llvm-project/llvm/lib/Target/AMDGPU/AMDGPUUnifyDivergentExitNod...

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//===- AMDGPUUnifyDivergentExitNodes.cpp ----------------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// 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/DivergenceAnalysis.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/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"
using namespace llvm;
#define DEBUG_TYPE "amdgpu-unify-divergent-exit-nodes"
namespace {
class AMDGPUUnifyDivergentExitNodes : public FunctionPass {
public:
static char ID; // Pass identification, replacement for typeid
AMDGPUUnifyDivergentExitNodes() : FunctionPass(ID) {
initializeAMDGPUUnifyDivergentExitNodesPass(*PassRegistry::getPassRegistry());
}
// 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;
INITIALIZE_PASS_BEGIN(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
"Unify divergent function exit nodes", false, false)
INITIALIZE_PASS_DEPENDENCY(PostDominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DivergenceAnalysis)
INITIALIZE_PASS_END(AMDGPUUnifyDivergentExitNodes, DEBUG_TYPE,
"Unify divergent function exit nodes", false, false)
void AMDGPUUnifyDivergentExitNodes::getAnalysisUsage(AnalysisUsage &AU) const{
// TODO: Preserve dominator tree.
AU.addRequired<PostDominatorTreeWrapperPass>();
AU.addRequired<DivergenceAnalysis>();
// No divergent values are changed, only blocks and branch edges.
AU.addPreserved<DivergenceAnalysis>();
// We preserve the non-critical-edgeness property
AU.addPreservedID(BreakCriticalEdgesID);
// This is a cluster of orthogonal Transforms
AU.addPreservedID(LowerSwitchID);
FunctionPass::getAnalysisUsage(AU);
AU.addRequired<TargetTransformInfoWrapperPass>();
}
/// \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 DivergenceAnalysis &DA,
BasicBlock &BB) {
SmallVector<BasicBlock *, 8> Stack;
SmallPtrSet<BasicBlock *, 8> Visited;
for (BasicBlock *Pred : predecessors(&BB))
Stack.push_back(Pred);
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)
Stack.push_back(Pred);
}
}
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(),
"UnifiedRetVal");
NewRetBlock->getInstList().push_back(PN);
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);
BB->getInstList().pop_back(); // Remove the return insn
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;
DivergenceAnalysis &DA = getAnalysis<DivergenceAnalysis>();
// Loop over all of the blocks in a function, tracking all of the blocks that
// return.
SmallVector<BasicBlock *, 4> ReturningBlocks;
SmallVector<BasicBlock *, 4> UnreachableBlocks;
for (BasicBlock *BB : PDT.getRoots()) {
if (isa<ReturnInst>(BB->getTerminator())) {
if (!isUniformlyReached(DA, *BB))
ReturningBlocks.push_back(BB);
} else if (isa<UnreachableInst>(BB->getTerminator())) {
if (!isUniformlyReached(DA, *BB))
UnreachableBlocks.push_back(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) {
BB->getInstList().pop_back(); // Remove 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);
UnreachableBlock->getInstList().pop_back(); // Remove 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);
ReturningBlocks.push_back(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;
}