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//===- AMDGPUUnifyDivergentExitNodes.cpp ----------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This is a variant of the UnifyDivergentExitNodes pass. Rather than ensuring
// there is at most one ret and one unreachable instruction, it ensures there is
// at most one divergent exiting block.
// StructurizeCFG can't deal with multi-exit regions formed by branches to
// multiple return nodes. It is not desirable to structurize regions with
// uniform branches, so unifying those to the same return block as divergent
// branches inhibits use of scalar branching. It still can't deal with the case
// where one branch goes to return, and one unreachable. Replace unreachable in
// this case with a return.
#include "AMDGPU.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/LegacyDivergenceAnalysis.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Intrinsics.h"
#include "llvm/IR/Type.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
using namespace llvm;
#define DEBUG_TYPE "amdgpu-unify-divergent-exit-nodes"
namespace {
class AMDGPUUnifyDivergentExitNodes : public FunctionPass {
static char ID; // Pass identification, replacement for typeid
AMDGPUUnifyDivergentExitNodes() : FunctionPass(ID) {
// We can preserve non-critical-edgeness when we unify function exit nodes
void getAnalysisUsage(AnalysisUsage &AU) const override;
bool runOnFunction(Function &F) override;
} // end anonymous namespace
char AMDGPUUnifyDivergentExitNodes::ID = 0;
char &llvm::AMDGPUUnifyDivergentExitNodesID = AMDGPUUnifyDivergentExitNodes::ID;
"Unify divergent function exit nodes", false, false)
"Unify divergent function exit nodes", false, false)
void AMDGPUUnifyDivergentExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
// TODO: Preserve dominator tree.
// No divergent values are changed, only blocks and branch edges.
// We preserve the non-critical-edgeness property
// This is a cluster of orthogonal Transforms
/// \returns true if \p BB is reachable through only uniform branches.
/// XXX - Is there a more efficient way to find this?
static bool isUniformlyReached(const LegacyDivergenceAnalysis &DA,
BasicBlock &BB) {
SmallVector<BasicBlock *, 8> Stack;
SmallPtrSet<BasicBlock *, 8> Visited;
for (BasicBlock *Pred : predecessors(&BB))
while (!Stack.empty()) {
BasicBlock *Top = Stack.pop_back_val();
if (!DA.isUniform(Top->getTerminator()))
return false;
for (BasicBlock *Pred : predecessors(Top)) {
if (Visited.insert(Pred).second)
return true;
static BasicBlock *unifyReturnBlockSet(Function &F,
ArrayRef<BasicBlock *> ReturningBlocks,
const TargetTransformInfo &TTI,
StringRef Name) {
// Otherwise, we need to insert a new basic block into the function, add a PHI
// nodes (if the function returns values), and convert all of the return
// instructions into unconditional branches.
BasicBlock *NewRetBlock = BasicBlock::Create(F.getContext(), Name, &F);
PHINode *PN = nullptr;
if (F.getReturnType()->isVoidTy()) {
ReturnInst::Create(F.getContext(), nullptr, NewRetBlock);
} else {
// If the function doesn't return void... add a PHI node to the block...
PN = PHINode::Create(F.getReturnType(), ReturningBlocks.size(),
ReturnInst::Create(F.getContext(), PN, NewRetBlock);
// Loop over all of the blocks, replacing the return instruction with an
// unconditional branch.
for (BasicBlock *BB : ReturningBlocks) {
// Add an incoming element to the PHI node for every return instruction that
// is merging into this new block...
if (PN)
PN->addIncoming(BB->getTerminator()->getOperand(0), BB);
// Remove and delete the return inst.
BranchInst::Create(NewRetBlock, BB);
for (BasicBlock *BB : ReturningBlocks) {
// Cleanup possible branch to unconditional branch to the return.
simplifyCFG(BB, TTI, {2});
return NewRetBlock;
bool AMDGPUUnifyDivergentExitNodes::runOnFunction(Function &F) {
auto &PDT = getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
if (PDT.getRoots().size() <= 1)
return false;
LegacyDivergenceAnalysis &DA = getAnalysis<LegacyDivergenceAnalysis>();
// Loop over all of the blocks in a function, tracking all of the blocks that
// return.
SmallVector<BasicBlock *, 4> ReturningBlocks;
SmallVector<BasicBlock *, 4> UnreachableBlocks;
// Dummy return block for infinite loop.
BasicBlock *DummyReturnBB = nullptr;
for (BasicBlock *BB : PDT.getRoots()) {
if (isa<ReturnInst>(BB->getTerminator())) {
if (!isUniformlyReached(DA, *BB))
} else if (isa<UnreachableInst>(BB->getTerminator())) {
if (!isUniformlyReached(DA, *BB))
} else if (BranchInst *BI = dyn_cast<BranchInst>(BB->getTerminator())) {
ConstantInt *BoolTrue = ConstantInt::getTrue(F.getContext());
if (DummyReturnBB == nullptr) {
DummyReturnBB = BasicBlock::Create(F.getContext(),
"DummyReturnBlock", &F);
Type *RetTy = F.getReturnType();
Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
ReturnInst::Create(F.getContext(), RetVal, DummyReturnBB);
if (BI->isUnconditional()) {
BasicBlock *LoopHeaderBB = BI->getSuccessor(0);
BI->eraseFromParent(); // Delete the unconditional branch.
// Add a new conditional branch with a dummy edge to the return block.
BranchInst::Create(LoopHeaderBB, DummyReturnBB, BoolTrue, BB);
} else { // Conditional branch.
// Create a new transition block to hold the conditional branch.
BasicBlock *TransitionBB = BB->splitBasicBlock(BI, "TransitionBlock");
// Create a branch that will always branch to the transition block and
// references DummyReturnBB.
BranchInst::Create(TransitionBB, DummyReturnBB, BoolTrue, BB);
if (!UnreachableBlocks.empty()) {
BasicBlock *UnreachableBlock = nullptr;
if (UnreachableBlocks.size() == 1) {
UnreachableBlock = UnreachableBlocks.front();
} else {
UnreachableBlock = BasicBlock::Create(F.getContext(),
"UnifiedUnreachableBlock", &F);
new UnreachableInst(F.getContext(), UnreachableBlock);
for (BasicBlock *BB : UnreachableBlocks) {
// Remove and delete the unreachable inst.
BranchInst::Create(UnreachableBlock, BB);
if (!ReturningBlocks.empty()) {
// Don't create a new unreachable inst if we have a return. The
// structurizer/annotator can't handle the multiple exits
Type *RetTy = F.getReturnType();
Value *RetVal = RetTy->isVoidTy() ? nullptr : UndefValue::get(RetTy);
// Remove and delete the unreachable inst.
Function *UnreachableIntrin =
Intrinsic::getDeclaration(F.getParent(), Intrinsic::amdgcn_unreachable);
// Insert a call to an intrinsic tracking that this is an unreachable
// point, in case we want to kill the active lanes or something later.
CallInst::Create(UnreachableIntrin, {}, "", UnreachableBlock);
// Don't create a scalar trap. We would only want to trap if this code was
// really reached, but a scalar trap would happen even if no lanes
// actually reached here.
ReturnInst::Create(F.getContext(), RetVal, UnreachableBlock);
// Now handle return blocks.
if (ReturningBlocks.empty())
return false; // No blocks return
if (ReturningBlocks.size() == 1)
return false; // Already has a single return block
const TargetTransformInfo &TTI
= getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
unifyReturnBlockSet(F, ReturningBlocks, TTI, "UnifiedReturnBlock");
return true;