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178 lines
6.2 KiB
178 lines
6.2 KiB
4 months ago
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# Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
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# Use of this source code is governed by a BSD-style license that can be
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# found in the LICENSE file.
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"""Iterative flags elimination.
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Part of the Chrome build flags optimization.
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This module implements the flag iterative elimination algorithm (IE) adopted
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from the paper
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Z. Pan et al. Fast and Effective Orchestration of Compiler Optimizations for
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Automatic Performance Tuning.
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IE begins with the base line that turns on all the optimizations flags and
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setting the numeric flags to their highest values. IE turns off the one boolean
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flag or lower the value of a numeric flag with the most negative effect from the
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baseline. This process repeats with all remaining flags, until none of them
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causes performance degradation. The complexity of IE is O(n^2).
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For example, -fstrict-aliasing and -ftree-vectorize. The base line is
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b=[-fstrict-aliasing, -ftree-vectorize]. The two tasks in the first iteration
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are t0=[-fstrict-aliasing] and t1=[-ftree-vectorize]. The algorithm compares b
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with t0 and t1, respectively, and see whether setting the numeric flag with a
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lower value or removing the boolean flag -fstrict-aliasing produce a better
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fitness value.
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"""
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__author__ = 'yuhenglong@google.com (Yuheng Long)'
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import flags
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from generation import Generation
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import task
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def _DecreaseFlag(flags_dict, spec):
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"""Decrease the value of the flag that has the specification spec.
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If the flag that contains the spec is a boolean flag, it is eliminated.
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Otherwise the flag is a numeric flag, its value will be reduced by one.
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Args:
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flags_dict: The dictionary containing the original flags whose neighbors are
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to be explored.
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spec: The spec in the flags_dict is to be changed.
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Returns:
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Dictionary of neighbor flag that is only different from the original
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dictionary by the spec.
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"""
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# The specification must be held by one of the flags.
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assert spec in flags_dict
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# The results this method returns.
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results = flags_dict.copy()
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# This method searches for a pattern [start-end] in the spec. If the spec
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# contains this pattern, it is a numeric flag. Otherwise it is a boolean flag.
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# For example, -finline-limit=[1-1000] is a numeric flag and -falign-jumps is
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# a boolean flag.
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numeric_flag_match = flags.Search(spec)
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if numeric_flag_match:
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# numeric flag
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val = results[spec].GetValue()
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# If the value of the flag is the lower boundary of the specification, this
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# flag will be turned off. Because it already contains the lowest value and
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# can not be decreased any more.
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if val == int(numeric_flag_match.group('start')):
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# Turn off the flag. A flag is turned off if it is not presented in the
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# flags_dict.
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del results[spec]
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else:
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results[spec] = flags.Flag(spec, val - 1)
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else:
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# Turn off the flag. A flag is turned off if it is not presented in the
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# flags_dict.
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del results[spec]
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return results
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class IterativeEliminationGeneration(Generation):
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"""The negative flag iterative elimination algorithm."""
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def __init__(self, exe_set, parent_task):
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"""Set up the base line parent task.
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The parent task is the base line against which the new tasks are compared.
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The new tasks are only different from the base line from one flag f by
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either turning this flag f off, or lower the flag value by 1.
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If a new task is better than the base line, one flag is identified that
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gives degradation. The flag that give the worst degradation will be removed
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or lower the value by 1 in the base in each iteration.
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Args:
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exe_set: A set of tasks to be run. Each one only differs from the
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parent_task by one flag.
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parent_task: The base line task, against which the new tasks in exe_set
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are compared.
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"""
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Generation.__init__(self, exe_set, None)
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self._parent_task = parent_task
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def IsImproved(self):
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"""Whether any new task has improvement upon the parent task."""
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parent = self._parent_task
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# Whether there is any new task that has improvement over the parent base
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# line task.
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for curr in [curr for curr in self.Pool() if curr != parent]:
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if curr.IsImproved(parent):
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return True
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return False
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def Next(self, cache):
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"""Find out the flag that gives the worst degradation.
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Found out the flag that gives the worst degradation. Turn that flag off from
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the base line and use the new base line for the new generation.
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Args:
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cache: A set of tasks that have been generated before.
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Returns:
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A set of new generations.
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"""
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parent_task = self._parent_task
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# Find out the task that gives the worst degradation.
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worst_task = parent_task
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for curr in [curr for curr in self.Pool() if curr != parent_task]:
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# The method IsImproved, which is supposed to be called before, ensures
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# that there is at least a task that improves upon the parent_task.
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if curr.IsImproved(worst_task):
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worst_task = curr
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assert worst_task != parent_task
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# The flags_set of the worst task.
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work_flags_set = worst_task.GetFlags().GetFlags()
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results = set([])
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# If the flags_set contains no flag, i.e., all the flags have been
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# eliminated, the algorithm stops.
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if not work_flags_set:
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return []
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# Turn of the remaining flags one by one for the next generation.
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for spec in work_flags_set:
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flag_set = flags.FlagSet(_DecreaseFlag(work_flags_set, spec).values())
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new_task = task.Task(flag_set)
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if new_task not in cache:
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results.add(new_task)
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return [IterativeEliminationGeneration(results, worst_task)]
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class IterativeEliminationFirstGeneration(IterativeEliminationGeneration):
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"""The first iteration of the iterative elimination algorithm.
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The first iteration also evaluates the base line task. The base line tasks in
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the subsequent iterations have been evaluated. Therefore,
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IterativeEliminationGeneration does not include the base line task in the
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execution set.
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"""
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def IsImproved(self):
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# Find out the base line task in the execution set.
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parent = next(task for task in self.Pool() if task == self._parent_task)
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self._parent_task = parent
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return IterativeEliminationGeneration.IsImproved(self)
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