SimplifyCFG: Use parallel-and and parallel-or mode to consolidate branch conditions
Merge consecutive if-regions if they contain identical statements.
Both transformations reduce number of branches. The transformation
is guarded by a target-hook, and is currently enabled only for +R600,
but the correctness has been tested on X86 target using a variety of
CPU benchmarks.
Patch by: Mei Ye
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187278 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Transforms/Utils/SimplifyCFG.cpp b/lib/Transforms/Utils/SimplifyCFG.cpp
index eaeb19f..084b74c 100644
--- a/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -19,6 +19,7 @@
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
+#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
@@ -65,6 +66,10 @@
HoistCondStores("simplifycfg-hoist-cond-stores", cl::Hidden, cl::init(true),
cl::desc("Hoist conditional stores if an unconditional store preceeds"));
+static cl::opt<bool>
+ParallelAndOr("simplifycfg-parallel-and-or", cl::Hidden, cl::init(true),
+ cl::desc("Use parallel-and-or mode for branch conditions"));
+
STATISTIC(NumBitMaps, "Number of switch instructions turned into bitmaps");
STATISTIC(NumLookupTables, "Number of switch instructions turned into lookup tables");
STATISTIC(NumSinkCommons, "Number of common instructions sunk down to the end block");
@@ -90,6 +95,7 @@
class SimplifyCFGOpt {
const TargetTransformInfo &TTI;
const DataLayout *const TD;
+ AliasAnalysis *AA;
Value *isValueEqualityComparison(TerminatorInst *TI);
BasicBlock *GetValueEqualityComparisonCases(TerminatorInst *TI,
@@ -107,10 +113,25 @@
bool SimplifyIndirectBr(IndirectBrInst *IBI);
bool SimplifyUncondBranch(BranchInst *BI, IRBuilder <> &Builder);
bool SimplifyCondBranch(BranchInst *BI, IRBuilder <>&Builder);
+ /// \brief Use parallel-and or parallel-or to generate conditions for
+ /// conditional branches.
+ bool SimplifyParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder, Pass *P = 0);
+ /// \brief If \param BB is the merge block of an if-region, attempt to merge
+ /// the if-region with an adjacent if-region upstream if two if-regions
+ /// contain identical instructions.
+ bool MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder, Pass *P = 0);
+ /// \brief Compare a pair of blocks: \param Block1 and \param Block2, which
+ /// are from two if-regions whose entry blocks are \param Head1 and \param
+ /// Head2. \returns true if \param Block1 and \param Block2 contain identical
+ /// instructions, and have no memory reference alias with \param Head2.
+ /// This is used as a legality check for merging if-regions.
+ bool CompareIfRegionBlock(BasicBlock *Head1, BasicBlock *Head2,
+ BasicBlock *Block1, BasicBlock *Block2);
public:
- SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout *TD)
- : TTI(TTI), TD(TD) {}
+ SimplifyCFGOpt(const TargetTransformInfo &TTI, const DataLayout *TD,
+ AliasAnalysis *AA)
+ : TTI(TTI), TD(TD), AA(AA) {}
bool run(BasicBlock *BB);
};
}
@@ -197,8 +218,8 @@
}
-/// GetIfCondition - Given a basic block (BB) with two predecessors (and at
-/// least one PHI node in it), check to see if the merge at this block is due
+/// GetIfCondition - Given a basic block (BB) with two predecessors,
+/// check to see if the merge at this block is due
/// to an "if condition". If so, return the boolean condition that determines
/// which entry into BB will be taken. Also, return by references the block
/// that will be entered from if the condition is true, and the block that will
@@ -208,11 +229,26 @@
/// instructions in them.
static Value *GetIfCondition(BasicBlock *BB, BasicBlock *&IfTrue,
BasicBlock *&IfFalse) {
- PHINode *SomePHI = cast<PHINode>(BB->begin());
- assert(SomePHI->getNumIncomingValues() == 2 &&
- "Function can only handle blocks with 2 predecessors!");
- BasicBlock *Pred1 = SomePHI->getIncomingBlock(0);
- BasicBlock *Pred2 = SomePHI->getIncomingBlock(1);
+ PHINode *SomePHI = dyn_cast<PHINode>(BB->begin());
+ BasicBlock *Pred1 = NULL;
+ BasicBlock *Pred2 = NULL;
+
+ if (SomePHI) {
+ if (SomePHI->getNumIncomingValues() != 2)
+ return NULL;
+ Pred1 = SomePHI->getIncomingBlock(0);
+ Pred2 = SomePHI->getIncomingBlock(1);
+ } else {
+ pred_iterator PI = pred_begin(BB), PE = pred_end(BB);
+ if (PI == PE) // No predecessor
+ return NULL;
+ Pred1 = *PI++;
+ if (PI == PE) // Only one predecessor
+ return NULL;
+ Pred2 = *PI++;
+ if (PI != PE) // More than two predecessors
+ return NULL;
+ }
// We can only handle branches. Other control flow will be lowered to
// branches if possible anyway.
@@ -4066,6 +4102,383 @@
return false;
}
+/// If \param [in] BB has more than one predecessor that is a conditional
+/// branch, attempt to use parallel and/or for the branch condition. \returns
+/// true on success.
+///
+/// Before:
+/// ......
+/// %cmp10 = fcmp une float %tmp1, %tmp2
+/// br i1 %cmp1, label %if.then, label %lor.rhs
+///
+/// lor.rhs:
+/// ......
+/// %cmp11 = fcmp une float %tmp3, %tmp4
+/// br i1 %cmp11, label %if.then, label %ifend
+///
+/// if.end: // the merge block
+/// ......
+///
+/// if.then: // has two predecessors, both of them contains conditional branch.
+/// ......
+/// br label %if.end;
+///
+/// After:
+/// ......
+/// %cmp10 = fcmp une float %tmp1, %tmp2
+/// ......
+/// %cmp11 = fcmp une float %tmp3, %tmp4
+/// %cmp12 = or i1 %cmp10, %cmp11 // parallel-or mode.
+/// br i1 %cmp12, label %if.then, label %ifend
+///
+/// if.end:
+/// ......
+///
+/// if.then:
+/// ......
+/// br label %if.end;
+///
+/// Current implementation handles two cases.
+/// Case 1: \param BB is on the else-path.
+///
+/// BB1
+/// / |
+/// BB2 |
+/// / \ |
+/// BB3 \ | where, BB1, BB2 contain conditional branches.
+/// \ | / BB3 contains unconditional branch.
+/// \ | / BB4 corresponds to \param BB which is also the merge.
+/// BB => BB4
+///
+///
+/// Corresponding source code:
+///
+/// if (a == b && c == d)
+/// statement; // BB3
+///
+/// Case 2: \param BB BB is on the then-path.
+///
+/// BB1
+/// / |
+/// | BB2
+/// \ / | where BB1, BB2 contain conditional branches.
+/// BB => BB3 | BB3 contains unconditiona branch and corresponds
+/// \ / to \param BB. BB4 is the merge.
+/// BB4
+///
+/// Corresponding source code:
+///
+/// if (a == b || c == d)
+/// statement; // BB3
+///
+/// In both cases, \param BB is the common successor of conditional branches.
+/// In Case 1, \param BB (BB4) has an unconditional branch (BB3) as
+/// its predecessor. In Case 2, \param BB (BB3) only has conditional branches
+/// as its predecessors.
+///
+bool SimplifyCFGOpt::SimplifyParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder,
+ Pass *P) {
+ PHINode *PHI = dyn_cast<PHINode>(BB->begin());
+ if (PHI)
+ return false; // For simplicity, avoid cases containing PHI nodes.
+
+ BasicBlock *LastCondBlock = NULL;
+ BasicBlock *FirstCondBlock = NULL;
+ BasicBlock *UnCondBlock = NULL;
+ int Idx = -1;
+
+ // Check predecessors of \param BB.
+ SmallPtrSet<BasicBlock*, 16> Preds(pred_begin(BB), pred_end(BB));
+ for (SmallPtrSetIterator<BasicBlock*> PI = Preds.begin(), PE = Preds.end();
+ PI != PE; ++PI) {
+ BasicBlock *Pred = *PI;
+ BranchInst *PBI = dyn_cast<BranchInst>(Pred->getTerminator());
+
+ // All predecessors should terminate with a branch.
+ if (!PBI)
+ return false;
+
+ BasicBlock *PP = Pred->getSinglePredecessor();
+
+ if (PBI->isUnconditional()) {
+ // Case 1: Pred (BB3) is an unconditional block, it should
+ // have a single predecessor (BB2) that is also a predecessor
+ // of \param BB (BB4) and should not have address-taken.
+ // There should exist only one such unconditional
+ // branch among the predecessors.
+ if (UnCondBlock || !PP || (Preds.count(PP) == 0) ||
+ Pred->hasAddressTaken())
+ return false;
+
+ UnCondBlock = Pred;
+ continue;
+ }
+
+ // Only conditional branches are allowed beyond this point.
+ assert(PBI->isConditional());
+
+ // Condition's unique use should be the branch instruction.
+ Value *PC = PBI->getCondition();
+ if (!PC || !PC->hasOneUse())
+ return false;
+
+ if (PP && Preds.count(PP)) {
+ // These are internal condition blocks to be merged from, e.g.,
+ // BB2 in both cases.
+ // Should not be address-taken.
+ if (Pred->hasAddressTaken())
+ return false;
+
+ // Instructions in the internal condition blocks should be safe
+ // to hoist up.
+ for (BasicBlock::iterator BI = Pred->begin(), BE = PBI; BI != BE;) {
+ Instruction *CI = BI++;
+ if (isa<PHINode>(CI) ||
+ !isSafeToSpeculativelyExecute(CI))
+ return false;
+ }
+ } else {
+ // This is the condition block to be merged into, e.g. BB1 in
+ // both cases.
+ if (FirstCondBlock)
+ return false;
+ FirstCondBlock = Pred;
+ }
+
+ // Find whether BB is uniformly on the true (or false) path
+ // for all of its predecessors.
+ BasicBlock *PS1 = PBI->getSuccessor(0);
+ BasicBlock *PS2 = PBI->getSuccessor(1);
+ BasicBlock *PS = (PS1 == BB) ? PS2 : PS1;
+ int CIdx = (PS1 == BB) ? 0 : 1;
+
+ if (Idx == -1)
+ Idx = CIdx;
+ else if (CIdx != Idx)
+ return false;
+
+ // PS is the successor which is not BB. Check successors to identify
+ // the last conditional branch.
+ if (Preds.count(PS) == 0) {
+ // Case 2.
+ // BB must have an unique successor.
+ TerminatorInst *TBB = BB->getTerminator();
+ if (TBB->getNumSuccessors() != 1)
+ return false;
+
+ BasicBlock *SBB = TBB->getSuccessor(0);
+ PHI = dyn_cast<PHINode>(SBB->begin());
+ if (PHI)
+ return false;
+
+ // PS (BB4) should be BB's successor.
+ if (SBB != PS)
+ return false;
+ LastCondBlock = Pred;
+ } else {
+ BranchInst *BPS = dyn_cast<BranchInst>(PS->getTerminator());
+ if (BPS && BPS->isUnconditional()) {
+ // Case 1: PS(BB3) should be an unconditional branch.
+ LastCondBlock = Pred;
+ }
+ }
+ }
+
+ if (!FirstCondBlock || !LastCondBlock || (FirstCondBlock == LastCondBlock))
+ return false;
+
+ // Do the transformation.
+ BasicBlock *CB;
+ bool Iteration = true;
+ BasicBlock::iterator ItOld = Builder.GetInsertPoint();
+ BranchInst *PBI = dyn_cast<BranchInst>(FirstCondBlock->getTerminator());
+ Value *PC = PBI->getCondition();
+ do {
+ CB = PBI->getSuccessor(1 - Idx);
+ // Delete the conditional branch.
+ FirstCondBlock->getInstList().pop_back();
+ FirstCondBlock->getInstList().splice(FirstCondBlock->end(), CB->getInstList());
+ PBI = cast<BranchInst>(FirstCondBlock->getTerminator());
+ Value *CC = PBI->getCondition();
+ // Merge conditions.
+ Builder.SetInsertPoint(PBI);
+ Value *NC;
+ if (Idx == 0)
+ // Case 2, use parallel or.
+ NC = Builder.CreateOr(PC, CC);
+ else
+ // Case 1, use parallel and.
+ NC = Builder.CreateAnd(PC, CC);
+
+ PBI->replaceUsesOfWith(CC, NC);
+ PC = NC;
+ if (CB == LastCondBlock)
+ Iteration = false;
+ // Remove internal conditional branches.
+ CB->dropAllReferences();
+ // make CB unreachable and let downstream to delete the block.
+ new UnreachableInst(CB->getContext(), CB);
+ } while (Iteration);
+
+ Builder.SetInsertPoint(ItOld);
+ DEBUG(dbgs() << "Use parallel and/or in:\n" << *FirstCondBlock);
+ return true;
+}
+
+/// Compare blocks from two if-regions, where \param Head1 is the entry of the
+/// 1st if-region. \param Head2 is the entry of the 2nd if-region. \param
+/// Block1 is a block in the 1st if-region to compare. \param Block2 is a block
+// in the 2nd if-region to compare. \returns true if \param Block1 and \param
+/// Block2 have identical instructions and do not have memory reference alias
+/// with \param Head2.
+///
+bool SimplifyCFGOpt::CompareIfRegionBlock(BasicBlock *Head1, BasicBlock *Head2,
+ BasicBlock *Block1, BasicBlock *Block2) {
+ TerminatorInst *PTI2 = Head2->getTerminator();
+ Instruction *PBI2 = Head2->begin();
+
+ bool eq1 = (Block1 == Head1);
+ bool eq2 = (Block2 == Head2);
+ if (eq1 || eq2) {
+ // An empty then-path or else-path.
+ return (eq1 == eq2);
+ }
+
+ // Check whether instructions in Block1 and Block2 are identical
+ // and do not alias with instructions in Head2.
+ BasicBlock::iterator iter1 = Block1->begin();
+ BasicBlock::iterator end1 = Block1->getTerminator();
+ BasicBlock::iterator iter2 = Block2->begin();
+ BasicBlock::iterator end2 = Block2->getTerminator();
+
+ while (1) {
+ if (iter1 == end1) {
+ if (iter2 != end2)
+ return false;
+ break;
+ }
+
+ if (!iter1->isIdenticalTo(iter2))
+ return false;
+
+ // Illegal to remove instructions with side effects except
+ // non-volatile stores.
+ if (iter1->mayHaveSideEffects()) {
+ Instruction *CurI = &*iter1;
+ StoreInst *SI = dyn_cast<StoreInst>(CurI);
+ if (!SI || SI->isVolatile())
+ return false;
+ }
+
+ // For simplicity and speed, data dependency check can be
+ // avoided if read from memory doesn't exist.
+ if (iter1->mayReadFromMemory())
+ return false;
+
+ if (iter1->mayWriteToMemory()) {
+ for (BasicBlock::iterator BI = PBI2, BE = PTI2; BI != BE; ++BI) {
+ if (BI->mayReadFromMemory() || BI->mayWriteToMemory()) {
+ // Check alias with Head2.
+ if (!AA || AA->alias(iter1, BI))
+ return false;
+ }
+ }
+ }
+ ++iter1;
+ ++iter2;
+ }
+
+ return true;
+}
+
+/// Check whether \param BB is the merge block of a if-region. If yes, check
+/// whether there exists an adjacent if-region upstream, the two if-regions
+/// contain identical instuctions and can be legally merged. \returns true if
+/// the two if-regions are merged.
+///
+/// From:
+/// if (a)
+/// statement;
+/// if (b)
+/// statement;
+///
+/// To:
+/// if (a || b)
+/// statement;
+///
+bool SimplifyCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder,
+ Pass *P) {
+ BasicBlock *IfTrue2, *IfFalse2;
+ Value *IfCond2 = GetIfCondition(BB, IfTrue2, IfFalse2);
+ Instruction *CInst2 = dyn_cast_or_null<Instruction>(IfCond2);
+ if (!CInst2)
+ return false;
+
+ BasicBlock *SecondEntryBlock = CInst2->getParent();
+ if (SecondEntryBlock->hasAddressTaken())
+ return false;
+
+ BasicBlock *IfTrue1, *IfFalse1;
+ Value *IfCond1 = GetIfCondition(SecondEntryBlock, IfTrue1, IfFalse1);
+ Instruction *CInst1 = dyn_cast_or_null<Instruction>(IfCond1);
+ if (!CInst1)
+ return false;
+
+ BasicBlock *FirstEntryBlock = CInst1->getParent();
+
+ // Either then-path or else-path should be empty.
+ if ((IfTrue1 != FirstEntryBlock) && (IfFalse1 != FirstEntryBlock))
+ return false;
+ if ((IfTrue2 != SecondEntryBlock) && (IfFalse2 != SecondEntryBlock))
+ return false;
+
+ TerminatorInst *PTI2 = SecondEntryBlock->getTerminator();
+ Instruction *PBI2 = SecondEntryBlock->begin();
+
+ if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfTrue1, IfTrue2))
+ return false;
+
+ if (!CompareIfRegionBlock(FirstEntryBlock, SecondEntryBlock, IfFalse1, IfFalse2))
+ return false;
+
+ // Check whether \param SecondEntryBlock has side-effect and is safe to speculate.
+ for (BasicBlock::iterator BI = PBI2, BE = PTI2; BI != BE; ++BI) {
+ Instruction *CI = BI;
+ if (isa<PHINode>(CI) || CI->mayHaveSideEffects() ||
+ !isSafeToSpeculativelyExecute(CI))
+ return false;
+ }
+
+ // Merge \param SecondEntryBlock into \param FirstEntryBlock.
+ FirstEntryBlock->getInstList().pop_back();
+ FirstEntryBlock->getInstList().splice(FirstEntryBlock->end(), SecondEntryBlock->getInstList());
+ BranchInst *PBI = dyn_cast<BranchInst>(FirstEntryBlock->getTerminator());
+ Value *CC = PBI->getCondition();
+ BasicBlock::iterator ItOld = Builder.GetInsertPoint();
+ Builder.SetInsertPoint(PBI);
+ Value *NC = Builder.CreateOr(CInst1, CC);
+ PBI->replaceUsesOfWith(CC, NC);
+ Builder.SetInsertPoint(ItOld);
+
+ // Remove IfTrue1
+ if (IfTrue1 != FirstEntryBlock) {
+ IfTrue1->dropAllReferences();
+ IfTrue1->eraseFromParent();
+ }
+
+ // Remove IfFalse1
+ if (IfFalse1 != FirstEntryBlock) {
+ IfFalse1->dropAllReferences();
+ IfFalse1->eraseFromParent();
+ }
+
+ // Remove \param SecondEntryBlock
+ SecondEntryBlock->dropAllReferences();
+ SecondEntryBlock->eraseFromParent();
+ DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
+ return true;
+}
+
/// Check if passing a value to an instruction will cause undefined behavior.
static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
Constant *C = dyn_cast<Constant>(V);
@@ -4168,6 +4581,11 @@
return true;
IRBuilder<> Builder(BB);
+ // Whether to optimize conditional branches.
+ bool OptCB = (ParallelAndOr && AA && TTI.hasBranchDivergence());
+
+ if (OptCB && SimplifyParallelAndOr(BB, Builder))
+ return true;
// If there is a trivial two-entry PHI node in this basic block, and we can
// eliminate it, do so now.
@@ -4196,6 +4614,9 @@
if (SimplifyIndirectBr(IBI)) return true;
}
+ if (OptCB && MergeIfRegion(BB, Builder))
+ return true;
+
return Changed;
}
@@ -4205,6 +4626,6 @@
/// of the CFG. It returns true if a modification was made.
///
bool llvm::SimplifyCFG(BasicBlock *BB, const TargetTransformInfo &TTI,
- const DataLayout *TD) {
- return SimplifyCFGOpt(TTI, TD).run(BB);
+ const DataLayout *TD, AliasAnalysis *AA) {
+ return SimplifyCFGOpt(TTI, TD, AA).run(BB);
}