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

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)));
+  if (gimple_phi_num_args (phi) != 2)
+    return false;
+
+  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;
+
+  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;
+    }
+
+  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)));
+    }
+  else
+    {
+      new_arg0 = build_int_cst (type, tree_to_uhwi (arg0) - 1);
+      new_arg1 = build_int_cst (type, tree_to_uhwi (arg1) - 1);
+    }
+
+  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);
+
+  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