summaryrefslogtreecommitdiffstats
path: root/lib/Transforms/Utils/SimplifyCFG.cpp
diff options
context:
space:
mode:
Diffstat (limited to 'lib/Transforms/Utils/SimplifyCFG.cpp')
-rw-r--r--lib/Transforms/Utils/SimplifyCFG.cpp288
1 files changed, 148 insertions, 140 deletions
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index 28d3988..f12ad9b 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -20,12 +20,14 @@
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Metadata.h"
+#include "llvm/Operator.h"
#include "llvm/Type.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
@@ -207,6 +209,42 @@ static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
return BI->getCondition();
}
+/// ComputeSpeculuationCost - Compute an abstract "cost" of speculating the
+/// given instruction, which is assumed to be safe to speculate. 1 means
+/// cheap, 2 means less cheap, and UINT_MAX means prohibitively expensive.
+static unsigned ComputeSpeculationCost(const User *I) {
+ assert(isSafeToSpeculativelyExecute(I) &&
+ "Instruction is not safe to speculatively execute!");
+ switch (Operator::getOpcode(I)) {
+ default:
+ // In doubt, be conservative.
+ return UINT_MAX;
+ case Instruction::GetElementPtr:
+ // GEPs are cheap if all indices are constant.
+ if (!cast<GEPOperator>(I)->hasAllConstantIndices())
+ return UINT_MAX;
+ return 1;
+ case Instruction::Load:
+ case Instruction::Add:
+ case Instruction::Sub:
+ case Instruction::And:
+ case Instruction::Or:
+ case Instruction::Xor:
+ case Instruction::Shl:
+ case Instruction::LShr:
+ case Instruction::AShr:
+ case Instruction::ICmp:
+ case Instruction::Trunc:
+ case Instruction::ZExt:
+ case Instruction::SExt:
+ return 1; // These are all cheap.
+
+ case Instruction::Call:
+ case Instruction::Select:
+ return 2;
+ }
+}
+
/// DominatesMergePoint - If we have a merge point of an "if condition" as
/// accepted above, return true if the specified value dominates the block. We
/// don't handle the true generality of domination here, just a special case
@@ -262,44 +300,7 @@ static bool DominatesMergePoint(Value *V, BasicBlock *BB,
if (!isSafeToSpeculativelyExecute(I))
return false;
- unsigned Cost = 0;
-
- switch (I->getOpcode()) {
- default: return false; // Cannot hoist this out safely.
- case Instruction::Load:
- // We have to check to make sure there are no instructions before the
- // load in its basic block, as we are going to hoist the load out to its
- // predecessor.
- if (PBB->getFirstNonPHIOrDbg() != I)
- return false;
- Cost = 1;
- break;
- case Instruction::GetElementPtr:
- // GEPs are cheap if all indices are constant.
- if (!cast<GetElementPtrInst>(I)->hasAllConstantIndices())
- return false;
- Cost = 1;
- break;
- case Instruction::Add:
- case Instruction::Sub:
- case Instruction::And:
- case Instruction::Or:
- case Instruction::Xor:
- case Instruction::Shl:
- case Instruction::LShr:
- case Instruction::AShr:
- case Instruction::ICmp:
- case Instruction::Trunc:
- case Instruction::ZExt:
- case Instruction::SExt:
- Cost = 1;
- break; // These are all cheap and non-trapping instructions.
-
- case Instruction::Call:
- case Instruction::Select:
- Cost = 2;
- break;
- }
+ unsigned Cost = ComputeSpeculationCost(I);
if (Cost > CostRemaining)
return false;
@@ -954,6 +955,20 @@ HoistTerminator:
/// and an BB2 and the only successor of BB1 is BB2, hoist simple code
/// (for now, restricted to a single instruction that's side effect free) from
/// the BB1 into the branch block to speculatively execute it.
+///
+/// Turn
+/// BB:
+/// %t1 = icmp
+/// br i1 %t1, label %BB1, label %BB2
+/// BB1:
+/// %t3 = add %t2, c
+/// br label BB2
+/// BB2:
+/// =>
+/// BB:
+/// %t1 = icmp
+/// %t4 = add %t2, c
+/// %t3 = select i1 %t1, %t2, %t3
static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
// Only speculatively execution a single instruction (not counting the
// terminator) for now.
@@ -970,8 +985,29 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
return false;
HInst = I;
}
- if (!HInst)
- return false;
+
+ BasicBlock *BIParent = BI->getParent();
+
+ // Check the instruction to be hoisted, if there is one.
+ if (HInst) {
+ // Don't hoist the instruction if it's unsafe or expensive.
+ if (!isSafeToSpeculativelyExecute(HInst))
+ return false;
+ if (ComputeSpeculationCost(HInst) > PHINodeFoldingThreshold)
+ return false;
+
+ // Do not hoist the instruction if any of its operands are defined but not
+ // used in this BB. The transformation will prevent the operand from
+ // being sunk into the use block.
+ for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
+ i != e; ++i) {
+ Instruction *OpI = dyn_cast<Instruction>(*i);
+ if (OpI && OpI->getParent() == BIParent &&
+ !OpI->mayHaveSideEffects() &&
+ !OpI->isUsedInBasicBlock(BIParent))
+ return false;
+ }
+ }
// Be conservative for now. FP select instruction can often be expensive.
Value *BrCond = BI->getCondition();
@@ -986,106 +1022,78 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *BB1) {
Invert = true;
}
- // Turn
- // BB:
- // %t1 = icmp
- // br i1 %t1, label %BB1, label %BB2
- // BB1:
- // %t3 = add %t2, c
- // br label BB2
- // BB2:
- // =>
- // BB:
- // %t1 = icmp
- // %t4 = add %t2, c
- // %t3 = select i1 %t1, %t2, %t3
- switch (HInst->getOpcode()) {
- default: return false; // Not safe / profitable to hoist.
- case Instruction::Add:
- case Instruction::Sub:
- // Not worth doing for vector ops.
- if (HInst->getType()->isVectorTy())
- return false;
- break;
- case Instruction::And:
- case Instruction::Or:
- case Instruction::Xor:
- case Instruction::Shl:
- case Instruction::LShr:
- case Instruction::AShr:
- // Don't mess with vector operations.
- if (HInst->getType()->isVectorTy())
- return false;
- break; // These are all cheap and non-trapping instructions.
- }
-
- // If the instruction is obviously dead, don't try to predicate it.
- if (HInst->use_empty()) {
- HInst->eraseFromParent();
- return true;
+ // Collect interesting PHIs, and scan for hazards.
+ SmallSetVector<std::pair<Value *, Value *>, 4> PHIs;
+ BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
+ for (BasicBlock::iterator I = BB2->begin();
+ PHINode *PN = dyn_cast<PHINode>(I); ++I) {
+ Value *BB1V = PN->getIncomingValueForBlock(BB1);
+ Value *BIParentV = PN->getIncomingValueForBlock(BIParent);
+
+ // Skip PHIs which are trivial.
+ if (BB1V == BIParentV)
+ continue;
+
+ // Check for saftey.
+ if (ConstantExpr *CE = dyn_cast<ConstantExpr>(BB1V)) {
+ // An unfolded ConstantExpr could end up getting expanded into
+ // Instructions. Don't speculate this and another instruction at
+ // the same time.
+ if (HInst)
+ return false;
+ if (!isSafeToSpeculativelyExecute(CE))
+ return false;
+ if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
+ return false;
+ }
+
+ // Ok, we may insert a select for this PHI.
+ PHIs.insert(std::make_pair(BB1V, BIParentV));
}
- // Can we speculatively execute the instruction? And what is the value
- // if the condition is false? Consider the phi uses, if the incoming value
- // from the "if" block are all the same V, then V is the value of the
- // select if the condition is false.
- BasicBlock *BIParent = BI->getParent();
- SmallVector<PHINode*, 4> PHIUses;
- Value *FalseV = NULL;
+ // If there are no PHIs to process, bail early. This helps ensure idempotence
+ // as well.
+ if (PHIs.empty())
+ return false;
- BasicBlock *BB2 = BB1->getTerminator()->getSuccessor(0);
- for (Value::use_iterator UI = HInst->use_begin(), E = HInst->use_end();
- UI != E; ++UI) {
- // Ignore any user that is not a PHI node in BB2. These can only occur in
- // unreachable blocks, because they would not be dominated by the instr.
- PHINode *PN = dyn_cast<PHINode>(*UI);
- if (!PN || PN->getParent() != BB2)
- return false;
- PHIUses.push_back(PN);
-
- Value *PHIV = PN->getIncomingValueForBlock(BIParent);
- if (!FalseV)
- FalseV = PHIV;
- else if (FalseV != PHIV)
- return false; // Inconsistent value when condition is false.
- }
-
- assert(FalseV && "Must have at least one user, and it must be a PHI");
-
- // Do not hoist the instruction if any of its operands are defined but not
- // used in this BB. The transformation will prevent the operand from
- // being sunk into the use block.
- for (User::op_iterator i = HInst->op_begin(), e = HInst->op_end();
- i != e; ++i) {
- Instruction *OpI = dyn_cast<Instruction>(*i);
- if (OpI && OpI->getParent() == BIParent &&
- !OpI->isUsedInBasicBlock(BIParent))
- return false;
- }
+ // If we get here, we can hoist the instruction and if-convert.
+ DEBUG(dbgs() << "SPECULATIVELY EXECUTING BB" << *BB1 << "\n";);
- // If we get here, we can hoist the instruction.
- BIParent->getInstList().splice(BI, BB1->getInstList(), HInst);
+ // Hoist the instruction.
+ if (HInst)
+ BIParent->getInstList().splice(BI, BB1->getInstList(), HInst);
- // Create a select whose true value is the speculatively executed value and
- // false value is the previously determined FalseV.
+ // Insert selects and rewrite the PHI operands.
IRBuilder<true, NoFolder> Builder(BI);
- SelectInst *SI;
- if (Invert)
- SI = cast<SelectInst>
- (Builder.CreateSelect(BrCond, FalseV, HInst,
- FalseV->getName() + "." + HInst->getName()));
- else
- SI = cast<SelectInst>
- (Builder.CreateSelect(BrCond, HInst, FalseV,
- HInst->getName() + "." + FalseV->getName()));
-
- // Make the PHI node use the select for all incoming values for "then" and
- // "if" blocks.
- for (unsigned i = 0, e = PHIUses.size(); i != e; ++i) {
- PHINode *PN = PHIUses[i];
- for (unsigned j = 0, ee = PN->getNumIncomingValues(); j != ee; ++j)
- if (PN->getIncomingBlock(j) == BB1 || PN->getIncomingBlock(j) == BIParent)
- PN->setIncomingValue(j, SI);
+ for (unsigned i = 0, e = PHIs.size(); i != e; ++i) {
+ Value *TrueV = PHIs[i].first;
+ Value *FalseV = PHIs[i].second;
+
+ // Create a select whose true value is the speculatively executed value and
+ // false value is the previously determined FalseV.
+ SelectInst *SI;
+ if (Invert)
+ SI = cast<SelectInst>
+ (Builder.CreateSelect(BrCond, FalseV, TrueV,
+ FalseV->getName() + "." + TrueV->getName()));
+ else
+ SI = cast<SelectInst>
+ (Builder.CreateSelect(BrCond, TrueV, FalseV,
+ TrueV->getName() + "." + FalseV->getName()));
+
+ // Make the PHI node use the select for all incoming values for "then" and
+ // "if" blocks.
+ for (BasicBlock::iterator I = BB2->begin();
+ PHINode *PN = dyn_cast<PHINode>(I); ++I) {
+ unsigned BB1I = PN->getBasicBlockIndex(BB1);
+ unsigned BIParentI = PN->getBasicBlockIndex(BIParent);
+ Value *BB1V = PN->getIncomingValue(BB1I);
+ Value *BIParentV = PN->getIncomingValue(BIParentI);
+ if (TrueV == BB1V && FalseV == BIParentV) {
+ PN->setIncomingValue(BB1I, SI);
+ PN->setIncomingValue(BIParentI, SI);
+ }
+ }
}
++NumSpeculations;
@@ -2772,6 +2780,12 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
if (SimplifyBranchOnICmpChain(BI, TD, Builder))
return true;
+ // If this basic block is ONLY a compare and a branch, and if a predecessor
+ // branches to us and one of our successors, fold the comparison into the
+ // predecessor and use logical operations to pick the right destination.
+ if (FoldBranchToCommonDest(BI))
+ return SimplifyCFG(BB) | true;
+
// We have a conditional branch to two blocks that are only reachable
// from BI. We know that the condbr dominates the two blocks, so see if
// there is any identical code in the "then" and "else" blocks. If so, we
@@ -2806,12 +2820,6 @@ bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
if (FoldCondBranchOnPHI(BI, TD))
return SimplifyCFG(BB) | true;
- // If this basic block is ONLY a compare and a branch, and if a predecessor
- // branches to us and one of our successors, fold the comparison into the
- // predecessor and use logical operations to pick the right destination.
- if (FoldBranchToCommonDest(BI))
- return SimplifyCFG(BB) | true;
-
// Scan predecessor blocks for conditional branches.
for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
if (BranchInst *PBI = dyn_cast<BranchInst>((*PI)->getTerminator()))
generated by cgit v1.1 at 2024-10-03 05:21:27 +0000