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