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https://github.com/KhronosGroup/SPIRV-Tools
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1dc2458060
This pass will look for adjacent loops that are compatible and legal to
be fused.
Loops are compatible if:
- they both have one induction variable
- they have the same upper and lower bounds
- same initial value
- same condition
- they have the same update step
- they are adjacent
- there are no break/continue in either of them
Fusion is legal if:
- fused loops do not have any dependencies with dependence distance
greater than 0 that did not exist in the original loops.
- there are no function calls in the loops (could have side-effects)
- there are no barriers in the loops
It will fuse all such loops as long as the number of registers used for
the fused loop stays under the threshold defined by
max_registers_per_loop.
71 lines
2.2 KiB
C++
71 lines
2.2 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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#include "opt/loop_fusion_pass.h"
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#include "opt/ir_context.h"
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#include "opt/loop_descriptor.h"
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#include "opt/loop_fusion.h"
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#include "opt/register_pressure.h"
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namespace spvtools {
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namespace opt {
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Pass::Status LoopFusionPass::Process(ir::IRContext* c) {
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bool modified = false;
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ir::Module* module = c->module();
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// Process each function in the module
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for (ir::Function& f : *module) {
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modified |= ProcessFunction(&f);
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}
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return modified ? Status::SuccessWithChange : Status::SuccessWithoutChange;
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}
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bool LoopFusionPass::ProcessFunction(ir::Function* function) {
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auto c = function->context();
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ir::LoopDescriptor& ld = *c->GetLoopDescriptor(function);
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// If a loop doesn't have a preheader needs then it needs to be created. Make
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// sure to return Status::SuccessWithChange in that case.
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auto modified = ld.CreatePreHeaderBlocksIfMissing();
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// TODO(tremmelg): Could the only loop that |loop| could possibly be fused be
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// picked out so don't have to check every loop
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for (auto& loop_0 : ld) {
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for (auto& loop_1 : ld) {
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LoopFusion fusion(c, &loop_0, &loop_1);
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if (fusion.AreCompatible() && fusion.IsLegal()) {
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RegisterLiveness liveness(c, function);
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RegisterLiveness::RegionRegisterLiveness reg_pressure{};
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liveness.SimulateFusion(loop_0, loop_1, ®_pressure);
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if (reg_pressure.used_registers_ <= max_registers_per_loop_) {
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fusion.Fuse();
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// Recurse, as the current iterators will have been invalidated.
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ProcessFunction(function);
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return true;
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}
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}
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}
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}
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return modified;
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}
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} // namespace opt
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} // namespace spvtools
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