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// Implementation of function-related RTL SSA functions -*- C++ -*-
// Copyright (C) 2020-2022 Free Software Foundation, Inc.
// This file is part of GCC.
// GCC is free software; you can redistribute it and/or modify it under
// the terms of the GNU General Public License as published by the Free
// Software Foundation; either version 3, or (at your option) any later
// version.
// GCC is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or
// FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
// for more details.
// You should have received a copy of the GNU General Public License
// along with GCC; see the file COPYING3. If not see
// <>.
#include "config.h"
#include "system.h"
#include "coretypes.h"
#include "backend.h"
#include "rtl.h"
#include "df.h"
#include "rtl-ssa.h"
#include "rtl-ssa/internals.h"
#include "rtl-ssa/internals.inl"
using namespace rtl_ssa;
function_info::function_info (function *fn)
: m_fn (fn)
// Force the alignment to be obstack_alignment. Everything else is normal.
obstack_specify_allocation (&m_obstack, OBSTACK_CHUNK_SIZE,
obstack_alignment, obstack_chunk_alloc,
obstack_specify_allocation (&m_temp_obstack, OBSTACK_CHUNK_SIZE,
obstack_alignment, obstack_chunk_alloc,
// Record the start of the obstacks.
m_obstack_start = XOBNEWVAR (&m_obstack, char, 0);
m_temp_obstack_start = XOBNEWVAR (&m_temp_obstack, char, 0);
init_function_data ();
process_all_blocks ();
simplify_phis ();
function_info::~function_info ()
// Anything using the temporary obstack should free it afterwards,
// preferably via temp_watermark ().
gcc_assert (XOBNEWVAR (&m_temp_obstack, char, 0) == m_temp_obstack_start);
obstack_free (&m_temp_obstack, nullptr);
obstack_free (&m_obstack, nullptr);
// See the comment above the declaration.
function_info::print (pretty_printer *pp) const
pp_string (pp, "Function: ");
pp_string (pp, function_name (m_fn));
for (ebb_info *ebb : ebbs ())
pp_newline (pp);
pp_newline_and_indent (pp, 0);
pp_ebb (pp, ebb);
// Initialize all member variables in preparation for (re)building
// SSA form from scratch.
function_info::init_function_data ()
m_next_artificial_uid = -1;
m_next_phi_uid = 0;
m_num_regs = max_reg_num ();
m_defs.safe_grow_cleared (m_num_regs + 1);
m_bbs.safe_grow_cleared (last_basic_block_for_fn (m_fn));
m_first_bb = nullptr;
m_last_bb = nullptr;
m_first_insn = nullptr;
m_last_insn = nullptr;
m_last_nondebug_insn = nullptr;
m_free_phis = nullptr;
// The initial phase of the phi simplification process. The cumulative
// effect of the initial phase is to set up ASSUMED_VALUES such that,
// for a phi P with uid ID:
// - if we think all inputs to P have the same value, ASSUMED_VALUES[ID]
// is that value
// - otherwise, ASSUMED_VALUES[ID] is P.
// This has already been done for phis with a lower uid than PHI,
// initially making optimistic assumptions about backedge inputs.
// Now do the same for PHI. If this might invalidate any assumptions
// made for earlier phis, add the uids of those phis to WORKLIST.
function_info::simplify_phi_setup (phi_info *phi, set_info **assumed_values,
bitmap worklist)
// If all non-backedge inputs have the same value, set NEW_VALUE
// to that value. Otherwise set NEW_VALUE to PHI, to indicate
// that PHI cannot be simplified.
unsigned int phi_uid = phi->uid ();
bool is_first_input = true;
set_info *new_value = nullptr;
machine_mode phi_mode = phi->mode ();
for (use_info *input : phi->inputs ())
set_info *def = input->def ();
if (auto *input_phi = safe_dyn_cast<phi_info *> (def))
// Ignore backedges for now.
unsigned int input_phi_uid = input_phi->uid ();
if (phi_uid <= input_phi_uid)
def = assumed_values[input_phi_uid];
// Compare this definition with previous ones.
if (is_first_input)
new_value = def;
is_first_input = false;
else if (new_value != def)
new_value = phi;
// If the input has a known mode (i.e. not BLKmode), make sure
// that the phi's mode is at least as large.
if (def)
phi_mode = combine_modes (phi_mode, def->mode ());
if (phi->mode () != phi_mode)
phi->set_mode (phi_mode);
// Since we use a reverse postorder traversal, no phi can consist
// entirely of backedges.
gcc_checking_assert (!is_first_input);
assumed_values[phi_uid] = new_value;
// See whether any assumptions for earlier phis are now invalid.
simplify_phi_propagate (phi, assumed_values, nullptr, worklist);
// The propagation phase of the phi simplification process, with
// ASSUMED_VALUES as described above simplify_phi_setup. Iteratively
// update the phis that use PHI based on PHI's entry in ASSUMED_VALUES.
// If CURR_WORKLIST is null, consider only phi uses with a lower uid
// than PHI, otherwise consider all phi uses.
// If a phi with a higher uid than PHI needs updating, add its uid to
// CURR_WORKLIST; if a phi with a lower uid than PHI needs updating,
// add its uid to NEXT_WORKLIST.
function_info::simplify_phi_propagate (phi_info *phi,
set_info **assumed_values,
bitmap curr_worklist,
bitmap next_worklist)
// Go through each phi user of PHI to see whether it needs updating.
unsigned int phi_uid = phi->uid ();
machine_mode phi_mode = phi->mode ();
set_info *phi_value = assumed_values[phi_uid];
for (use_info *use : phi->phi_uses ())
phi_info *user_phi = use->phi ();
// Propagate the phi's new mode to all phi users. Insn uses should
// not be updated, since their modes reflect a property of the insns
// rather than the phi.
if (use->mode () != phi_mode)
use->set_mode (phi_mode);
if (user_phi == phi)
// If this is a phi we should be looking at, see whether it needs
// an update.
unsigned int user_phi_uid = user_phi->uid ();
if (user_phi_uid < phi_uid || curr_worklist)
bool needs_update = false;
// Make sure that USER_PHI's mode is at least as big as PHI_MODE.
machine_mode user_phi_mode = user_phi->mode ();
machine_mode new_mode = combine_modes (user_phi_mode, phi_mode);
if (user_phi_mode != new_mode)
user_phi->set_mode (new_mode);
needs_update = true;
// If USER_PHI optimistically assumed an incorrect value,
// adjust it now.
if (assumed_values[user_phi_uid] != user_phi
&& assumed_values[user_phi_uid] != phi_value)
assumed_values[user_phi_uid] = user_phi;
needs_update = true;
if (needs_update)
if (user_phi_uid < phi_uid)
bitmap_set_bit (next_worklist, user_phi_uid);
bitmap_set_bit (curr_worklist, user_phi_uid);
// Update the modes of all phis so that they are at least as big as
// all inputs. Remove any non-degenerate phis whose inputs are all equal.
function_info::simplify_phis ()
auto temps = temp_watermark ();
// See the comment above simplify_phi_setup for details about this array.
auto *assumed_values = XOBNEWVEC (&m_temp_obstack, set_info *,
// An array of all phis, indexed by uid.
auto *phis = XOBNEWVEC (&m_temp_obstack, phi_info *, m_next_phi_uid);
// Which phi uids are actually in use.
auto_sbitmap valid_phi_uids (m_next_phi_uid);
bitmap_clear (valid_phi_uids);
// Bitmaps used for the main double-queue propagation phase.
auto_bitmap worklist1;
auto_bitmap worklist2;
bitmap curr_worklist = worklist1;
bitmap next_worklist = worklist2;
// Perform the set-up phase; see simplify_phi_setup for details.
for (ebb_info *ebb : ebbs ())
for (phi_info *phi : ebb->phis ())
bitmap_set_bit (valid_phi_uids, phi->uid ());
phis[phi->uid ()] = phi;
simplify_phi_setup (phi, assumed_values, curr_worklist);
// Iteratively process any phis that need updating; see
// simplify_phi_propagate for details. Using a double queue
// should reduce the number of times that any given phi node
// needs to be revisited.
while (!bitmap_empty_p (curr_worklist))
unsigned int uid = bitmap_first_set_bit (curr_worklist);
bitmap_clear_bit (curr_worklist, uid);
simplify_phi_propagate (phis[uid], assumed_values,
curr_worklist, next_worklist);
while (!bitmap_empty_p (curr_worklist));
std::swap (next_worklist, curr_worklist);
// Make sure that assumed_values is a transitive closure. This ensures
// that each use_info is only updated once.
if (flag_checking)
for (unsigned int i = 0; i < m_next_phi_uid; ++i)
if (bitmap_bit_p (valid_phi_uids, i))
if (auto *new_phi = safe_dyn_cast<phi_info *> (assumed_values[i]))
gcc_assert (assumed_values[new_phi->uid ()] == new_phi);
// Update any phis that turned out to be equivalent to a single input.
for (unsigned int i = 0; i < m_next_phi_uid; ++i)
if (bitmap_bit_p (valid_phi_uids, i) && phis[i] != assumed_values[i])
replace_phi (phis[i], assumed_values[i]);
// Print a description of FUNCTION to PP.
rtl_ssa::pp_function (pretty_printer *pp, const function_info *function)
function->print (pp);
// Print a description of FUNCTION to FILE.
dump (FILE *file, const function_info *function)
dump_using (file, pp_function, function);
// Debug interface to the dump routine above.
void debug (const function_info *x) { dump (stderr, x); }