Re: [PATCH v3] forwprop: add simplify_phi_result_op() [PR101179]

On Fri, Jul 31, 2026 at 4:57 AM Daniel Barboza
<[email protected]> wrote:
>
> This new forwprop  step is my attempt to implement Richi's suggestions from
> v1 of this work [1] where he suggested to push things out of match.pd.
>
> The idea is to simplify DIV/MOD into RSHIFT/BIT_AND ops in which the
> divisor are pow2 integers in a PHI.  E.g.:
>
> phi_var = PHI <16,4>
> _x = _y % phi_var
>
> Can be turned into:
>
> phi_var = PHI <15,3>
> _x = _y & phi_var
>
> As long as we know that _y is a positive number or '_x' is a single use
> with a zero comparison.
>
> Most of 101179 use cases are solved by this change.
>
> Boostrapped and regression tested with x86_64, aarch64 and riscv64.
>
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2026-May/716303.html
>
>         PR tree-optimization/101179
>
> gcc/ChangeLog:
>
>         * tree-ssa-forwprop.cc (simplify_phi_result_op): New phiopt step
>         where MOD/DIV ops with pow2 divisors can be simplified to
>         BIT_AND/RSHIFT.
>         (pass_forwprop::execute): Call simplify_phi_result_op.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/pr101179.c: New test.
> ---
>
> Changes from v2:
> - moved the implementation from phiopt to forwprop.  It resulted in a
>   smaller and a bit cleaner logic.
> - v2 link: https://gcc.gnu.org/pipermail/gcc-patches/2026-July/724039.html
>
>  gcc/testsuite/gcc.dg/tree-ssa/pr101179.c |  56 ++++++++++++
>  gcc/tree-ssa-forwprop.cc                 | 110 +++++++++++++++++++++++
>  2 files changed, 166 insertions(+)
>  create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/pr101179.c
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/pr101179.c b/gcc/testsuite/gcc.dg/tree-ssa/pr101179.c
> new file mode 100644
> index 00000000000..478bdc6fcd3
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/pr101179.c
> @@ -0,0 +1,56 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O1 -fdump-tree-phiopt1" } */
> +
> +typedef unsigned uint;
> +
> +int f1 (int y, _Bool x)
> +{
> +  return y % (x ? 16 : 4) == 0;
> +}
> +
> +/* We can't turn this into bit_and because there's no
> +   guarantee 'y' is a positive val.  */
> +int f2 (int y, _Bool x)
> +{
> +  return y % (x ? 16 : 4);
> +}
> +
> +uint f3 (uint y, _Bool x)
> +{
> +  return y % (x ? 16 : 4) == 0;
> +}
> +
> +uint f4 (uint y, _Bool x)
> +{
> +  return y % (x ? 16 : 4);
> +}
> +
> +int g1 (int y, _Bool x)
> +{
> +  return y / (x ? 16 : 4) == 0;
> +}
> +
> +/* We can't turn this into rshift because there's no
> +   guarantee 'y' is a positive val.  */
> +int g2 (int y, _Bool x)
> +{
> +  return y / (x ? 16 : 4);
> +}
> +
> +/* This will be turned by match.pd into:
> +   "(X / Y) == 0 -> X < Y if X, Y are unsigned."
> +   We're adding it here for completioness.  */
> +uint g3 (uint y, _Bool x)
> +{
> +  return y / (x ? 16 : 4) == 0;
> +}
> +
> +uint g4 (uint y, _Bool x)
> +{
> +  return y / (x ? 16 : 4);
> +}
> +
> +/* { dg-final { scan-tree-dump-times " \& " 3 "phiopt1" } } */
> +/* { dg-final { scan-tree-dump-times " \% " 1 "phiopt1" } } */
> +/* { dg-final { scan-tree-dump-times " >> " 2 "phiopt1" } } */
> +/* { dg-final { scan-tree-dump-times " \\/ " 1 "phiopt1" } } */
> diff --git a/gcc/tree-ssa-forwprop.cc b/gcc/tree-ssa-forwprop.cc
> index 75f06c6ba41..32a9fee560c 100644
> --- a/gcc/tree-ssa-forwprop.cc
> +++ b/gcc/tree-ssa-forwprop.cc
> @@ -3623,6 +3623,108 @@ simplify_count_zeroes (gimple_stmt_iterator *gsi)
>    return true;
>  }
>
> +/* Verify if we have the following structure:
> +
> +   iftmp1 = PHI <pow2a, pow2b>
> +   _ssa1 = _ssa2 MOD|DIV iftmp1;
> +   _ssa3 = _ssa1 EQ|NE 0;
> +
> +   And, as long as "_ssa2" is either known to be positive or
> +   "_ssa1" is single use in a zero comparison, change the PHI
> +   args and "_ssa1" stmt to a cheaper alternative.
> +
> +   For MOD:
> +
> +   iftmp1 = PHI <(pow2a - 1), (pow2b - 1)>
> +   _ssa1 = _ssa2 & iftmp1;
> +
> +   For DIV:
> +
> +   iftmp1 = PHI <log2 (pow2a), log2 (pow2b)>
> +   _ssa1 = _ssa2 >> iftmp1;  */
> +static bool
> +simplify_phi_result_op (gimple *stmt, tree_code code)
> +{
> +  gphi *phi = dyn_cast<gphi *> (SSA_NAME_DEF_STMT (gimple_assign_rhs2 (stmt)));
Need a check for nullptr after dyn_cast.

> +  if (gimple_phi_num_args (phi) != 2)
> +    return false;

Instead of just handling 2 argument phis. why not handle N argument phis here.


> +
> +  tree arg0 = gimple_phi_arg_def (phi, 0);
> +  tree arg1 = gimple_phi_arg_def (phi, 1);
> +
> +  if (!integer_pow2p (arg0) || !tree_fits_uhwi_p (arg0)
> +      || !integer_pow2p (arg1) || !tree_fits_uhwi_p (arg1))
> +    return false;

That is 2 loops.
One to reject if the argument is not a power of 2. I don't think you
need tree_fits_uhwi_p (see below on why).

> +
> +  tree_code new_code;
> +  switch (code)
> +    {
> +      case TRUNC_MOD_EXPR:
> +      case CEIL_MOD_EXPR:
> +      case FLOOR_MOD_EXPR:
> +      case ROUND_MOD_EXPR:
> +       new_code = BIT_AND_EXPR;
> +       break;
> +      case TRUNC_DIV_EXPR:
> +      case CEIL_DIV_EXPR:
> +      case FLOOR_DIV_EXPR:
> +      case ROUND_DIV_EXPR:
> +       new_code = RSHIFT_EXPR;
> +       break;
> +
> +     default:
> +       return false;
> +    }
> +
> +  /* If rhs1 is a known positive value we can always apply these
> +     simplification.  Otherwise see if lhs is single_use with a
> +     EQ|NE 0 cmp.  */
> +  tree rhs1 = gimple_assign_rhs1 (stmt);
> +  if (!tree_expr_nonnegative_p (rhs1))
> +    {
> +      gimple *cmp_stmt;
> +      use_operand_p use_p;
> +
> +      if (!single_imm_use (gimple_assign_lhs (stmt), &use_p, &cmp_stmt)
> +         || !cmp_stmt
> +         || !is_gimple_assign (cmp_stmt))
> +       return false;
> +
> +      if (!(gimple_assign_rhs_code (cmp_stmt) == NE_EXPR
> +           || gimple_assign_rhs_code (cmp_stmt) == EQ_EXPR))
> +       return false;
> +
> +      if (!integer_zerop (gimple_assign_rhs2 (cmp_stmt)))
> +       return false;
> +    }

Put this before the phi check loop.

> +
> +  tree phires = gimple_phi_result (phi);
> +  tree type = TREE_TYPE (phires);
> +  tree new_arg0, new_arg1;
> +
> +  if (new_code == RSHIFT_EXPR)
> +    {
> +      new_arg0 = build_int_cst (type, wi::exact_log2 (tree_to_uhwi (arg0)));
> +      new_arg1 = build_int_cst (type, wi::exact_log2 (tree_to_uhwi (arg1)));

Use wi::to_wide here.
For RSHIFT_EXPR, add a gimple_convert to integer_type_node for the new
rhs. That is it might be a long long which we want to convert into an
integer or a bit_int that we want to convert into an integer.

> +    }
> +  else
> +    {
> +      new_arg0 = build_int_cst (type, tree_to_uhwi (arg0) - 1);
> +      new_arg1 = build_int_cst (type, tree_to_uhwi (arg1) - 1);

Likewise. (this removes the need for the tree_fits_uhwi_p check)

> +    }
> +
> +  SET_PHI_ARG_DEF (phi, 0, new_arg0);
> +  SET_PHI_ARG_DEF (phi, 1, new_arg1);
> +  if (SSA_NAME_RANGE_INFO (phires))
> +    reset_flow_sensitive_info (phires);

Maybe it is better to insert a new phi rather than reuse the current
phi for debug info reasons. You might want to fixup the debug info
too.

Thanks,
Andrea

> +
> +  gimple_assign_set_rhs1 (stmt, rhs1);
> +  gimple_assign_set_rhs2 (stmt, phires);
> +  gimple_assign_set_rhs_code (stmt, new_code);
> +  update_stmt (stmt);
> +
> +  return true;
> +}
>
>  /* Determine whether applying the 2 permutations (mask1 then mask2)
>     gives back one of the input.  */
> @@ -5894,6 +5996,14 @@ pass_forwprop::execute (function *fun)
>                       changed |= simplify_vector_constructor (&gsi);
>                     else if (code == ARRAY_REF)
>                       changed |= simplify_count_zeroes (&gsi);
> +                   else if (get_gimple_rhs_class (code) == GIMPLE_BINARY_RHS
> +                            && TREE_CODE (
> +                                   gimple_assign_rhs2 (stmt)) == SSA_NAME
> +                            && has_single_use (gimple_assign_rhs2 (stmt))
> +                            && SSA_NAME_DEF_STMT (gimple_assign_rhs2 (stmt))
> +                            && is_a<gphi*> (SSA_NAME_DEF_STMT (
> +                                               gimple_assign_rhs2 (stmt))))
> +                     changed |= simplify_phi_result_op (stmt, code);
>                     break;
>                   }
>
> --
> 2.43.0
>