Re: [PATCH 7/7] ifcvt: add if-conversion to conditional-zero instructions

[Philipp Tomsich <philipp.tomsich@vrull.eu>]

On Sat, 12 Nov 2022 at 22:47, Andrew Pinski <pinskia@gmail.com> wrote:
>
> On Sat, Nov 12, 2022 at 1:34 PM Philipp Tomsich
> <philipp.tomsich@vrull.eu> wrote:
> >
> > Some architectures, as it the case on RISC-V with the proposed
> > ZiCondOps and the vendor-defined XVentanaCondOps, define a
> > conditional-zero instruction that is equivalent to:
> >  - the positive form:  rd = (rc != 0) ? rs : 0
> >  - the negated form:   rd = (rc == 0) ? rs : 0
> >
> > While noce_try_store_flag_mask will somewhat work for this case, it
> > will generate a number of atomic RTX that will misdirect the cost
> > calculation and may be too long (i.e., 4 RTX and more) to successfully
> > merge at combine-time.
> >
> > Instead, we add two new transforms that attempt to build up what we
> > define as the canonical form of a conditional-zero expression:
> >
> >   (set (match_operand 0 "register_operand" "=r")
> >        (and (neg (eq_or_ne (match_operand 1 "register_operand" "r")
> >                            (const_int 0)))
> >             (match_operand 2 "register_operand" "r")))
>
>
> Why is it not:
> (set x (if_then_else (eq_or_ne y (0)) z (0))
> (set x (if_then_else (ne y (0)) (0) z)
>
> That seems simpler to expression and is the normal a==0?0:z expression.

Having an if_then_else come out of if-conversion would be a bit unusual, as
transformation to branchless is the intent of the entire exercise.

Existing if-conversion via noce_try_store_flag_mask and noce_try_store_flag
already catch these sequences—if that happens, the above representation
will be present during combine: i.e., we need to implement this match anyway
(and it also matches the typical idiom, if a programmer tries to express the
idiom in a branchless way).

Consequently, we decided to use the same pattern as the canonical
representation in case that if-conversion had occurred prior.

> Also all canonical forms of RTL should be documented too.
> They are documented here:
> https://gcc.gnu.org/onlinedocs/gcc-12.2.0/gccint/Insn-Canonicalizations.html
> https://gcc.gnu.org/onlinedocs/gccint/machine-descriptions/canonicalization-of-instructions.html
> gcc/doc/gccint/machine-descriptions/canonicalization-of-instructions.rst
>
>
> Thanks,
> Andrew Pinski
>
> >
> > Architectures that provide a conditional-zero are thus expected to
> > define an instruction matching this pattern in their backend.
> >
> > Based on this, we support the following cases:
> >  - noce_try_condzero:
> >       a ? a : b
> >       a ? b : 0  (and then/else swapped)
> >      !a ? b : 0  (and then/else swapped)
> >  - noce_try_condzero_arith:
> >      conditional-plus, conditional-minus, conditional-and,
> >      conditional-or, conditional-xor, conditional-shift,
> >      conditional-and
> >
> > Given that this is hooked into the CE passes, it is less powerful than
> > a tree-pass (e.g., it can not transform cases where an extension, such
> > as for uint16_t operations is in either the then or else-branch
> > together with the arithmetic) but already covers a good array of cases
> > and triggers across SPEC CPU 2017.
> > Adding transofmrations in a tree pass will be considered as a future
> > improvement.
> >
> > gcc/ChangeLog:
> >
> >         * ifcvt.cc (noce_emit_insn): Add prototype.
> >         (noce_emit_condzero): Helper for noce_try_condzero and
> >         noce_try_condzero_arith transforms.
> >         (noce_try_condzero): New transform.
> >         (noce_try_condzero_arith): New transform for conditional
> >         arithmetic that can be built up by exploiting that the
> >         conditional-zero instruction will inject 0, which acts
> >         as the neutral element for operations.
> >         (noce_process_if_block): Call noce_try_condzero and
> >         noce_try_condzero_arith.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * gcc.target/riscv/xventanacondops-and-01.c: New test.
> >         * gcc.target/riscv/xventanacondops-and-02.c: New test.
> >         * gcc.target/riscv/xventanacondops-eq-01.c: New test.
> >         * gcc.target/riscv/xventanacondops-eq-02.c: New test.
> >         * gcc.target/riscv/xventanacondops-lt-01.c: New test.
> >         * gcc.target/riscv/xventanacondops-ne-01.c: New test.
> >         * gcc.target/riscv/xventanacondops-xor-01.c: New test.
> >
> > Signed-off-by: Philipp Tomsich <philipp.tomsich@vrull.eu>
> > ---
> >
> >  gcc/ifcvt.cc                                  | 214 ++++++++++++++++++
> >  .../gcc.target/riscv/xventanacondops-and-01.c |  16 ++
> >  .../gcc.target/riscv/xventanacondops-and-02.c |  15 ++
> >  .../gcc.target/riscv/xventanacondops-eq-01.c  |  11 +
> >  .../gcc.target/riscv/xventanacondops-eq-02.c  |  14 ++
> >  .../gcc.target/riscv/xventanacondops-lt-01.c  |  16 ++
> >  .../gcc.target/riscv/xventanacondops-ne-01.c  |  11 +
> >  .../gcc.target/riscv/xventanacondops-xor-01.c |  14 ++
> >  8 files changed, 311 insertions(+)
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-and-01.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-and-02.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-eq-01.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-eq-02.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-lt-01.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-ne-01.c
> >  create mode 100644 gcc/testsuite/gcc.target/riscv/xventanacondops-xor-01.c
> >
> > diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
> > index eb8efb89a89..41c58876d05 100644
> > --- a/gcc/ifcvt.cc
> > +++ b/gcc/ifcvt.cc
> > @@ -97,6 +97,7 @@ static int find_if_case_2 (basic_block, edge, edge);
> >  static int dead_or_predicable (basic_block, basic_block, basic_block,
> >                                edge, int);
> >  static void noce_emit_move_insn (rtx, rtx);
> > +static rtx_insn *noce_emit_insn (rtx);
> >  static rtx_insn *block_has_only_trap (basic_block);
> >  static void need_cmov_or_rewire (basic_block, hash_set<rtx_insn *> *,
> >                                  hash_map<rtx_insn *, int> *);
> > @@ -787,6 +788,9 @@ static rtx noce_get_alt_condition (struct noce_if_info *, rtx, rtx_insn **);
> >  static int noce_try_minmax (struct noce_if_info *);
> >  static int noce_try_abs (struct noce_if_info *);
> >  static int noce_try_sign_mask (struct noce_if_info *);
> > +static rtx noce_emit_condzero (struct noce_if_info *, rtx, bool = false);
> > +static int noce_try_condzero (struct noce_if_info *);
> > +static int noce_try_condzero_arith (struct noce_if_info *);
> >
> >  /* Return the comparison code for reversed condition for IF_INFO,
> >     or UNKNOWN if reversing the condition is not possible.  */
> > @@ -1664,6 +1668,212 @@ noce_try_addcc (struct noce_if_info *if_info)
> >    return FALSE;
> >  }
> >
> > +/* Helper to noce_try_condzero: cond ? a : 0. */
> > +static rtx
> > +noce_emit_condzero (struct noce_if_info *if_info, rtx a, bool reverse)
> > +{
> > +  /* The canonical form for a conditional-zero-or-value is:
> > +       (set (match_operand 0 "register_operand" "=r")
> > +           (and (neg (eq_or_ne (match_operand 1 "register_operand" "r")
> > +                               (const_int 0)))
> > +                (match_operand 2 "register_operand" "r")))
> > +   */
> > +
> > +  machine_mode opmode = GET_MODE (if_info->x);
> > +  enum rtx_code code = GET_CODE (if_info->cond);
> > +  rtx cond;
> > +  rtx op_a = XEXP (if_info->cond, 0);
> > +  rtx op_b = XEXP (if_info->cond, 1);
> > +
> > +  /* If it is not a EQ/NE comparison against const0_rtx, canonicalize
> > +     by first synthesizing a truth-value and then building a NE
> > +     condition around it. */
> > +  if ((code != EQ && code != NE) || XEXP (if_info->cond, 1) != const0_rtx)
> > +    {
> > +      rtx tmp = gen_reg_rtx (opmode);
> > +
> > +      start_sequence ();
> > +      cond = gen_rtx_fmt_ee (code, opmode, op_a, op_b);
> > +      if (!noce_emit_insn (gen_rtx_SET (tmp, cond)))
> > +       {
> > +         end_sequence ();
> > +
> > +         /* If we can't emit this pattern, try to reverse it and
> > +            invert the polarity of the second test. */
> > +         start_sequence ();
> > +         cond = gen_rtx_fmt_ee (reverse_condition (code), opmode, op_a, op_b);
> > +         if (!noce_emit_insn (gen_rtx_SET (tmp, cond))) {
> > +           end_sequence ();
> > +           return NULL_RTX;
> > +         }
> > +
> > +         /* We have recovered by reversing the first comparison,
> > +            so we need change the second one around as well... */
> > +         reverse = !reverse;
> > +       }
> > +      rtx_insn *seq = get_insns ();
> > +      end_sequence ();
> > +      emit_insn (seq);
> > +
> > +      /* Set up the second comparison that will be embedded in the
> > +        canonical conditional-zero-or-value RTX. */
> > +      code = NE;
> > +      op_a = tmp;
> > +      op_b = const0_rtx;
> > +    }
> > +
> > +  cond = gen_rtx_fmt_ee (reverse ? reverse_condition (code) : code,
> > +                        opmode, op_a, op_b);
> > +
> > +  /* Build (and (neg (eq_or_ne ... const0_rtx)) (reg <a>)) */
> > +  rtx target = gen_reg_rtx (opmode);
> > +  rtx czero = gen_rtx_AND (opmode, gen_rtx_NEG (opmode, cond), a);
> > +  noce_emit_move_insn (target, czero);
> > +
> > +  return target;
> > +}
> > +
> > +/* Use a conditional-zero instruction for "if (test) x = 0;", if available. */
> > +static int
> > +noce_try_condzero (struct noce_if_info *if_info)
> > +{
> > +  rtx target;
> > +  rtx_insn *seq;
> > +  int reversep = 0;
> > +  rtx orig_b = NULL_RTX;
> > +  rtx cond = if_info->cond;
> > +  enum rtx_code code = GET_CODE (cond);
> > +  rtx cond_arg0 = XEXP (cond, 0);
> > +  rtx cond_arg1 = XEXP (cond, 1);
> > +
> > +  if (!noce_simple_bbs (if_info))
> > +    return FALSE;
> > +
> > +  /* We may encounter the form "(a != 0) ? a : b", which can be
> > +     simplified to "a | ((a != 0) ? 0 : b)". */
> > +  if (code == NE && cond_arg1 == const0_rtx &&
> > +      REG_P (if_info->b) && rtx_equal_p (if_info->b, cond_arg0))
> > +    {
> > +      orig_b = if_info->b;
> > +      if_info->b = const0_rtx;
> > +    }
> > +
> > +  /* We may encounter the form "(a != 0) ? b : a", which can be
> > +     simplied to "(a != 0) ? b : 0". */
> > +  if (code == EQ && cond_arg1 == const0_rtx &&
> > +      REG_P (if_info->b) && rtx_equal_p (if_info->b, cond_arg0))
> > +    {
> > +      /* We know that cond_arg0 is const_0, if the THEN branch is
> > +        taken... so if it is the same as if_info->b (yes, things are
> > +        backwards!), we can rewrite it with that knowledge.  */
> > +      if_info->b = const0_rtx;
> > +    }
> > +
> > +  start_sequence ();
> > +
> > +  if ((if_info->a == const0_rtx
> > +       && (REG_P (if_info->b) || rtx_equal_p (if_info->b, if_info->x)))
> > +      || ((reversep = (noce_reversed_cond_code (if_info) != UNKNOWN))
> > +         && if_info->b == const0_rtx
> > +         && (REG_P (if_info->a) || rtx_equal_p (if_info->a, if_info->x))))
> > +    {
> > +      target = noce_emit_condzero(if_info,
> > +                                 reversep ? if_info->a : if_info->b,
> > +                                 reversep);
> > +
> > +      if (orig_b && target)
> > +       target = expand_simple_binop (GET_MODE (if_info->x), IOR, orig_b,
> > +                                     target, if_info->x, 0, OPTAB_WIDEN);
> > +
> > +      if (target)
> > +       {
> > +         if (target != if_info->x)
> > +           noce_emit_move_insn (if_info->x, target);
> > +
> > +         seq = end_ifcvt_sequence (if_info);
> > +         if (!seq || !targetm.noce_conversion_profitable_p (seq, if_info))
> > +           return FALSE;
> > +
> > +         emit_insn_before_setloc (seq, if_info->jump,
> > +                                  INSN_LOCATION (if_info->insn_a));
> > +         if_info->transform_name = "noce_try_condzero";
> > +
> > +         return TRUE;
> > +       }
> > +    }
> > +
> > +  end_sequence ();
> > +
> > +  return FALSE;
> > +}
> > +
> > +/* Convert "if (test) x op= a;" to a branchless sequence using the
> > +   canonical form for a conditional-zero. */
> > +static int
> > +noce_try_condzero_arith (struct noce_if_info *if_info)
> > +{
> > +  rtx target;
> > +  rtx_insn *seq;
> > +  rtx_code op = GET_CODE (if_info->a);
> > +  const rtx arg0 = XEXP (if_info->a, 0);
> > +  const rtx arg1 = XEXP (if_info->a, 1);
> > +
> > +  if (!noce_simple_bbs (if_info))
> > +    return FALSE;
> > +
> > +  /* Check for no else condition.  */
> > +  if (!rtx_equal_p (if_info->x, if_info->b))
> > +    return FALSE;
> > +
> > +  if (op != PLUS && op != MINUS && op != IOR && op != XOR &&
> > +      op != ASHIFT && op != ASHIFTRT && op != LSHIFTRT && op != AND)
> > +    return FALSE;
> > +
> > +  if (!rtx_equal_p (if_info->x, arg0))
> > +    return FALSE;
> > +
> > +  start_sequence ();
> > +
> > +  target = noce_emit_condzero(if_info, arg1, op != AND ? true : false);
> > +
> > +  if (target)
> > +    {
> > +      rtx op1 = if_info->x;
> > +
> > +      if (op == AND)
> > +       {
> > +         /* Emit "tmp = x & val;" followed by "tmp |= !cond ? x : 0;" */
> > +         op1 = expand_simple_binop (GET_MODE (if_info->x), AND, op1,
> > +                                    arg1, NULL_RTX, 0, OPTAB_WIDEN);
> > +         op = IOR;
> > +       }
> > +
> > +      if (op1)
> > +       target = expand_simple_binop (GET_MODE (if_info->x), op, op1,
> > +                                     target, if_info->x, 0, OPTAB_WIDEN);
> > +    }
> > +
> > +  if (target)
> > +    {
> > +      if (target != if_info->x)
> > +       noce_emit_move_insn (if_info->x, target);
> > +
> > +      seq = end_ifcvt_sequence (if_info);
> > +      if (!seq || !targetm.noce_conversion_profitable_p (seq, if_info))
> > +       return FALSE;
> > +
> > +      emit_insn_before_setloc(seq, if_info->jump,
> > +                             INSN_LOCATION(if_info->insn_a));
> > +      if_info->transform_name = "noce_try_condzero_arith";
> > +
> > +      return TRUE;
> > +    }
> > +
> > +  end_sequence ();
> > +
> > +  return FALSE;
> > +}
> > +
> >  /* Convert "if (test) x = 0;" to "x &= -(test == 0);"  */
> >
> >  static int
> > @@ -3967,8 +4177,12 @@ noce_process_if_block (struct noce_if_info *if_info)
> >      {
> >        if (noce_try_addcc (if_info))
> >         goto success;
> > +      if (noce_try_condzero (if_info))
> > +       goto success;
> >        if (noce_try_store_flag_mask (if_info))
> >         goto success;
> > +      if (noce_try_condzero_arith (if_info))
> > +       goto success;
> >        if (HAVE_conditional_move
> >           && noce_try_cmove_arith (if_info))
> >         goto success;
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-and-01.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-and-01.c
> > new file mode 100644
> > index 00000000000..9b26cdf0513
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-and-01.c
> > @@ -0,0 +1,16 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-Os" "-Oz" } } */
> > +
> > +long and1(long a, long b, long c, long d)
> > +{
> > +  if (c < d)
> > +    a &= b;
> > +
> > +  return a;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "and\t" 1 } } */
> > +/* { dg-final { scan-assembler-times "slt" 1 } } */
> > +/* { dg-final { scan-assembler-times "vt.maskcn" 1 } } */
> > +/* { dg-final { scan-assembler-times "or\t" 1 } } */
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-and-02.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-and-02.c
> > new file mode 100644
> > index 00000000000..66d2ec10211
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-and-02.c
> > @@ -0,0 +1,15 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-Os" "-Oz" } } */
> > +
> > +int and2(int a, int b, long c)
> > +{
> > +  if (c)
> > +    a &= b;
> > +
> > +  return a;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "and\t" 1 } } */
> > +/* { dg-final { scan-assembler-times "vt.maskcn" 1 } } */
> > +/* { dg-final { scan-assembler-times "or\t" 1 } } */
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-01.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-01.c
> > new file mode 100644
> > index 00000000000..bc877d9e81b
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-01.c
> > @@ -0,0 +1,11 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" } } */
> > +
> > +long
> > +eq1 (long a, long b)
> > +{
> > +  return (a == 0) ? b : 0;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "vt.maskcn" 1 } } */
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-02.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-02.c
> > new file mode 100644
> > index 00000000000..28317613ba8
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-eq-02.c
> > @@ -0,0 +1,14 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" } } */
> > +
> > +long
> > +eq2 (long a, long b)
> > +{
> > +  if (a == 0)
> > +    return b;
> > +
> > +  return 0;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "vt.maskcn" 1 } } */
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-lt-01.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-lt-01.c
> > new file mode 100644
> > index 00000000000..db7498801f9
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-lt-01.c
> > @@ -0,0 +1,16 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-Os" "-Oz" } } */
> > +
> > +long long sink (long long);
> > +
> > +long long lt3 (long long a, long long b)
> > +{
> > +  if (a < b)
> > +    b = 0;
> > +
> > +  return sink(b);
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "vt.maskcn\" 1 } } */
> > +/* { dg-final { scan-assembler-times "slt\t" 1 } } */
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-ne-01.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-ne-01.c
> > new file mode 100644
> > index 00000000000..eff1486828c
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-ne-01.c
> > @@ -0,0 +1,11 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" } } */
> > +
> > +long long ne1(long long a, long long b)
> > +{
> > +  return (a != 0) ? b : 0;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "vt.maskc" 1 } } */
> > +
> > diff --git a/gcc/testsuite/gcc.target/riscv/xventanacondops-xor-01.c b/gcc/testsuite/gcc.target/riscv/xventanacondops-xor-01.c
> > new file mode 100644
> > index 00000000000..43020790a22
> > --- /dev/null
> > +++ b/gcc/testsuite/gcc.target/riscv/xventanacondops-xor-01.c
> > @@ -0,0 +1,14 @@
> > +/* { dg-do compile } */
> > +/* { dg-options "-march=rv64gc_xventanacondops -mabi=lp64" } */
> > +/* { dg-skip-if "" { *-*-* } { "-O0" "-Og" "-Os" "-Oz" } } */
> > +
> > +long xor1(long crc, long poly)
> > +{
> > +  if (crc & 1)
> > +    crc ^= poly;
> > +
> > +  return crc;
> > +}
> > +
> > +/* { dg-final { scan-assembler-times "vt.maskc" 1 } } */
> > +/* { dg-final { scan-assembler-times "xor\t" 1 } } */
> > --
> > 2.34.1
> >