337 lines
13 KiB
C++
337 lines
13 KiB
C++
// Copyright (c) 2018 Google LLC.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef SOURCE_OPT_LOOP_PEELING_H_
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#define SOURCE_OPT_LOOP_PEELING_H_
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#include <algorithm>
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#include <limits>
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#include <memory>
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#include <tuple>
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#include <unordered_map>
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#include <unordered_set>
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#include <utility>
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#include <vector>
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#include "source/opt/ir_context.h"
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#include "source/opt/loop_descriptor.h"
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#include "source/opt/loop_utils.h"
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#include "source/opt/pass.h"
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#include "source/opt/scalar_analysis.h"
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namespace spvtools {
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namespace opt {
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// Utility class to perform the peeling of a given loop.
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// The loop peeling transformation make a certain amount of a loop iterations to
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// be executed either before (peel before) or after (peel after) the transformed
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// loop.
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//
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// For peeling cases the transformation does the following steps:
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// - It clones the loop and inserts the cloned loop before the original loop;
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// - It connects all iterating values of the cloned loop with the
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// corresponding original loop values so that the second loop starts with
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// the appropriate values.
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// - It inserts a new induction variable "i" is inserted into the cloned that
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// starts with the value 0 and increment by step of one.
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//
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// The last step is specific to each case:
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// - Peel before: the transformation is to peel the "N" first iterations.
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// The exit condition of the cloned loop is changed so that the loop
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// exits when "i < N" becomes false. The original loop is then protected to
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// only execute if there is any iteration left to do.
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// - Peel after: the transformation is to peel the "N" last iterations,
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// then the exit condition of the cloned loop is changed so that the loop
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// exits when "i + N < max_iteration" becomes false, where "max_iteration"
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// is the upper bound of the loop. The cloned loop is then protected to
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// only execute if there is any iteration left to do no covered by the
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// second.
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//
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// To be peelable:
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// - The loop must be in LCSSA form;
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// - The loop must not contain any breaks;
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// - The loop must not have any ambiguous iterators updates (see
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// "CanPeelLoop").
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// The method "CanPeelLoop" checks that those constrained are met.
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class LoopPeeling {
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public:
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// LoopPeeling constructor.
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// |loop| is the loop to peel.
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// |loop_iteration_count| is the instruction holding the |loop| iteration
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// count, must be invariant for |loop| and must be of an int 32 type (signed
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// or unsigned).
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// |canonical_induction_variable| is an induction variable that can be used to
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// count the number of iterations, must be of the same type as
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// |loop_iteration_count| and start at 0 and increase by step of one at each
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// iteration. The value nullptr is interpreted as no suitable variable exists
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// and one will be created.
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LoopPeeling(Loop* loop, Instruction* loop_iteration_count,
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Instruction* canonical_induction_variable = nullptr)
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: context_(loop->GetContext()),
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loop_utils_(loop->GetContext(), loop),
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loop_(loop),
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loop_iteration_count_(!loop->IsInsideLoop(loop_iteration_count)
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? loop_iteration_count
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: nullptr),
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int_type_(nullptr),
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original_loop_canonical_induction_variable_(
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canonical_induction_variable),
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canonical_induction_variable_(nullptr) {
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if (loop_iteration_count_) {
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int_type_ = context_->get_type_mgr()
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->GetType(loop_iteration_count_->type_id())
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->AsInteger();
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if (canonical_induction_variable_) {
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assert(canonical_induction_variable_->type_id() ==
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loop_iteration_count_->type_id() &&
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"loop_iteration_count and canonical_induction_variable do not "
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"have the same type");
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}
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}
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GetIteratingExitValues();
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}
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// Returns true if the loop can be peeled.
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// To be peelable, all operation involved in the update of the loop iterators
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// must not dominates the exit condition. This restriction is a work around to
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// not miss compile code like:
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//
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// for (int i = 0; i + 1 < N; i++) {}
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// for (int i = 0; ++i < N; i++) {}
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//
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// The increment will happen before the test on the exit condition leading to
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// very look-a-like code.
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//
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// This restriction will not apply if a loop rotate is applied before (i.e.
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// becomes a do-while loop).
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bool CanPeelLoop() const {
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CFG& cfg = *context_->cfg();
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if (!loop_iteration_count_) {
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return false;
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}
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if (!int_type_) {
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return false;
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}
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if (int_type_->width() != 32) {
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return false;
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}
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if (!loop_->IsLCSSA()) {
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return false;
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}
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if (!loop_->GetMergeBlock()) {
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return false;
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}
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if (cfg.preds(loop_->GetMergeBlock()->id()).size() != 1) {
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return false;
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}
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if (!IsConditionCheckSideEffectFree()) {
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return false;
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}
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return !std::any_of(exit_value_.cbegin(), exit_value_.cend(),
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[](std::pair<uint32_t, Instruction*> it) {
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return it.second == nullptr;
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});
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}
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// Moves the execution of the |factor| first iterations of the loop into a
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// dedicated loop.
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void PeelBefore(uint32_t factor);
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// Moves the execution of the |factor| last iterations of the loop into a
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// dedicated loop.
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void PeelAfter(uint32_t factor);
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// Returns the cloned loop.
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Loop* GetClonedLoop() { return cloned_loop_; }
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// Returns the original loop.
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Loop* GetOriginalLoop() { return loop_; }
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private:
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IRContext* context_;
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LoopUtils loop_utils_;
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// The original loop.
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Loop* loop_;
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// The initial |loop_| upper bound.
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Instruction* loop_iteration_count_;
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// The int type to use for the canonical_induction_variable_.
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analysis::Integer* int_type_;
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// The cloned loop.
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Loop* cloned_loop_;
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// This is set to true when the exit and back-edge branch instruction is the
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// same.
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bool do_while_form_;
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// The canonical induction variable from the original loop if it exists.
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Instruction* original_loop_canonical_induction_variable_;
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// The canonical induction variable of the cloned loop. The induction variable
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// is initialized to 0 and incremented by step of 1.
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Instruction* canonical_induction_variable_;
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// Map between loop iterators and exit values. Loop iterators
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std::unordered_map<uint32_t, Instruction*> exit_value_;
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// Duplicate |loop_| and place the new loop before the cloned loop. Iterating
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// values from the cloned loop are then connected to the original loop as
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// initializer.
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void DuplicateAndConnectLoop(LoopUtils::LoopCloningResult* clone_results);
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// Insert the canonical induction variable into the first loop as a simplified
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// counter.
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void InsertCanonicalInductionVariable(
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LoopUtils::LoopCloningResult* clone_results);
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// Fixes the exit condition of the before loop. The function calls
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// |condition_builder| to get the condition to use in the conditional branch
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// of the loop exit. The loop will be exited if the condition evaluate to
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// true. |condition_builder| takes an Instruction* that represent the
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// insertion point.
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void FixExitCondition(
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const std::function<uint32_t(Instruction*)>& condition_builder);
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// Gathers all operations involved in the update of |iterator| into
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// |operations|.
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void GetIteratorUpdateOperations(
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const Loop* loop, Instruction* iterator,
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std::unordered_set<Instruction*>* operations);
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// Gathers exiting iterator values. The function builds a map between each
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// iterating value in the loop (a phi instruction in the loop header) and its
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// SSA value when it exit the loop. If no exit value can be accurately found,
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// it is map to nullptr (see comment on CanPeelLoop).
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void GetIteratingExitValues();
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// Returns true if a for-loop has no instruction with effects before the
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// condition check.
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bool IsConditionCheckSideEffectFree() const;
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// Creates a new basic block and insert it between |bb| and the predecessor of
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// |bb|.
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BasicBlock* CreateBlockBefore(BasicBlock* bb);
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// Inserts code to only execute |loop| only if the given |condition| is true.
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// |if_merge| is a suitable basic block to be used by the if condition as
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// merge block.
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// The function returns the if block protecting the loop.
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BasicBlock* ProtectLoop(Loop* loop, Instruction* condition,
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BasicBlock* if_merge);
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};
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// Implements a loop peeling optimization.
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// For each loop, the pass will try to peel it if there is conditions that
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// are true for the "N" first or last iterations of the loop.
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// To avoid code size explosion, too large loops will not be peeled.
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class LoopPeelingPass : public Pass {
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public:
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// Describes the peeling direction.
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enum class PeelDirection {
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kNone, // Cannot peel
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kBefore, // Can peel before
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kAfter // Can peel last
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};
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// Holds some statistics about peeled function.
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struct LoopPeelingStats {
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std::vector<std::tuple<const Loop*, PeelDirection, uint32_t>> peeled_loops_;
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};
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LoopPeelingPass(LoopPeelingStats* stats = nullptr) : stats_(stats) {}
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// Sets the loop peeling growth threshold. If the code size increase is above
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// |code_grow_threshold|, the loop will not be peeled. The code size is
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// measured in terms of SPIR-V instructions.
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static void SetLoopPeelingThreshold(size_t code_grow_threshold) {
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code_grow_threshold_ = code_grow_threshold;
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}
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// Returns the loop peeling code growth threshold.
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static size_t GetLoopPeelingThreshold() { return code_grow_threshold_; }
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const char* name() const override { return "loop-peeling"; }
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// Processes the given |module|. Returns Status::Failure if errors occur when
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// processing. Returns the corresponding Status::Success if processing is
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// successful to indicate whether changes have been made to the module.
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Pass::Status Process() override;
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private:
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// Describes the peeling direction.
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enum class CmpOperator {
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kLT, // less than
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kGT, // greater than
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kLE, // less than or equal
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kGE, // greater than or equal
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};
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class LoopPeelingInfo {
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public:
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using Direction = std::pair<PeelDirection, uint32_t>;
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LoopPeelingInfo(Loop* loop, size_t loop_max_iterations,
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ScalarEvolutionAnalysis* scev_analysis)
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: context_(loop->GetContext()),
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loop_(loop),
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scev_analysis_(scev_analysis),
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loop_max_iterations_(loop_max_iterations) {}
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// Returns by how much and to which direction a loop should be peeled to
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// make the conditional branch of the basic block |bb| an unconditional
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// branch. If |bb|'s terminator is not a conditional branch or the condition
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// is not workable then it returns PeelDirection::kNone and a 0 factor.
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Direction GetPeelingInfo(BasicBlock* bb) const;
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private:
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// Returns the id of the loop invariant operand of the conditional
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// expression |condition|. It returns if no operand is invariant.
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uint32_t GetFirstLoopInvariantOperand(Instruction* condition) const;
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// Returns the id of the non loop invariant operand of the conditional
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// expression |condition|. It returns if all operands are invariant.
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uint32_t GetFirstNonLoopInvariantOperand(Instruction* condition) const;
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// Returns the value of |rec| at the first loop iteration.
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SExpression GetValueAtFirstIteration(SERecurrentNode* rec) const;
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// Returns the value of |rec| at the given |iteration|.
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SExpression GetValueAtIteration(SERecurrentNode* rec,
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int64_t iteration) const;
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// Returns the value of |rec| at the last loop iteration.
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SExpression GetValueAtLastIteration(SERecurrentNode* rec) const;
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bool EvalOperator(CmpOperator cmp_op, SExpression lhs, SExpression rhs,
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bool* result) const;
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Direction HandleEquality(SExpression lhs, SExpression rhs) const;
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Direction HandleInequality(CmpOperator cmp_op, SExpression lhs,
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SERecurrentNode* rhs) const;
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static Direction GetNoneDirection() {
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return Direction{LoopPeelingPass::PeelDirection::kNone, 0};
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}
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IRContext* context_;
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Loop* loop_;
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ScalarEvolutionAnalysis* scev_analysis_;
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size_t loop_max_iterations_;
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};
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// Peel profitable loops in |f|.
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bool ProcessFunction(Function* f);
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// Peel |loop| if profitable.
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std::pair<bool, Loop*> ProcessLoop(Loop* loop, CodeMetrics* loop_size);
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static size_t code_grow_threshold_;
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LoopPeelingStats* stats_;
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};
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} // namespace opt
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} // namespace spvtools
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#endif // SOURCE_OPT_LOOP_PEELING_H_
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