1 //===- AMDGPUUnifyDivergentExitNodes.cpp ----------------------------------===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This is a variant of the UnifyDivergentExitNodes pass. Rather than ensuring
11 // there is at most one ret and one unreachable instruction, it ensures there is
12 // at most one divergent exiting block.
13 //
14 // StructurizeCFG can't deal with multi-exit regions formed by branches to
15 // multiple return nodes. It is not desirable to structurize regions with
16 // uniform branches, so unifying those to the same return block as divergent
17 // branches inhibits use of scalar branching. It still can't deal with the case
18 // where one branch goes to return, and one unreachable. Replace unreachable in
19 // this case with a return.
20 //
21 //===----------------------------------------------------------------------===//
22 
23 #include "AMDGPU.h"
24 #include "llvm/ADT/ArrayRef.h"
25 #include "llvm/ADT/SmallPtrSet.h"
26 #include "llvm/ADT/SmallVector.h"
27 #include "llvm/ADT/StringRef.h"
28 #include "llvm/Analysis/DivergenceAnalysis.h"
29 #include "llvm/Analysis/PostDominators.h"
30 #include "llvm/Analysis/TargetTransformInfo.h"
31 #include "llvm/IR/BasicBlock.h"
32 #include "llvm/IR/CFG.h"
33 #include "llvm/IR/Constants.h"
34 #include "llvm/IR/Function.h"
35 #include "llvm/IR/InstrTypes.h"
36 #include "llvm/IR/Instructions.h"
37 #include "llvm/IR/Intrinsics.h"
38 #include "llvm/IR/Type.h"
39 #include "llvm/Pass.h"
40 #include "llvm/Support/Casting.h"
41 #include "llvm/Transforms/Scalar.h"
42 #include "llvm/Transforms/Utils/Local.h"
43 
44 using namespace llvm;
45 
46 #define DEBUG_TYPE "amdgpu-unify-divergent-exit-nodes"
47 
48 namespace {
49 
50 class AMDGPUUnifyDivergentExitNodes : public FunctionPass {
51 public:
52   static char ID; // Pass identification, replacement for typeid
53 
54   AMDGPUUnifyDivergentExitNodes() : FunctionPass(ID) {
55     initializeAMDGPUUnifyDivergentExitNodesPass(*PassRegistry::getPassRegistry());
56   }
57 
58   // We can preserve non-critical-edgeness when we unify function exit nodes
59   void getAnalysisUsage(AnalysisUsage &AU) const override;
60   bool runOnFunction(Function &F) override;
61 };
62 
63 } // end anonymous namespace
64 
65 char AMDGPUUnifyDivergentExitNodes::ID = 0;
66 
67 char &llvm::AMDGPUUnifyDivergentExitNodesID = AMDGPUUnifyDivergentExitNodes::ID;
68 
69 INITIALIZE_PASS_BEGIN(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
70                      "Unify divergent function exit nodes", false, false)
71 INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
72 INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
73 INITIALIZE_PASS_END(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
74                     "Unify divergent function exit nodes", false, false)
75 
76 void AMDGPUUnifyDivergentExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
77   // TODO: Preserve dominator tree.
78   AU.addRequired<PostDominatorTreeWrapperPass>();
79 
80   AU.addRequired<DivergenceAnalysis>();
81 
82   // No divergent values are changed, only blocks and branch edges.
83   AU.addPreserved<DivergenceAnalysis>();
84 
85   // We preserve the non-critical-edgeness property
86   AU.addPreservedID(BreakCriticalEdgesID);
87 
88   // This is a cluster of orthogonal Transforms
89   AU.addPreservedID(LowerSwitchID);
90   FunctionPass::getAnalysisUsage(AU);
91 
92   AU.addRequired<TargetTransformInfoWrapperPass>();
93 }
94 
95 /// \returns true if \p BB is reachable through only uniform branches.
96 /// XXX - Is there a more efficient way to find this?
97 static bool isUniformlyReached(const DivergenceAnalysis &DA,
98                                BasicBlock &BB) {
99   SmallVector<BasicBlock *, 8> Stack;
100   SmallPtrSet<BasicBlock *, 8> Visited;
101 
102   for (BasicBlock *Pred : predecessors(&BB))
103     Stack.push_back(Pred);
104 
105   while (!Stack.empty()) {
106     BasicBlock *Top = Stack.pop_back_val();
107     if (!DA.isUniform(Top->getTerminator()))
108       return false;
109 
110     for (BasicBlock *Pred : predecessors(Top)) {
111       if (Visited.insert(Pred).second)
112         Stack.push_back(Pred);
113     }
114   }
115 
116   return true;
117 }
118 
119 static BasicBlock *unifyReturnBlockSet(Function &F,
120                                        ArrayRef<BasicBlock *> ReturningBlocks,
121                                        const TargetTransformInfo &TTI,
122                                        StringRef Name) {
123   // Otherwise, we need to insert a new basic block into the function, add a PHI
124   // nodes (if the function returns values), and convert all of the return
125   // instructions into unconditional branches.
126   BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), Name, &F);
127 
128   PHINode *PN = nullptr;
129   if (F.getReturnType()->isVoidTy()) {
130     ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
131   } else {
132     // If the function doesn't return void... add a PHI node to the block...
133     PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
134                          "UnifiedRetVal");
135     NewRetBlock->getInstList().push_back(PN);
136     ReturnInst::Create(F.getContext(), PN, NewRetBlock);
137   }
138 
139   // Loop over all of the blocks, replacing the return instruction with an
140   // unconditional branch.
141   for (BasicBlock *BB : ReturningBlocks) {
142     // Add an incoming element to the PHI node for every return instruction that
143     // is merging into this new block...
144     if (PN)
145       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
146 
147     BB->getInstList().pop_back();  // Remove the return insn
148     BranchInst::Create(NewRetBlock, BB);
149   }
150 
151   for (BasicBlock *BB : ReturningBlocks) {
152     // Cleanup possible branch to unconditional branch to the return.
153     simplifyCFG(BB, TTI, {2});
154   }
155 
156   return NewRetBlock;
157 }
158 
159 bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
160   auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
161   if (PDT.getRoots().size() <= 1)
162     return false;
163 
164   DivergenceAnalysis &DA = getAnalysis<DivergenceAnalysis>();
165 
166   // Loop over all of the blocks in a function, tracking all of the blocks that
167   // return.
168   SmallVector<BasicBlock *, 4> ReturningBlocks;
169   SmallVector<BasicBlock *, 4> UnreachableBlocks;
170 
171   for (BasicBlock *BB : PDT.getRoots()) {
172     if (isa<ReturnInst>(BB->getTerminator())) {
173       if (!isUniformlyReached(DA, *BB))
174         ReturningBlocks.push_back(BB);
175     } else if (isa<UnreachableInst>(BB->getTerminator())) {
176       if (!isUniformlyReached(DA, *BB))
177         UnreachableBlocks.push_back(BB);
178     }
179   }
180 
181   if (!UnreachableBlocks.empty()) {
182     BasicBlock *UnreachableBlock = nullptr;
183 
184     if (UnreachableBlocks.size() == 1) {
185       UnreachableBlock = UnreachableBlocks.front();
186     } else {
187       UnreachableBlock = BasicBlock::Create(F.getContext(),
188                                             "UnifiedUnreachableBlock", &F);
189       new UnreachableInst(F.getContext(), UnreachableBlock);
190 
191       for (BasicBlock *BB : UnreachableBlocks) {
192         BB->getInstList().pop_back();  // Remove the unreachable inst.
193         BranchInst::Create(UnreachableBlock, BB);
194       }
195     }
196 
197     if (!ReturningBlocks.empty()) {
198       // Don't create a new unreachable inst if we have a return. The
199       // structurizer/annotator can't handle the multiple exits
200 
201       Type *RetTy = F.getReturnType();
202       Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
203       UnreachableBlock->getInstList().pop_back();  // Remove the unreachable inst.
204 
205       Function *UnreachableIntrin =
206         Intrinsic::getDeclaration(F.getParent(), Intrinsic::amdgcn_unreachable);
207 
208       // Insert a call to an intrinsic tracking that this is an unreachable
209       // point, in case we want to kill the active lanes or something later.
210       CallInst::Create(UnreachableIntrin, {}, "", UnreachableBlock);
211 
212       // Don't create a scalar trap. We would only want to trap if this code was
213       // really reached, but a scalar trap would happen even if no lanes
214       // actually reached here.
215       ReturnInst::Create(F.getContext(), RetVal, UnreachableBlock);
216       ReturningBlocks.push_back(UnreachableBlock);
217     }
218   }
219 
220   // Now handle return blocks.
221   if (ReturningBlocks.empty())
222     return false; // No blocks return
223 
224   if (ReturningBlocks.size() == 1)
225     return false; // Already has a single return block
226 
227   const TargetTransformInfo &TTI
228     = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
229 
230   unifyReturnBlockSet(F, ReturningBlocks, TTI, "UnifiedReturnBlock");
231   return true;
232 }
233