1 // Copyright (c) 2019 Google LLC
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // http://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #include "source/reduce/merge_blocks_reduction_opportunity.h"
16
17 #include "source/opt/block_merge_util.h"
18 #include "source/opt/ir_context.h"
19
20 namespace spvtools {
21 namespace reduce {
22
MergeBlocksReductionOpportunity(opt::IRContext * context,opt::Function * function,opt::BasicBlock * block)23 MergeBlocksReductionOpportunity::MergeBlocksReductionOpportunity(
24 opt::IRContext* context, opt::Function* function, opt::BasicBlock* block) {
25 // Precondition: the terminator has to be OpBranch.
26 assert(block->terminator()->opcode() == SpvOpBranch);
27 context_ = context;
28 function_ = function;
29 // Get the successor block associated with the OpBranch.
30 successor_block_ =
31 context->cfg()->block(block->terminator()->GetSingleWordInOperand(0));
32 }
33
PreconditionHolds()34 bool MergeBlocksReductionOpportunity::PreconditionHolds() {
35 // Merge block opportunities can disable each other.
36 // Example: Given blocks: A->B->C.
37 // A is a loop header; B and C are blocks in the loop; C ends with OpReturn.
38 // There are two opportunities: B and C can be merged with their predecessors.
39 // Merge C. B now ends with OpReturn. We now just have: A->B.
40 // Merge B is now disabled, as this would lead to A, a loop header, ending
41 // with an OpReturn, which is invalid.
42
43 const auto predecessors = context_->cfg()->preds(successor_block_->id());
44 assert(1 == predecessors.size() &&
45 "For a successor to be merged into its predecessor, exactly one "
46 "predecessor must be present.");
47 const uint32_t predecessor_id = predecessors[0];
48 opt::BasicBlock* predecessor_block =
49 context_->get_instr_block(predecessor_id);
50 return opt::blockmergeutil::CanMergeWithSuccessor(context_,
51 predecessor_block);
52 }
53
Apply()54 void MergeBlocksReductionOpportunity::Apply() {
55 // While the original block that targeted the successor may not exist anymore
56 // (it might have been merged with another block), some block must exist that
57 // targets the successor. Find it.
58
59 const auto predecessors = context_->cfg()->preds(successor_block_->id());
60 assert(1 == predecessors.size() &&
61 "For a successor to be merged into its predecessor, exactly one "
62 "predecessor must be present.");
63 const uint32_t predecessor_id = predecessors[0];
64
65 // We need an iterator pointing to the predecessor, hence the loop.
66 for (auto bi = function_->begin(); bi != function_->end(); ++bi) {
67 if (bi->id() == predecessor_id) {
68 opt::blockmergeutil::MergeWithSuccessor(context_, function_, bi);
69 // Block merging changes the control flow graph, so invalidate it.
70 context_->InvalidateAnalysesExceptFor(
71 opt::IRContext::Analysis::kAnalysisNone);
72 return;
73 }
74 }
75
76 assert(false &&
77 "Unreachable: we should have found a block with the desired id.");
78 }
79
80 } // namespace reduce
81 } // namespace spvtools
82