Re: [PATCH] i386: Improve extensions of __builtin_clz and constant - __builtin_clz for -mno-lzcnt [PR78103]

[Uros Bizjak <ubizjak@gmail.com>]

On Wed, Jul 28, 2021 at 10:36 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> Hi!
>
> This patch improves emitted code for the non-TARGET_LZCNT case.
> As __builtin_clz* is UB on 0 argument and for !TARGET_LZCNT
> CLZ_VALUE_DEFINED_AT_ZERO is 0, it is UB even at RTL time and so we
> can take advantage of that and assume the result will be 0 to 31 or
> 0 to 63.
> Given that, sign or zero extension of that result are the same and
> are actually already performed by bsrl or xorl instructions.
> And constant - __builtin_clz* can be simplified into
> bsr + constant - bitmask.
> For TARGET_LZCNT, a lot of this is already fine as is (e.g. the sign or
> zero extensions), and other optimizations are IMHO not possible
> (if we have lzcnt, we've lost information on whether it is UB at
> zero or not and so can't transform it into bsr even when that is
> 1-2 insns shorter).
> The changes on the 3 testcases between unpatched and patched gcc
> are for -m64:
> pr78103-1.s:
>         bsrq    %rdi, %rax
> -       xorq    $63, %rax
> -       cltq
> +       xorl    $63, %eax
> ...
>         bsrq    %rdi, %rax
> -       xorq    $63, %rax
> -       cltq
> +       xorl    $63, %eax
> ...
>         bsrl    %edi, %eax
>         xorl    $31, %eax
> -       cltq
> ...
>         bsrl    %edi, %eax
>         xorl    $31, %eax
> -       cltq
> pr78103-2.s:
>         bsrl    %edi, %edi
> -       movl    $32, %eax
> -       xorl    $31, %edi
> -       subl    %edi, %eax
> +       leal    1(%rdi), %eax
> ...
> -       bsrl    %edi, %edi
> -       movl    $31, %eax
> -       xorl    $31, %edi
> -       subl    %edi, %eax
> +       bsrl    %edi, %eax
> ...
>         bsrq    %rdi, %rdi
> -       movl    $64, %eax
> -       xorq    $63, %rdi
> -       subl    %edi, %eax
> +       leal    1(%rdi), %eax
> ...
> -       bsrq    %rdi, %rdi
> -       movl    $63, %eax
> -       xorq    $63, %rdi
> -       subl    %edi, %eax
> +       bsrq    %rdi, %rax
> pr78103-3.s:
>         bsrl    %edi, %edi
> -       movl    $32, %eax
> -       xorl    $31, %edi
> -       movslq  %edi, %rdi
> -       subq    %rdi, %rax
> +       leaq    1(%rdi), %rax
> ...
> -       bsrl    %edi, %edi
> -       movl    $31, %eax
> -       xorl    $31, %edi
> -       movslq  %edi, %rdi
> -       subq    %rdi, %rax
> +       bsrl    %edi, %eax
> ...
>         bsrq    %rdi, %rdi
> -       movl    $64, %eax
> -       xorq    $63, %rdi
> -       movslq  %edi, %rdi
> -       subq    %rdi, %rax
> +       leaq    1(%rdi), %rax
> ...
> -       bsrq    %rdi, %rdi
> -       movl    $63, %eax
> -       xorq    $63, %rdi
> -       movslq  %edi, %rdi
> -       subq    %rdi, %rax
> +       bsrq    %rdi, %rax
>
> Most of the changes are done with combine splitters, but for
> *bsr_rex64_2 and *bsr_2 I had to use define_insn_and_split, because
> as mentioned in the PR the combiner unfortunately doesn't create LOG_LINKS
> in between the two insns created by combine splitter, so it can't be
> combined further with following instructions.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
>
> 2021-07-28  Jakub Jelinek  <jakub@redhat.com>
>
>         PR target/78103
>         * config/i386/i386.md (*bsr_rex64_1, *bsr_1, *bsr_zext_1): New
>         define_insn patterns.
>         (*bsr_rex64_2, *bsr_2): New define_insn_and_split patterns.
>         Add combine splitters for constant - clz.
>         (clz<mode>2): Use a temporary pseudo for bsr result.
>
>         * gcc.target/i386/pr78103-1.c: New test.
>         * gcc.target/i386/pr78103-2.c: New test.
>         * gcc.target/i386/pr78103-3.c: New test.
>
> --- gcc/config/i386/i386.md.jj  2021-07-27 09:47:30.311970004 +0200
> +++ gcc/config/i386/i386.md     2021-07-27 15:37:59.011394624 +0200
> @@ -14761,6 +14761,18 @@ (define_insn "bsr_rex64"
>     (set_attr "znver1_decode" "vector")
>     (set_attr "mode" "DI")])
>
> +(define_insn "*bsr_rex64_1"
> +  [(set (match_operand:DI 0 "register_operand" "=r")
> +       (minus:DI (const_int 63)
> +                 (clz:DI (match_operand:DI 1 "nonimmediate_operand" "rm"))))
> +   (clobber (reg:CC FLAGS_REG))]
> +  "!TARGET_LZCNT && TARGET_64BIT"
> +  "bsr{q}\t{%1, %0|%0, %1}"
> +  [(set_attr "type" "alu1")
> +   (set_attr "prefix_0f" "1")
> +   (set_attr "znver1_decode" "vector")
> +   (set_attr "mode" "DI")])
> +
>  (define_insn "bsr"
>    [(set (reg:CCZ FLAGS_REG)
>         (compare:CCZ (match_operand:SI 1 "nonimmediate_operand" "rm")
> @@ -14775,17 +14787,210 @@ (define_insn "bsr"
>     (set_attr "znver1_decode" "vector")
>     (set_attr "mode" "SI")])
>
> +(define_insn "*bsr_1"
> +  [(set (match_operand:SI 0 "register_operand" "=r")
> +       (minus:SI (const_int 31)
> +                 (clz:SI (match_operand:SI 1 "nonimmediate_operand" "rm"))))
> +   (clobber (reg:CC FLAGS_REG))]
> +  "!TARGET_LZCNT"
> +  "bsr{l}\t{%1, %0|%0, %1}"
> +  [(set_attr "type" "alu1")
> +   (set_attr "prefix_0f" "1")
> +   (set_attr "znver1_decode" "vector")
> +   (set_attr "mode" "SI")])
> +
> +(define_insn "*bsr_zext_1"
> +  [(set (match_operand:DI 0 "register_operand" "=r")
> +       (zero_extend:DI
> +         (minus:SI
> +           (const_int 31)
> +           (clz:SI (match_operand:SI 1 "nonimmediate_operand" "rm")))))
> +   (clobber (reg:CC FLAGS_REG))]
> +  "!TARGET_LZCNT && TARGET_64BIT"
> +  "bsr{l}\t{%1, %k0|%k0, %1}"
> +  [(set_attr "type" "alu1")
> +   (set_attr "prefix_0f" "1")
> +   (set_attr "znver1_decode" "vector")
> +   (set_attr "mode" "SI")])
> +
> +; As bsr is undefined behavior on zero and for other input
> +; values it is in range 0 to 63, we can optimize away sign-extends.
> +(define_insn_and_split "*bsr_rex64_2"
> +  [(set (match_operand:DI 0 "register_operand")
> +       (xor:DI
> +         (sign_extend:DI
> +           (minus:SI
> +             (const_int 63)
> +             (subreg:SI (clz:DI (match_operand:DI 1 "nonimmediate_operand"))
> +                        0)))
> +         (const_int 63)))
> +    (clobber (reg:CC FLAGS_REG))]
> +  "!TARGET_LZCNT && TARGET_64BIT && ix86_pre_reload_split ()"
> +  "#"
> +  "&& 1"
> +  [(parallel [(set (reg:CCZ FLAGS_REG)
> +                  (compare:CCZ (match_dup 1) (const_int 0)))
> +             (set (match_dup 2)
> +                  (minus:DI (const_int 63) (clz:DI (match_dup 1))))])
> +   (parallel [(set (match_dup 0)
> +                  (zero_extend:DI (xor:SI (match_dup 3) (const_int 63))))
> +             (clobber (reg:CC FLAGS_REG))])]
> +{
> +  operands[2] = gen_reg_rtx (DImode);
> +  operands[3] = lowpart_subreg (SImode, operands[2], DImode);
> +})
> +
> +(define_insn_and_split "*bsr_2"
> +  [(set (match_operand:DI 0 "register_operand")
> +       (sign_extend:DI
> +         (xor:SI
> +           (minus:SI
> +             (const_int 31)
> +             (clz:SI (match_operand:SI 1 "nonimmediate_operand")))
> +           (const_int 31))))
> +   (clobber (reg:CC FLAGS_REG))]
> +  "!TARGET_LZCNT && TARGET_64BIT && ix86_pre_reload_split ()"
> +  "#"
> +  "&& 1"
> +  [(parallel [(set (reg:CCZ FLAGS_REG)
> +                  (compare:CCZ (match_dup 1) (const_int 0)))
> +             (set (match_dup 2)
> +                  (minus:SI (const_int 31) (clz:SI (match_dup 1))))])
> +   (parallel [(set (match_dup 0)
> +                  (zero_extend:DI (xor:SI (match_dup 2) (const_int 31))))
> +             (clobber (reg:CC FLAGS_REG))])]
> +  "operands[2] = gen_reg_rtx (SImode);")
> +
> +; Splitters to optimize 64 - __builtin_clzl (x) or 32 - __builtin_clz (x).
> +; Again, as for !TARGET_LZCNT CLZ is UB at zero, CLZ is guaranteed to be
> +; in [0, 63] or [0, 31] range.
> +(define_split
> +  [(set (match_operand:SI 0 "register_operand")
> +       (minus:SI
> +         (match_operand:SI 2 "const_int_operand")
> +         (xor:SI
> +           (minus:SI (const_int 63)
> +                     (subreg:SI
> +                       (clz:DI (match_operand:DI 1 "nonimmediate_operand"))
> +                       0))
> +           (const_int 63))))]
> +  "!TARGET_LZCNT && TARGET_64BIT && ix86_pre_reload_split ()"
> +  [(set (match_dup 3) (minus:DI (const_int 63) (clz:DI (match_dup 1))))
> +   (set (match_dup 0) (plus:SI (match_dup 5) (match_dup 4)))]

Please put some space here, e.g.:

[(set (match_dup 3)
        (minus:DI (const_int 63) (clz:DI (match_dup 1))))
 (set (match_dup 0)
        (plus:SI (match_dup 5) (match_dup 4)))]

OK with other patterns changed in a similar way.

> +{
> +  operands[3] = gen_reg_rtx (DImode);
> +  operands[5] = lowpart_subreg (SImode, operands[3], DImode);
> +  if (INTVAL (operands[2]) == 63)
> +    {
> +      rtx tem = gen_rtx_CLZ (DImode, operands[1]);
> +      tem = gen_rtx_MINUS (DImode, operands[2], tem);
> +      tem = gen_rtx_SET (operands[3], tem);
> +      emit_insn (tem);

Can you just name the relevant insn pattern and use

emit_insn (gen_bsr_1)?

This is much more friendly and descriptive to readers. (The combiner
adds clobber automatically, but only after it fails to recognize insn
without clobber. We don't have insn without clobber, so we can help
the combiner a bit.)

Thanks,
Uros.

> +      emit_move_insn (operands[0], operands[5]);
> +      DONE;
> +    }
> +  operands[4] = gen_int_mode (UINTVAL (operands[2]) - 63, SImode);
> +})
> +
> +(define_split
> +  [(set (match_operand:SI 0 "register_operand")
> +       (minus:SI
> +         (match_operand:SI 2 "const_int_operand")
> +         (xor:SI
> +           (minus:SI (const_int 31)
> +                     (clz:SI (match_operand:SI 1 "nonimmediate_operand")))
> +           (const_int 31))))]
> +  "!TARGET_LZCNT && ix86_pre_reload_split ()"
> +  [(set (match_dup 3) (minus:SI (const_int 31) (clz:SI (match_dup 1))))
> +   (set (match_dup 0) (plus:SI (match_dup 3) (match_dup 4)))]
> +{
> +  if (INTVAL (operands[2]) == 31)
> +    {
> +      rtx tem = gen_rtx_CLZ (SImode, operands[1]);
> +      tem = gen_rtx_MINUS (SImode, operands[2], tem);
> +      tem = gen_rtx_SET (operands[0], tem);
> +      emit_insn (tem);
> +      DONE;
> +    }
> +  operands[3] = gen_reg_rtx (SImode);
> +  operands[4] = gen_int_mode (UINTVAL (operands[2]) - 31, SImode);
> +})
> +
> +(define_split
> +  [(set (match_operand:DI 0 "register_operand")
> +       (minus:DI
> +         (match_operand:DI 2 "const_int_operand")
> +         (xor:DI
> +           (sign_extend:DI
> +             (minus:SI (const_int 63)
> +                       (subreg:SI
> +                         (clz:DI (match_operand:DI 1 "nonimmediate_operand"))
> +                         0)))
> +           (const_int 63))))]
> +  "!TARGET_LZCNT
> +   && TARGET_64BIT
> +   && ix86_pre_reload_split ()
> +   && ((unsigned HOST_WIDE_INT)
> +       trunc_int_for_mode (UINTVAL (operands[2]) - 63, SImode)
> +       == UINTVAL (operands[2]) - 63)"
> +  [(set (match_dup 3) (minus:DI (const_int 63) (clz:DI (match_dup 1))))
> +   (set (match_dup 0) (plus:DI (match_dup 3) (match_dup 4)))]
> +{
> +  if (INTVAL (operands[2]) == 63)
> +    {
> +      rtx tem = gen_rtx_CLZ (DImode, operands[1]);
> +      tem = gen_rtx_MINUS (DImode, operands[2], tem);
> +      tem = gen_rtx_SET (operands[0], tem);
> +      emit_insn (tem);
> +      DONE;
> +    }
> +  operands[3] = gen_reg_rtx (DImode);
> +  operands[4] = GEN_INT (UINTVAL (operands[2]) - 63);
> +})
> +
> +(define_split
> +  [(set (match_operand:DI 0 "register_operand")
> +       (minus:DI
> +         (match_operand:DI 2 "const_int_operand")
> +         (sign_extend:DI
> +           (xor:SI
> +             (minus:SI (const_int 31)
> +                       (clz:SI (match_operand:SI 1 "nonimmediate_operand")))
> +             (const_int 31)))))]
> +  "!TARGET_LZCNT
> +   && TARGET_64BIT
> +   && ix86_pre_reload_split ()
> +   && ((unsigned HOST_WIDE_INT)
> +       trunc_int_for_mode (UINTVAL (operands[2]) - 31, SImode)
> +       == UINTVAL (operands[2]) - 31)"
> +  [(set (match_dup 3)
> +       (zero_extend:DI (minus:SI (const_int 31) (clz:SI (match_dup 1)))))
> +   (set (match_dup 0) (plus:DI (match_dup 3) (match_dup 4)))]
> +{
> +  if (INTVAL (operands[2]) == 31)
> +    {
> +      rtx tem = gen_rtx_CLZ (SImode, operands[1]);
> +      tem = gen_rtx_MINUS (SImode, operands[2], tem);
> +      tem = gen_rtx_ZERO_EXTEND (DImode, tem);
> +      tem = gen_rtx_SET (operands[0], tem);
> +      emit_insn (tem);
> +      DONE;
> +    }
> +  operands[3] = gen_reg_rtx (DImode);
> +  operands[4] = GEN_INT (UINTVAL (operands[2]) - 31);
> +})
> +
>  (define_expand "clz<mode>2"
>    [(parallel
>       [(set (reg:CCZ FLAGS_REG)
>         (compare:CCZ (match_operand:SWI48 1 "nonimmediate_operand" "rm")
>                      (const_int 0)))
> -      (set (match_operand:SWI48 0 "register_operand")
> -          (minus:SWI48
> -            (match_dup 2)
> -            (clz:SWI48 (match_dup 1))))])
> +      (set (match_dup 3) (minus:SWI48
> +                          (match_dup 2)
> +                          (clz:SWI48 (match_dup 1))))])
>     (parallel
> -     [(set (match_dup 0) (xor:SWI48 (match_dup 0) (match_dup 2)))
> +     [(set (match_operand:SWI48 0 "register_operand")
> +          (xor:SWI48 (match_dup 3) (match_dup 2)))
>        (clobber (reg:CC FLAGS_REG))])]
>    ""
>  {
> @@ -14795,6 +15000,7 @@ (define_expand "clz<mode>2"
>        DONE;
>      }
>    operands[2] = GEN_INT (GET_MODE_BITSIZE (<MODE>mode)-1);
> +  operands[3] = gen_reg_rtx (<MODE>mode);
>  })
>
>  (define_insn_and_split "clz<mode>2_lzcnt"
> --- gcc/testsuite/gcc.target/i386/pr78103-1.c.jj        2021-07-27 10:29:13.278547362 +0200
> +++ gcc/testsuite/gcc.target/i386/pr78103-1.c   2021-07-27 10:29:13.278547362 +0200
> @@ -0,0 +1,28 @@
> +/* PR target/78103 */
> +/* { dg-do compile { target { ! ia32 } } } */
> +/* { dg-options "-O2 -mno-lzcnt" } */
> +/* { dg-final { scan-assembler-not {\mcltq\M} } } */
> +
> +long long
> +foo (long long x)
> +{
> +  return __builtin_clzll (x);
> +}
> +
> +long long
> +bar (long long x)
> +{
> +  return (unsigned int) __builtin_clzll (x);
> +}
> +
> +long long
> +baz (int x)
> +{
> +  return __builtin_clz (x);
> +}
> +
> +long long
> +qux (int x)
> +{
> +  return (unsigned int) __builtin_clz (x);
> +}
> --- gcc/testsuite/gcc.target/i386/pr78103-2.c.jj        2021-07-27 10:29:13.278547362 +0200
> +++ gcc/testsuite/gcc.target/i386/pr78103-2.c   2021-07-27 10:29:13.278547362 +0200
> @@ -0,0 +1,33 @@
> +/* PR target/78103 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -mno-lzcnt" } */
> +/* { dg-final { scan-assembler-not {\mmovl\M} } } */
> +/* { dg-final { scan-assembler-not {\mxor[lq]\M} } } */
> +/* { dg-final { scan-assembler-not {\msubl\M} } } */
> +/* { dg-final { scan-assembler {\m(leal|addl)\M} } } */
> +
> +unsigned int
> +foo (unsigned int x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned int) - __builtin_clz (x);
> +}
> +
> +unsigned int
> +bar (unsigned int x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned int) - 1 - __builtin_clz (x);
> +}
> +
> +#ifdef __x86_64__
> +unsigned int
> +baz (unsigned long long x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned long long) - __builtin_clzll (x);
> +}
> +
> +unsigned int
> +qux (unsigned long long x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned long long) - 1 - __builtin_clzll (x);
> +}
> +#endif
> --- gcc/testsuite/gcc.target/i386/pr78103-3.c.jj        2021-07-27 15:45:51.421113690 +0200
> +++ gcc/testsuite/gcc.target/i386/pr78103-3.c   2021-07-27 15:45:35.678323393 +0200
> @@ -0,0 +1,32 @@
> +/* PR target/78103 */
> +/* { dg-do compile { target { ! ia32 } } } */
> +/* { dg-options "-O2 -mno-lzcnt" } */
> +/* { dg-final { scan-assembler-not {\mmovl\M} } } */
> +/* { dg-final { scan-assembler-not {\mmovslq\M} } } */
> +/* { dg-final { scan-assembler-not {\mxor[lq]\M} } } */
> +/* { dg-final { scan-assembler-not {\msubq\M} } } */
> +/* { dg-final { scan-assembler {\m(leaq|addq)\M} } } */
> +
> +unsigned long long
> +foo (unsigned int x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned int) - __builtin_clz (x);
> +}
> +
> +unsigned long long
> +bar (unsigned int x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned int) - 1 - __builtin_clz (x);
> +}
> +
> +unsigned long long
> +baz (unsigned long long x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned long long) - __builtin_clzll (x);
> +}
> +
> +unsigned long long
> +qux (unsigned long long x)
> +{
> +  return __CHAR_BIT__ * sizeof (unsigned long long) - 1 - __builtin_clzll (x);
> +}
>
>         Jakub
>