LCOV - code coverage report
Current view: top level - lib/Target/AMDGPU - AMDGPUUnifyDivergentExitNodes.cpp (source / functions) Hit Total Coverage
Test: llvm-toolchain.info Lines: 80 80 100.0 %
Date: 2018-07-13 00:08:38 Functions: 7 8 87.5 %
Legend: Lines: hit not hit

          Line data    Source code
       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/Transforms/Utils/Local.h"
      32             : #include "llvm/IR/BasicBlock.h"
      33             : #include "llvm/IR/CFG.h"
      34             : #include "llvm/IR/Constants.h"
      35             : #include "llvm/IR/Function.h"
      36             : #include "llvm/IR/InstrTypes.h"
      37             : #include "llvm/IR/Instructions.h"
      38             : #include "llvm/IR/Intrinsics.h"
      39             : #include "llvm/IR/Type.h"
      40             : #include "llvm/Pass.h"
      41             : #include "llvm/Support/Casting.h"
      42             : #include "llvm/Transforms/Scalar.h"
      43             : #include "llvm/Transforms/Utils.h"
      44             : 
      45             : using namespace llvm;
      46             : 
      47             : #define DEBUG_TYPE "amdgpu-unify-divergent-exit-nodes"
      48             : 
      49             : namespace {
      50             : 
      51        3578 : class AMDGPUUnifyDivergentExitNodes : public FunctionPass {
      52             : public:
      53             :   static char ID; // Pass identification, replacement for typeid
      54             : 
      55        1797 :   AMDGPUUnifyDivergentExitNodes() : FunctionPass(ID) {
      56        1797 :     initializeAMDGPUUnifyDivergentExitNodesPass(*PassRegistry::getPassRegistry());
      57             :   }
      58             : 
      59             :   // We can preserve non-critical-edgeness when we unify function exit nodes
      60             :   void getAnalysisUsage(AnalysisUsage &AU) const override;
      61             :   bool runOnFunction(Function &F) override;
      62             : };
      63             : 
      64             : } // end anonymous namespace
      65             : 
      66             : char AMDGPUUnifyDivergentExitNodes::ID = 0;
      67             : 
      68             : char &llvm::AMDGPUUnifyDivergentExitNodesID = AMDGPUUnifyDivergentExitNodes::ID;
      69             : 
      70       73254 : INITIALIZE_PASS_BEGIN(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
      71             :                      "Unify divergent function exit nodes", false, false)
      72       73254 : INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
      73       73254 : INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
      74      346164 : INITIALIZE_PASS_END(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
      75             :                     "Unify divergent function exit nodes", false, false)
      76             : 
      77        1785 : void AMDGPUUnifyDivergentExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
      78             :   // TODO: Preserve dominator tree.
      79             :   AU.addRequired<PostDominatorTreeWrapperPass>();
      80             : 
      81             :   AU.addRequired<DivergenceAnalysis>();
      82             : 
      83             :   // No divergent values are changed, only blocks and branch edges.
      84             :   AU.addPreserved<DivergenceAnalysis>();
      85             : 
      86             :   // We preserve the non-critical-edgeness property
      87        1785 :   AU.addPreservedID(BreakCriticalEdgesID);
      88             : 
      89             :   // This is a cluster of orthogonal Transforms
      90        1785 :   AU.addPreservedID(LowerSwitchID);
      91        1785 :   FunctionPass::getAnalysisUsage(AU);
      92             : 
      93             :   AU.addRequired<TargetTransformInfoWrapperPass>();
      94        1785 : }
      95             : 
      96             : /// \returns true if \p BB is reachable through only uniform branches.
      97             : /// XXX - Is there a more efficient way to find this?
      98         178 : static bool isUniformlyReached(const DivergenceAnalysis &DA,
      99             :                                BasicBlock &BB) {
     100             :   SmallVector<BasicBlock *, 8> Stack;
     101             :   SmallPtrSet<BasicBlock *, 8> Visited;
     102             : 
     103         818 :   for (BasicBlock *Pred : predecessors(&BB))
     104         231 :     Stack.push_back(Pred);
     105             : 
     106         488 :   while (!Stack.empty()) {
     107             :     BasicBlock *Top = Stack.pop_back_val();
     108             :     if (!DA.isUniform(Top->getTerminator()))
     109             :       return false;
     110             : 
     111         466 :     for (BasicBlock *Pred : predecessors(Top)) {
     112          78 :       if (Visited.insert(Pred).second)
     113          61 :         Stack.push_back(Pred);
     114             :     }
     115             :   }
     116             : 
     117             :   return true;
     118             : }
     119             : 
     120          43 : static BasicBlock *unifyReturnBlockSet(Function &F,
     121             :                                        ArrayRef<BasicBlock *> ReturningBlocks,
     122             :                                        const TargetTransformInfo &TTI,
     123             :                                        StringRef Name) {
     124             :   // Otherwise, we need to insert a new basic block into the function, add a PHI
     125             :   // nodes (if the function returns values), and convert all of the return
     126             :   // instructions into unconditional branches.
     127          86 :   BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), Name, &F);
     128             : 
     129             :   PHINode *PN = nullptr;
     130          43 :   if (F.getReturnType()->isVoidTy()) {
     131          34 :     ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
     132             :   } else {
     133             :     // If the function doesn't return void... add a PHI node to the block...
     134          18 :     PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
     135             :                          "UnifiedRetVal");
     136           9 :     NewRetBlock->getInstList().push_back(PN);
     137           9 :     ReturnInst::Create(F.getContext(), PN, NewRetBlock);
     138             :   }
     139             : 
     140             :   // Loop over all of the blocks, replacing the return instruction with an
     141             :   // unconditional branch.
     142         219 :   for (BasicBlock *BB : ReturningBlocks) {
     143             :     // Add an incoming element to the PHI node for every return instruction that
     144             :     // is merging into this new block...
     145          88 :     if (PN)
     146          36 :       PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
     147             : 
     148             :     // Remove and delete the return inst.
     149          88 :     BB->getTerminator()->eraseFromParent();
     150          88 :     BranchInst::Create(NewRetBlock, BB);
     151             :   }
     152             : 
     153         219 :   for (BasicBlock *BB : ReturningBlocks) {
     154             :     // Cleanup possible branch to unconditional branch to the return.
     155          88 :     simplifyCFG(BB, TTI, {2});
     156             :   }
     157             : 
     158          43 :   return NewRetBlock;
     159             : }
     160             : 
     161       17751 : bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
     162       17751 :   auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
     163       17751 :   if (PDT.getRoots().size() <= 1)
     164             :     return false;
     165             : 
     166          93 :   DivergenceAnalysis &DA = getAnalysis<DivergenceAnalysis>();
     167             : 
     168             :   // Loop over all of the blocks in a function, tracking all of the blocks that
     169             :   // return.
     170             :   SmallVector<BasicBlock *, 4> ReturningBlocks;
     171             :   SmallVector<BasicBlock *, 4> UnreachableBlocks;
     172             : 
     173             :   // Dummy return block for infinite loop.
     174          93 :   BasicBlock *DummyReturnBB = nullptr;
     175             : 
     176         483 :   for (BasicBlock *BB : PDT.getRoots()) {
     177         195 :     if (isa<ReturnInst>(BB->getTerminator())) {
     178         125 :       if (!isUniformlyReached(DA, *BB))
     179          65 :         ReturningBlocks.push_back(BB);
     180          70 :     } else if (isa<UnreachableInst>(BB->getTerminator())) {
     181          53 :       if (!isUniformlyReached(DA, *BB))
     182          27 :         UnreachableBlocks.push_back(BB);
     183             :     } else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
     184             : 
     185          17 :       ConstantInt *BoolTrue = ConstantInt::getTrue(F.getContext());
     186          17 :       if (DummyReturnBB == nullptr) {
     187          30 :         DummyReturnBB = BasicBlock::Create(F.getContext(),
     188             :                                            "DummyReturnBlock", &F);
     189             :         Type *RetTy = F.getReturnType();
     190          15 :         Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
     191          15 :         ReturnInst::Create(F.getContext(), RetVal, DummyReturnBB);
     192          15 :         ReturningBlocks.push_back(DummyReturnBB);
     193             :       }
     194             : 
     195          17 :       if (BI->isUnconditional()) {
     196             :         BasicBlock *LoopHeaderBB = BI->getSuccessor(0);
     197          15 :         BI->eraseFromParent(); // Delete the unconditional branch.
     198             :         // Add a new conditional branch with a dummy edge to the return block.
     199          15 :         BranchInst::Create(LoopHeaderBB, DummyReturnBB, BoolTrue, BB);
     200             :       } else { // Conditional branch.
     201             :         // Create a new transition block to hold the conditional branch.
     202           4 :         BasicBlock *TransitionBB = BasicBlock::Create(F.getContext(),
     203             :                                                       "TransitionBlock", &F);
     204             : 
     205             :         // Move BI from BB to the new transition block.
     206           2 :         BI->removeFromParent();
     207           2 :         TransitionBB->getInstList().push_back(BI);
     208             : 
     209             :         // Create a branch that will always branch to the transition block.
     210           2 :         BranchInst::Create(TransitionBB, DummyReturnBB, BoolTrue, BB);
     211             :       }
     212             :     }
     213             :   }
     214             : 
     215          93 :   if (!UnreachableBlocks.empty()) {
     216          20 :     BasicBlock *UnreachableBlock = nullptr;
     217             : 
     218          20 :     if (UnreachableBlocks.size() == 1) {
     219          14 :       UnreachableBlock = UnreachableBlocks.front();
     220             :     } else {
     221          12 :       UnreachableBlock = BasicBlock::Create(F.getContext(),
     222             :                                             "UnifiedUnreachableBlock", &F);
     223          12 :       new UnreachableInst(F.getContext(), UnreachableBlock);
     224             : 
     225          32 :       for (BasicBlock *BB : UnreachableBlocks) {
     226             :         // Remove and delete the unreachable inst.
     227          13 :         BB->getTerminator()->eraseFromParent();
     228          13 :         BranchInst::Create(UnreachableBlock, BB);
     229             :       }
     230             :     }
     231             : 
     232          20 :     if (!ReturningBlocks.empty()) {
     233             :       // Don't create a new unreachable inst if we have a return. The
     234             :       // structurizer/annotator can't handle the multiple exits
     235             : 
     236             :       Type *RetTy = F.getReturnType();
     237          16 :       Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
     238             :       // Remove and delete the unreachable inst.
     239          32 :       UnreachableBlock->getTerminator()->eraseFromParent();
     240             : 
     241             :       Function *UnreachableIntrin =
     242          16 :         Intrinsic::getDeclaration(F.getParent(), Intrinsic::amdgcn_unreachable);
     243             : 
     244             :       // Insert a call to an intrinsic tracking that this is an unreachable
     245             :       // point, in case we want to kill the active lanes or something later.
     246          32 :       CallInst::Create(UnreachableIntrin, {}, "", UnreachableBlock);
     247             : 
     248             :       // Don't create a scalar trap. We would only want to trap if this code was
     249             :       // really reached, but a scalar trap would happen even if no lanes
     250             :       // actually reached here.
     251          16 :       ReturnInst::Create(F.getContext(), RetVal, UnreachableBlock);
     252          16 :       ReturningBlocks.push_back(UnreachableBlock);
     253             :     }
     254             :   }
     255             : 
     256             :   // Now handle return blocks.
     257          93 :   if (ReturningBlocks.empty())
     258             :     return false; // No blocks return
     259             : 
     260          51 :   if (ReturningBlocks.size() == 1)
     261             :     return false; // Already has a single return block
     262             : 
     263             :   const TargetTransformInfo &TTI
     264          43 :     = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
     265             : 
     266          43 :   unifyReturnBlockSet(F, ReturningBlocks, TTI, "UnifiedReturnBlock");
     267          43 :   return true;
     268             : }

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