> 
> The following patch implements that.
> 
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

Ok

Richard 
> Note, on some targets which implement both word_mode ctz and ffs patterns,
> it might be better to incrementally implement those double-word ffs expansion
> patterns in md files, because we aren't able to optimize it correctly;
> nothing can detect we have just made sure that argument is not 0 and so
> don't need to bother with handling that case.  So, on ia32 just using
> CTZ patterns would be better there, but I think we can even do better and
> instead of doing the comparisons of the operands against 0 do the CTZ
> expansion followed by testing of flags.
> 
> 2023-06-07  Jakub Jelinek  <jakub@redhat.com>
> 
>    * optabs.cc (expand_ffs): Add forward declaration.
>    (expand_doubleword_clz): Rename to ...
>    (expand_doubleword_clz_ctz_ffs): ... this.  Add UNOPTAB argument,
>    handle also doubleword CTZ and FFS in addition to CLZ.
>    (expand_unop): Adjust caller.  Also call it for doubleword
>    ctz_optab and ffs_optab.
> 
>    * gcc.target/i386/ctzll-1.c: New test.
>    * gcc.target/i386/ffsll-1.c: New test.
> 
> --- gcc/optabs.cc.jj    2023-06-07 09:42:14.701130305 +0200
> +++ gcc/optabs.cc    2023-06-07 14:35:04.909879272 +0200
> @@ -2697,10 +2697,14 @@ expand_clrsb_using_clz (scalar_int_mode
>   return temp;
> }
> 
> -/* Try calculating clz of a double-word quantity as two clz's of word-sized
> -   quantities, choosing which based on whether the high word is nonzero.  */
> +static rtx expand_ffs (scalar_int_mode, rtx, rtx);
> +
> +/* Try calculating clz, ctz or ffs of a double-word quantity as two clz, ctz or
> +   ffs operations on word-sized quantities, choosing which based on whether the
> +   high (for clz) or low (for ctz and ffs) word is nonzero.  */
> static rtx
> -expand_doubleword_clz (scalar_int_mode mode, rtx op0, rtx target)
> +expand_doubleword_clz_ctz_ffs (scalar_int_mode mode, rtx op0, rtx target,
> +                   optab unoptab)
> {
>   rtx xop0 = force_reg (mode, op0);
>   rtx subhi = gen_highpart (word_mode, xop0);
> @@ -2709,6 +2713,7 @@ expand_doubleword_clz (scalar_int_mode m
>   rtx_code_label *after_label = gen_label_rtx ();
>   rtx_insn *seq;
>   rtx temp, result;
> +  int addend = 0;
> 
>   /* If we were not given a target, use a word_mode register, not a
>      'mode' register.  The result will fit, and nobody is expecting
> @@ -2721,6 +2726,9 @@ expand_doubleword_clz (scalar_int_mode m
>      'target' to tag a REG_EQUAL note on.  */
>   result = gen_reg_rtx (word_mode);
> 
> +  if (unoptab != clz_optab)
> +    std::swap (subhi, sublo);
> +
>   start_sequence ();
> 
>   /* If the high word is not equal to zero,
> @@ -2728,7 +2736,13 @@ expand_doubleword_clz (scalar_int_mode m
>   emit_cmp_and_jump_insns (subhi, CONST0_RTX (word_mode), EQ, 0,
>               word_mode, true, hi0_label);
> 
> -  temp = expand_unop_direct (word_mode, clz_optab, subhi, result, true);
> +  if (optab_handler (unoptab, word_mode) != CODE_FOR_nothing)
> +    temp = expand_unop_direct (word_mode, unoptab, subhi, result, true);
> +  else
> +    {
> +      gcc_assert (unoptab == ffs_optab);
> +      temp = expand_ffs (word_mode, subhi, result);
> +    }
>   if (!temp)
>     goto fail;
> 
> @@ -2739,14 +2753,32 @@ expand_doubleword_clz (scalar_int_mode m
>   emit_barrier ();
> 
>   /* Else clz of the full value is clz of the low word plus the number
> -     of bits in the high word.  */
> +     of bits in the high word.  Similarly for ctz/ffs of the high word,
> +     except that ffs should be 0 when both words are zero.  */
>   emit_label (hi0_label);
> 
> -  temp = expand_unop_direct (word_mode, clz_optab, sublo, 0, true);
> +  if (unoptab == ffs_optab)
> +    {
> +      convert_move (result, const0_rtx, true);
> +      emit_cmp_and_jump_insns (sublo, CONST0_RTX (word_mode), EQ, 0,
> +                   word_mode, true, after_label);
> +    }
> +
> +  if (optab_handler (unoptab, word_mode) != CODE_FOR_nothing)
> +    temp = expand_unop_direct (word_mode, unoptab, sublo, NULL_RTX, true);
> +  else
> +    {
> +      gcc_assert (unoptab == ffs_optab);
> +      temp = expand_unop_direct (word_mode, ctz_optab, sublo, NULL_RTX, true);
> +      addend = 1;
> +    }
> +
>   if (!temp)
>     goto fail;
> +
>   temp = expand_binop (word_mode, add_optab, temp,
> -               gen_int_mode (GET_MODE_BITSIZE (word_mode), word_mode),
> +               gen_int_mode (GET_MODE_BITSIZE (word_mode) + addend,
> +                     word_mode),
>               result, true, OPTAB_DIRECT);
>   if (!temp)
>     goto fail;
> @@ -2759,7 +2791,7 @@ expand_doubleword_clz (scalar_int_mode m
>   seq = get_insns ();
>   end_sequence ();
> 
> -  add_equal_note (seq, target, CLZ, xop0, NULL_RTX, mode);
> +  add_equal_note (seq, target, optab_to_code (unoptab), xop0, NULL_RTX, mode);
>   emit_insn (seq);
>   return target;
> 
> @@ -3252,7 +3284,8 @@ expand_unop (machine_mode mode, optab un
>      if (GET_MODE_SIZE (int_mode) == 2 * UNITS_PER_WORD
>          && optab_handler (unoptab, word_mode) != CODE_FOR_nothing)
>        {
> -          temp = expand_doubleword_clz (int_mode, op0, target);
> +          temp = expand_doubleword_clz_ctz_ffs (int_mode, op0, target,
> +                            unoptab);
>          if (temp)
>        return temp;
>        }
> @@ -3499,6 +3532,18 @@ expand_unop (machine_mode mode, optab un
>       if (temp)
>    return temp;
>     }
> +
> +  if ((unoptab == ctz_optab || unoptab == ffs_optab)
> +      && optimize_insn_for_speed_p ()
> +      && is_a <scalar_int_mode> (mode, &int_mode)
> +      && GET_MODE_SIZE (int_mode) == 2 * UNITS_PER_WORD
> +      && (optab_handler (unoptab, word_mode) != CODE_FOR_nothing
> +      || optab_handler (ctz_optab, word_mode) != CODE_FOR_nothing))
> +    {
> +      temp = expand_doubleword_clz_ctz_ffs (int_mode, op0, target, unoptab);
> +      if (temp)
> +    return temp;
> +    }
> 
>  try_libcall:
>   /* Now try a library call in this mode.  */
> --- gcc/testsuite/gcc.target/i386/ctzll-1.c.jj    2023-06-07 14:38:58.749648164 +0200
> +++ gcc/testsuite/gcc.target/i386/ctzll-1.c    2023-06-07 14:41:22.676659439 +0200
> @@ -0,0 +1,9 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +/* { dg-final { scan-assembler-not "__ctzdi2" } } */
> +
> +int
> +foo (unsigned long long x)
> +{
> +  return __builtin_ctzll (x);
> +}
> --- gcc/testsuite/gcc.target/i386/ffsll-1.c.jj    2023-06-07 14:40:00.859789953 +0200
> +++ gcc/testsuite/gcc.target/i386/ffsll-1.c    2023-06-07 14:41:15.104764068 +0200
> @@ -0,0 +1,9 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +/* { dg-final { scan-assembler-not "__ffsdi2" } } */
> +
> +int
> +foo (unsigned long long x)
> +{
> +  return __builtin_ffsll (x);
> +}
> 
>    Jakub
>