mirror of
https://github.com/autc04/Retro68.git
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336 lines
11 KiB
C++
336 lines
11 KiB
C++
/* Copyright (C) 2008-2014 Free Software Foundation, Inc.
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Contributed by Richard Henderson <rth@redhat.com>.
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This file is part of the GNU Transactional Memory Library (libitm).
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Libitm is free software; you can redistribute it and/or modify it
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under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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Libitm is distributed in the hope that it will be useful, but WITHOUT ANY
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WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
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FOR A PARTICULAR PURPOSE. See the GNU General Public License for
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more details.
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Under Section 7 of GPL version 3, you are granted additional
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permissions described in the GCC Runtime Library Exception, version
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3.1, as published by the Free Software Foundation.
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You should have received a copy of the GNU General Public License and
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a copy of the GCC Runtime Library Exception along with this program;
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see the files COPYING3 and COPYING.RUNTIME respectively. If not, see
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<http://www.gnu.org/licenses/>. */
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#include <stdlib.h>
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#include <string.h>
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#include <ctype.h>
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#include "libitm_i.h"
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// The default TM method used when starting a new transaction. Initialized
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// in number_of_threads_changed() below.
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// Access to this variable is always synchronized with help of the serial
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// lock, except one read access that happens in decide_begin_dispatch() before
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// a transaction has become active (by acquiring the serial lock in read or
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// write mode). The default_dispatch is only changed and initialized in
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// serial mode. Transactions stay active when they restart (see beginend.cc),
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// thus decide_retry_strategy() can expect default_dispatch to be unmodified.
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// See decide_begin_dispatch() for further comments.
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static std::atomic<GTM::abi_dispatch*> default_dispatch;
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// The default TM method as requested by the user, if any.
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static GTM::abi_dispatch* default_dispatch_user = 0;
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void
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GTM::gtm_thread::decide_retry_strategy (gtm_restart_reason r)
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{
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struct abi_dispatch *disp = abi_disp ();
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this->restart_reason[r]++;
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this->restart_total++;
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if (r == RESTART_INIT_METHOD_GROUP)
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{
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// A re-initializations of the method group has been requested. Switch
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// to serial mode, initialize, and resume normal operation.
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if ((state & STATE_SERIAL) == 0)
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{
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// We have to eventually re-init the method group. Therefore,
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// we cannot just upgrade to a write lock here because this could
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// fail forever when other transactions execute in serial mode.
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// However, giving up the read lock then means that a change of the
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// method group could happen in-between, so check that we're not
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// re-initializing without a need.
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// ??? Note that we can still re-initialize too often, but avoiding
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// that would increase code complexity, which seems unnecessary
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// given that re-inits should be very infrequent.
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serial_lock.read_unlock(this);
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serial_lock.write_lock();
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if (disp->get_method_group()
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== default_dispatch.load(memory_order_relaxed)
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->get_method_group())
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// Still the same method group.
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disp->get_method_group()->reinit();
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serial_lock.write_unlock();
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// Also, we're making the transaction inactive, so when we become
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// active again, some other thread might have changed the default
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// dispatch, so we run the same code as for the first execution
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// attempt.
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disp = decide_begin_dispatch(prop);
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set_abi_disp(disp);
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}
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else
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// We are a serial transaction already, which makes things simple.
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disp->get_method_group()->reinit();
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return;
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}
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bool retry_irr = (r == RESTART_SERIAL_IRR);
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bool retry_serial = (retry_irr || this->restart_total > 100);
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// We assume closed nesting to be infrequently required, so just use
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// dispatch_serial (with undo logging) if required.
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if (r == RESTART_CLOSED_NESTING)
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retry_serial = true;
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if (retry_serial)
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{
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// In serialirr_mode we can succeed with the upgrade to
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// write-lock but fail the trycommit. In any case, if the
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// write lock is not yet held, grab it. Don't do this with
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// an upgrade, since we've no need to preserve the state we
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// acquired with the read.
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// Note that we will be restarting with either dispatch_serial or
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// dispatch_serialirr, which are compatible with all TM methods; if
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// we would retry with a different method, we would have to first check
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// whether the default dispatch or the method group have changed. Also,
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// the caller must have rolled back the previous transaction, so we
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// don't have to worry about things such as privatization.
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if ((this->state & STATE_SERIAL) == 0)
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{
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this->state |= STATE_SERIAL;
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serial_lock.read_unlock (this);
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serial_lock.write_lock ();
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}
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// We can retry with dispatch_serialirr if the transaction
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// doesn't contain an abort and if we don't need closed nesting.
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if ((this->prop & pr_hasNoAbort) && (r != RESTART_CLOSED_NESTING))
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retry_irr = true;
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}
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// Note that we can just use serial mode here without having to switch
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// TM method sets because serial mode is compatible with all of them.
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if (retry_irr)
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{
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this->state = (STATE_SERIAL | STATE_IRREVOCABLE);
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disp = dispatch_serialirr ();
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set_abi_disp (disp);
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}
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else if (retry_serial)
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{
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disp = dispatch_serial();
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set_abi_disp (disp);
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}
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}
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// Decides which TM method should be used on the first attempt to run this
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// transaction. Acquires the serial lock and sets transaction state
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// according to the chosen TM method.
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GTM::abi_dispatch*
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GTM::gtm_thread::decide_begin_dispatch (uint32_t prop)
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{
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abi_dispatch* dd;
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// TODO Pay more attention to prop flags (eg, *omitted) when selecting
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// dispatch.
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// ??? We go irrevocable eagerly here, which is not always good for
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// performance. Don't do this?
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if ((prop & pr_doesGoIrrevocable) || !(prop & pr_instrumentedCode))
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dd = dispatch_serialirr();
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else
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{
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// Load the default dispatch. We're not an active transaction and so it
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// can change concurrently but will still be some valid dispatch.
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// Relaxed memory order is okay because we expect each dispatch to be
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// constructed properly already (at least that its closed_nesting() and
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// closed_nesting_alternatives() will return sensible values). It is
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// harmless if we incorrectly chose the serial or serialirr methods, and
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// for all other methods we will acquire the serial lock in read mode
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// and load the default dispatch again.
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abi_dispatch* dd_orig = default_dispatch.load(memory_order_relaxed);
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dd = dd_orig;
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// If we might need closed nesting and the default dispatch has an
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// alternative that supports closed nesting, use it.
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// ??? We could choose another TM method that we know supports closed
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// nesting but isn't the default (e.g., dispatch_serial()). However, we
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// assume that aborts that need closed nesting are infrequent, so don't
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// choose a non-default method until we have to actually restart the
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// transaction.
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if (!(prop & pr_hasNoAbort) && !dd->closed_nesting()
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&& dd->closed_nesting_alternative())
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dd = dd->closed_nesting_alternative();
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if (!(dd->requires_serial() & STATE_SERIAL))
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{
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// The current dispatch is supposedly a non-serial one. Become an
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// active transaction and verify this. Relaxed memory order is fine
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// because the serial lock itself will have established
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// happens-before for any change to the selected dispatch.
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serial_lock.read_lock (this);
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if (default_dispatch.load(memory_order_relaxed) == dd_orig)
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return dd;
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// If we raced with a concurrent modification of default_dispatch,
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// just fall back to serialirr. The dispatch choice might not be
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// up-to-date anymore, but this is harmless.
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serial_lock.read_unlock (this);
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dd = dispatch_serialirr();
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}
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}
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// We are some kind of serial transaction.
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serial_lock.write_lock();
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state = dd->requires_serial();
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return dd;
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}
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void
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GTM::gtm_thread::set_default_dispatch(GTM::abi_dispatch* disp)
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{
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abi_dispatch* dd = default_dispatch.load(memory_order_relaxed);
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if (dd == disp)
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return;
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if (dd)
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{
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// If we are switching method groups, initialize and shut down properly.
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if (dd->get_method_group() != disp->get_method_group())
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{
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dd->get_method_group()->fini();
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disp->get_method_group()->init();
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}
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}
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else
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disp->get_method_group()->init();
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default_dispatch.store(disp, memory_order_relaxed);
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}
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static GTM::abi_dispatch*
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parse_default_method()
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{
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const char *env = getenv("ITM_DEFAULT_METHOD");
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GTM::abi_dispatch* disp = 0;
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if (env == NULL)
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return 0;
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while (isspace((unsigned char) *env))
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++env;
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if (strncmp(env, "serialirr_onwrite", 17) == 0)
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{
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disp = GTM::dispatch_serialirr_onwrite();
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env += 17;
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}
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else if (strncmp(env, "serialirr", 9) == 0)
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{
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disp = GTM::dispatch_serialirr();
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env += 9;
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}
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else if (strncmp(env, "serial", 6) == 0)
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{
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disp = GTM::dispatch_serial();
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env += 6;
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}
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else if (strncmp(env, "gl_wt", 5) == 0)
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{
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disp = GTM::dispatch_gl_wt();
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env += 5;
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}
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else if (strncmp(env, "ml_wt", 5) == 0)
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{
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disp = GTM::dispatch_ml_wt();
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env += 5;
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}
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else if (strncmp(env, "htm", 3) == 0)
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{
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disp = GTM::dispatch_htm();
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env += 3;
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}
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else
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goto unknown;
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while (isspace((unsigned char) *env))
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++env;
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if (*env == '\0')
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return disp;
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unknown:
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GTM::GTM_error("Unknown TM method in environment variable "
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"ITM_DEFAULT_METHOD\n");
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return 0;
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}
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// Gets notifications when the number of registered threads changes. This is
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// used to initialize the method set choice and trigger straightforward choice
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// adaption.
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// This must be called only by serial threads.
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void
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GTM::gtm_thread::number_of_threads_changed(unsigned previous, unsigned now)
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{
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if (previous == 0)
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{
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// No registered threads before, so initialize.
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static bool initialized = false;
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if (!initialized)
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{
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initialized = true;
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// Check for user preferences here.
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default_dispatch = 0;
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default_dispatch_user = parse_default_method();
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}
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}
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else if (now == 0)
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{
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// No registered threads anymore. The dispatch based on serial mode do
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// not have any global state, so this effectively shuts down properly.
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set_default_dispatch(dispatch_serialirr());
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}
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if (now == 1)
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{
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// Only one thread, so use a serializing method.
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// ??? If we don't have a fast serial mode implementation, it might be
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// better to use the global lock method set here.
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if (default_dispatch_user && default_dispatch_user->supports(now))
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set_default_dispatch(default_dispatch_user);
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else
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set_default_dispatch(dispatch_serialirr());
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}
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else if (now > 1 && previous <= 1)
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{
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// More than one thread, use the default method.
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if (default_dispatch_user && default_dispatch_user->supports(now))
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set_default_dispatch(default_dispatch_user);
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else
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{
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// If HTM is available, use it by default with serial mode as
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// fallback. Otherwise, use ml_wt because it probably scales best.
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abi_dispatch* a;
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#ifdef USE_HTM_FASTPATH
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if (htm_available())
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a = dispatch_htm();
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else
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#endif
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a = dispatch_ml_wt();
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if (a->supports(now))
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set_default_dispatch(a);
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else
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// Serial-irrevocable mode always works.
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set_default_dispatch(dispatch_serialirr());
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}
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}
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}
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