#include "llvm/IR/InlineAsm.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
+#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/IR/ValueMap.h"
bool DupRetToEnableTailCallOpts(BasicBlock *BB);
bool PlaceDbgValues(Function &F);
bool sinkAndCmp(Function &F);
+ bool splitBranchCondition(Function &F);
};
}
// into a single target instruction, push the mask and compare into branch
// users. Do this before OptimizeBlock -> OptimizeInst ->
// OptimizeCmpExpression, which perturbs the pattern being searched for.
- if (!DisableBranchOpts)
+ if (!DisableBranchOpts) {
EverMadeChange |= sinkAndCmp(F);
+ EverMadeChange |= splitBranchCondition(F);
+ }
bool MadeChange = true;
while (MadeChange) {
}
return MadeChange;
}
+
+/// \brief Retrieve the probabilities of a conditional branch. Returns true on
+/// success, or returns false if no or invalid metadata was found.
+static bool extractBranchMetadata(BranchInst *BI,
+ uint64_t &ProbTrue, uint64_t &ProbFalse) {
+ assert(BI->isConditional() &&
+ "Looking for probabilities on unconditional branch?");
+ auto *ProfileData = BI->getMetadata(LLVMContext::MD_prof);
+ if (!ProfileData || ProfileData->getNumOperands() != 3)
+ return false;
+
+ const auto *CITrue = dyn_cast<ConstantInt>(ProfileData->getOperand(1));
+ const auto *CIFalse = dyn_cast<ConstantInt>(ProfileData->getOperand(2));
+ if (!CITrue || !CIFalse)
+ return false;
+
+ ProbTrue = CITrue->getValue().getZExtValue();
+ ProbFalse = CIFalse->getValue().getZExtValue();
+
+ return true;
+}
+
+/// \brief Scale down both weights to fit into uint32_t.
+static void scaleWeights(uint64_t &NewTrue, uint64_t &NewFalse) {
+ uint64_t NewMax = (NewTrue > NewFalse) ? NewTrue : NewFalse;
+ uint32_t Scale = (NewMax / UINT32_MAX) + 1;
+ NewTrue = NewTrue / Scale;
+ NewFalse = NewFalse / Scale;
+}
+
+/// \brief Some targets prefer to split a conditional branch like:
+/// \code
+/// %0 = icmp ne i32 %a, 0
+/// %1 = icmp ne i32 %b, 0
+/// %or.cond = or i1 %0, %1
+/// br i1 %or.cond, label %TrueBB, label %FalseBB
+/// \endcode
+/// into multiple branch instructions like:
+/// \code
+/// bb1:
+/// %0 = icmp ne i32 %a, 0
+/// br i1 %0, label %TrueBB, label %bb2
+/// bb2:
+/// %1 = icmp ne i32 %b, 0
+/// br i1 %1, label %TrueBB, label %FalseBB
+/// \endcode
+/// This usually allows instruction selection to do even further optimizations
+/// and combine the compare with the branch instruction. Currently this is
+/// applied for targets which have "cheap" jump instructions.
+///
+/// FIXME: Remove the (equivalent?) implementation in SelectionDAG.
+///
+bool CodeGenPrepare::splitBranchCondition(Function &F) {
+ if (!TM || TM->Options.EnableFastISel != true ||
+ !TLI || TLI->isJumpExpensive())
+ return false;
+
+ bool MadeChange = false;
+ for (auto &BB : F) {
+ // Does this BB end with the following?
+ // %cond1 = icmp|fcmp|binary instruction ...
+ // %cond2 = icmp|fcmp|binary instruction ...
+ // %cond.or = or|and i1 %cond1, cond2
+ // br i1 %cond.or label %dest1, label %dest2"
+ BinaryOperator *LogicOp;
+ BasicBlock *TBB, *FBB;
+ if (!match(BB.getTerminator(), m_Br(m_OneUse(m_BinOp(LogicOp)), TBB, FBB)))
+ continue;
+
+ unsigned Opc;
+ Instruction *Cond1, *Cond2;
+ if (match(LogicOp, m_And(m_OneUse(m_Instruction(Cond1)),
+ m_OneUse(m_Instruction(Cond2)))))
+ Opc = Instruction::And;
+ else if (match(LogicOp, m_Or(m_OneUse(m_Instruction(Cond1)),
+ m_OneUse(m_Instruction(Cond2)))))
+ Opc = Instruction::Or;
+ else
+ continue;
+
+ if (!match(Cond1, m_CombineOr(m_Cmp(), m_BinOp())) ||
+ !match(Cond2, m_CombineOr(m_Cmp(), m_BinOp())) )
+ continue;
+
+ DEBUG(dbgs() << "Before branch condition splitting\n"; BB.dump());
+
+ // Create a new BB.
+ auto *InsertBefore = std::next(Function::iterator(BB))
+ .getNodePtrUnchecked();
+ auto TmpBB = BasicBlock::Create(BB.getContext(),
+ BB.getName() + ".cond.split",
+ BB.getParent(), InsertBefore);
+
+ // Update original basic block by using the first condition directly by the
+ // branch instruction and removing the no longer needed and/or instruction.
+ auto *Br1 = cast<BranchInst>(BB.getTerminator());
+ Br1->setCondition(Cond1);
+ LogicOp->eraseFromParent();
+ Cond2->removeFromParent();
+ // Depending on the conditon we have to either replace the true or the false
+ // successor of the original branch instruction.
+ if (Opc == Instruction::And)
+ Br1->setSuccessor(0, TmpBB);
+ else
+ Br1->setSuccessor(1, TmpBB);
+
+ // Fill in the new basic block.
+ auto *Br2 = IRBuilder<>(TmpBB).CreateCondBr(Cond2, TBB, FBB);
+ Cond2->insertBefore(Br2);
+
+ // Update PHI nodes in both successors. The original BB needs to be
+ // replaced in one succesor's PHI nodes, because the branch comes now from
+ // the newly generated BB (NewBB). In the other successor we need to add one
+ // incoming edge to the PHI nodes, because both branch instructions target
+ // now the same successor. Depending on the original branch condition
+ // (and/or) we have to swap the successors (TrueDest, FalseDest), so that
+ // we perfrom the correct update for the PHI nodes.
+ // This doesn't change the successor order of the just created branch
+ // instruction (or any other instruction).
+ if (Opc == Instruction::Or)
+ std::swap(TBB, FBB);
+
+ // Replace the old BB with the new BB.
+ for (auto &I : *TBB) {
+ PHINode *PN = dyn_cast<PHINode>(&I);
+ if (!PN)
+ break;
+ int i;
+ while ((i = PN->getBasicBlockIndex(&BB)) >= 0)
+ PN->setIncomingBlock(i, TmpBB);
+ }
+
+ // Add another incoming edge form the new BB.
+ for (auto &I : *FBB) {
+ PHINode *PN = dyn_cast<PHINode>(&I);
+ if (!PN)
+ break;
+ auto *Val = PN->getIncomingValueForBlock(&BB);
+ PN->addIncoming(Val, TmpBB);
+ }
+
+ // Update the branch weights (from SelectionDAGBuilder::
+ // FindMergedConditions).
+ if (Opc == Instruction::Or) {
+ // Codegen X | Y as:
+ // BB1:
+ // jmp_if_X TBB
+ // jmp TmpBB
+ // TmpBB:
+ // jmp_if_Y TBB
+ // jmp FBB
+ //
+
+ // We have flexibility in setting Prob for BB1 and Prob for NewBB.
+ // The requirement is that
+ // TrueProb for BB1 + (FalseProb for BB1 * TrueProb for TmpBB)
+ // = TrueProb for orignal BB.
+ // Assuming the orignal weights are A and B, one choice is to set BB1's
+ // weights to A and A+2B, and set TmpBB's weights to A and 2B. This choice
+ // assumes that
+ // TrueProb for BB1 == FalseProb for BB1 * TrueProb for TmpBB.
+ // Another choice is to assume TrueProb for BB1 equals to TrueProb for
+ // TmpBB, but the math is more complicated.
+ uint64_t TrueWeight, FalseWeight;
+ if (extractBranchMetadata(Br1, TrueWeight, FalseWeight)) {
+ uint64_t NewTrueWeight = TrueWeight;
+ uint64_t NewFalseWeight = TrueWeight + 2 * FalseWeight;
+ scaleWeights(NewTrueWeight, NewFalseWeight);
+ Br1->setMetadata(LLVMContext::MD_prof, MDBuilder(Br1->getContext())
+ .createBranchWeights(TrueWeight, FalseWeight));
+
+ NewTrueWeight = TrueWeight;
+ NewFalseWeight = 2 * FalseWeight;
+ scaleWeights(NewTrueWeight, NewFalseWeight);
+ Br2->setMetadata(LLVMContext::MD_prof, MDBuilder(Br2->getContext())
+ .createBranchWeights(TrueWeight, FalseWeight));
+ }
+ } else {
+ // Codegen X & Y as:
+ // BB1:
+ // jmp_if_X TmpBB
+ // jmp FBB
+ // TmpBB:
+ // jmp_if_Y TBB
+ // jmp FBB
+ //
+ // This requires creation of TmpBB after CurBB.
+
+ // We have flexibility in setting Prob for BB1 and Prob for TmpBB.
+ // The requirement is that
+ // FalseProb for BB1 + (TrueProb for BB1 * FalseProb for TmpBB)
+ // = FalseProb for orignal BB.
+ // Assuming the orignal weights are A and B, one choice is to set BB1's
+ // weights to 2A+B and B, and set TmpBB's weights to 2A and B. This choice
+ // assumes that
+ // FalseProb for BB1 == TrueProb for BB1 * FalseProb for TmpBB.
+ uint64_t TrueWeight, FalseWeight;
+ if (extractBranchMetadata(Br1, TrueWeight, FalseWeight)) {
+ uint64_t NewTrueWeight = 2 * TrueWeight + FalseWeight;
+ uint64_t NewFalseWeight = FalseWeight;
+ scaleWeights(NewTrueWeight, NewFalseWeight);
+ Br1->setMetadata(LLVMContext::MD_prof, MDBuilder(Br1->getContext())
+ .createBranchWeights(TrueWeight, FalseWeight));
+
+ NewTrueWeight = 2 * TrueWeight;
+ NewFalseWeight = FalseWeight;
+ scaleWeights(NewTrueWeight, NewFalseWeight);
+ Br2->setMetadata(LLVMContext::MD_prof, MDBuilder(Br2->getContext())
+ .createBranchWeights(TrueWeight, FalseWeight));
+ }
+ }
+
+ // Request DOM Tree update.
+ // Note: No point in getting fancy here, since the DT info is never
+ // available to CodeGenPrepare and the existing update code is broken
+ // anyways.
+ ModifiedDT = true;
+
+ MadeChange = true;
+
+ DEBUG(dbgs() << "After branch condition splitting\n"; BB.dump();
+ TmpBB->dump());
+ }
+ return MadeChange;
+}
projects / oota-llvm.git / blobdiff