mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-22 02:33:06 +01:00
fdb7d69784
Otherwise e.g. the FoldTwoEntryPHINode() has to do a lot of legwork to re-deduce what is the dominant block (i.e. for which block is this branch the terminator).
550 lines
17 KiB
C++
550 lines
17 KiB
C++
//===- FlatternCFG.cpp - Code to perform CFG flattening -------------------===//
|
|
//
|
|
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
|
|
// See https://llvm.org/LICENSE.txt for license information.
|
|
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
//
|
|
// Reduce conditional branches in CFG.
|
|
//
|
|
//===----------------------------------------------------------------------===//
|
|
|
|
#include "llvm/ADT/SmallPtrSet.h"
|
|
#include "llvm/Analysis/AliasAnalysis.h"
|
|
#include "llvm/Transforms/Utils/Local.h"
|
|
#include "llvm/Analysis/ValueTracking.h"
|
|
#include "llvm/IR/BasicBlock.h"
|
|
#include "llvm/IR/IRBuilder.h"
|
|
#include "llvm/IR/InstrTypes.h"
|
|
#include "llvm/IR/Instruction.h"
|
|
#include "llvm/IR/Instructions.h"
|
|
#include "llvm/IR/Value.h"
|
|
#include "llvm/Support/Casting.h"
|
|
#include "llvm/Support/Debug.h"
|
|
#include "llvm/Support/raw_ostream.h"
|
|
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
|
|
#include <cassert>
|
|
|
|
using namespace llvm;
|
|
|
|
#define DEBUG_TYPE "flattencfg"
|
|
|
|
namespace {
|
|
|
|
class FlattenCFGOpt {
|
|
AliasAnalysis *AA;
|
|
|
|
/// Use parallel-and or parallel-or to generate conditions for
|
|
/// conditional branches.
|
|
bool FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder);
|
|
|
|
/// 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);
|
|
|
|
/// Compare a pair of blocks: \p Block1 and \p Block2, which
|
|
/// are from two if-regions, where \p Head2 is the entry block of the 2nd
|
|
/// if-region. \returns true if \p Block1 and \p Block2 contain identical
|
|
/// instructions, and have no memory reference alias with \p Head2.
|
|
/// This is used as a legality check for merging if-regions.
|
|
bool CompareIfRegionBlock(BasicBlock *Block1, BasicBlock *Block2,
|
|
BasicBlock *Head2);
|
|
|
|
public:
|
|
FlattenCFGOpt(AliasAnalysis *AA) : AA(AA) {}
|
|
|
|
bool run(BasicBlock *BB);
|
|
};
|
|
|
|
} // end anonymous namespace
|
|
|
|
/// 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 %cmp10, 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: BB is on the else-path.
|
|
///
|
|
/// BB1
|
|
/// / |
|
|
/// BB2 |
|
|
/// / \ |
|
|
/// BB3 \ | where, BB1, BB2 contain conditional branches.
|
|
/// \ | / BB3 contains unconditional branch.
|
|
/// \ | / BB4 corresponds to BB which is also the merge.
|
|
/// BB => BB4
|
|
///
|
|
///
|
|
/// Corresponding source code:
|
|
///
|
|
/// if (a == b && c == d)
|
|
/// statement; // BB3
|
|
///
|
|
/// Case 2: BB is on the then-path.
|
|
///
|
|
/// BB1
|
|
/// / |
|
|
/// | BB2
|
|
/// \ / | where BB1, BB2 contain conditional branches.
|
|
/// BB => BB3 | BB3 contains unconditiona branch and corresponds
|
|
/// \ / to BB. BB4 is the merge.
|
|
/// BB4
|
|
///
|
|
/// Corresponding source code:
|
|
///
|
|
/// if (a == b || c == d)
|
|
/// statement; // BB3
|
|
///
|
|
/// In both cases, BB is the common successor of conditional branches.
|
|
/// In Case 1, BB (BB4) has an unconditional branch (BB3) as
|
|
/// its predecessor. In Case 2, BB (BB3) only has conditional branches
|
|
/// as its predecessors.
|
|
bool FlattenCFGOpt::FlattenParallelAndOr(BasicBlock *BB, IRBuilder<> &Builder) {
|
|
PHINode *PHI = dyn_cast<PHINode>(BB->begin());
|
|
if (PHI)
|
|
return false; // For simplicity, avoid cases containing PHI nodes.
|
|
|
|
BasicBlock *LastCondBlock = nullptr;
|
|
BasicBlock *FirstCondBlock = nullptr;
|
|
BasicBlock *UnCondBlock = nullptr;
|
|
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->getIterator();
|
|
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.
|
|
LastCondBlock = Pred;
|
|
} else {
|
|
// Case 1
|
|
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;
|
|
|
|
Instruction *TBB = LastCondBlock->getTerminator();
|
|
BasicBlock *PS1 = TBB->getSuccessor(0);
|
|
BasicBlock *PS2 = TBB->getSuccessor(1);
|
|
BranchInst *PBI1 = dyn_cast<BranchInst>(PS1->getTerminator());
|
|
BranchInst *PBI2 = dyn_cast<BranchInst>(PS2->getTerminator());
|
|
|
|
// If PS1 does not jump into PS2, but PS2 jumps into PS1,
|
|
// attempt branch inversion.
|
|
if (!PBI1 || !PBI1->isUnconditional() ||
|
|
(PS1->getTerminator()->getSuccessor(0) != PS2)) {
|
|
// Check whether PS2 jumps into PS1.
|
|
if (!PBI2 || !PBI2->isUnconditional() ||
|
|
(PS2->getTerminator()->getSuccessor(0) != PS1))
|
|
return false;
|
|
|
|
// Do branch inversion.
|
|
BasicBlock *CurrBlock = LastCondBlock;
|
|
bool EverChanged = false;
|
|
for (; CurrBlock != FirstCondBlock;
|
|
CurrBlock = CurrBlock->getSinglePredecessor()) {
|
|
auto *BI = cast<BranchInst>(CurrBlock->getTerminator());
|
|
auto *CI = dyn_cast<CmpInst>(BI->getCondition());
|
|
if (!CI)
|
|
continue;
|
|
|
|
CmpInst::Predicate Predicate = CI->getPredicate();
|
|
// Canonicalize icmp_ne -> icmp_eq, fcmp_one -> fcmp_oeq
|
|
if ((Predicate == CmpInst::ICMP_NE) || (Predicate == CmpInst::FCMP_ONE)) {
|
|
CI->setPredicate(ICmpInst::getInversePredicate(Predicate));
|
|
BI->swapSuccessors();
|
|
EverChanged = true;
|
|
}
|
|
}
|
|
return EverChanged;
|
|
}
|
|
|
|
// PS1 must have a conditional branch.
|
|
if (!PBI1 || !PBI1->isUnconditional())
|
|
return false;
|
|
|
|
// PS2 should not contain PHI node.
|
|
PHI = dyn_cast<PHINode>(PS2->begin());
|
|
if (PHI)
|
|
return false;
|
|
|
|
// Do the transformation.
|
|
BasicBlock *CB;
|
|
BranchInst *PBI = cast<BranchInst>(FirstCondBlock->getTerminator());
|
|
bool Iteration = true;
|
|
IRBuilder<>::InsertPointGuard Guard(Builder);
|
|
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);
|
|
|
|
LLVM_DEBUG(dbgs() << "Use parallel and/or in:\n" << *FirstCondBlock);
|
|
return true;
|
|
}
|
|
|
|
/// Compare blocks from two if-regions, where \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
|
|
/// Block1 and Block2 have identical instructions and do not have
|
|
/// memory reference alias with Head2.
|
|
bool FlattenCFGOpt::CompareIfRegionBlock(BasicBlock *Block1, BasicBlock *Block2,
|
|
BasicBlock *Head2) {
|
|
Instruction *PTI2 = Head2->getTerminator();
|
|
Instruction *PBI2 = &Head2->front();
|
|
|
|
// 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()->getIterator();
|
|
BasicBlock::iterator iter2 = Block2->begin();
|
|
BasicBlock::iterator end2 = Block2->getTerminator()->getIterator();
|
|
|
|
while (true) {
|
|
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->isNoAlias(&*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 instructions 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;
|
|
///
|
|
///
|
|
/// And from:
|
|
/// if (a)
|
|
/// ;
|
|
/// else
|
|
/// statement;
|
|
/// if (b)
|
|
/// ;
|
|
/// else
|
|
/// statement;
|
|
///
|
|
/// To:
|
|
/// if (a && b)
|
|
/// ;
|
|
/// else
|
|
/// statement;
|
|
///
|
|
/// We always take the form of the first if-region. This means that if the
|
|
/// statement in the first if-region, is in the "then-path", while in the second
|
|
/// if-region it is in the "else-path", then we convert the second to the first
|
|
/// form, by inverting the condition and the branch successors. The same
|
|
/// approach goes for the opposite case.
|
|
bool FlattenCFGOpt::MergeIfRegion(BasicBlock *BB, IRBuilder<> &Builder) {
|
|
BasicBlock *IfTrue2, *IfFalse2;
|
|
BranchInst *DomBI2 = GetIfCondition(BB, IfTrue2, IfFalse2);
|
|
if (!DomBI2)
|
|
return false;
|
|
Instruction *CInst2 = dyn_cast<Instruction>(DomBI2->getCondition());
|
|
if (!CInst2)
|
|
return false;
|
|
|
|
BasicBlock *SecondEntryBlock = CInst2->getParent();
|
|
if (SecondEntryBlock->hasAddressTaken())
|
|
return false;
|
|
|
|
BasicBlock *IfTrue1, *IfFalse1;
|
|
BranchInst *DomBI1 = GetIfCondition(SecondEntryBlock, IfTrue1, IfFalse1);
|
|
if (!DomBI1)
|
|
return false;
|
|
Instruction *CInst1 = dyn_cast<Instruction>(DomBI1->getCondition());
|
|
if (!CInst1)
|
|
return false;
|
|
|
|
BasicBlock *FirstEntryBlock = CInst1->getParent();
|
|
|
|
// Either then-path or else-path should be empty.
|
|
bool InvertCond2 = false;
|
|
BinaryOperator::BinaryOps CombineOp;
|
|
if (IfFalse1 == FirstEntryBlock) {
|
|
// The else-path is empty, so we must use "or" operation to combine the
|
|
// conditions.
|
|
CombineOp = BinaryOperator::Or;
|
|
if (IfFalse2 != SecondEntryBlock) {
|
|
if (IfTrue2 != SecondEntryBlock)
|
|
return false;
|
|
|
|
InvertCond2 = true;
|
|
std::swap(IfTrue2, IfFalse2);
|
|
}
|
|
|
|
if (!CompareIfRegionBlock(IfTrue1, IfTrue2, SecondEntryBlock))
|
|
return false;
|
|
} else if (IfTrue1 == FirstEntryBlock) {
|
|
// The then-path is empty, so we must use "and" operation to combine the
|
|
// conditions.
|
|
CombineOp = BinaryOperator::And;
|
|
if (IfTrue2 != SecondEntryBlock) {
|
|
if (IfFalse2 != SecondEntryBlock)
|
|
return false;
|
|
|
|
InvertCond2 = true;
|
|
std::swap(IfTrue2, IfFalse2);
|
|
}
|
|
|
|
if (!CompareIfRegionBlock(IfFalse1, IfFalse2, SecondEntryBlock))
|
|
return false;
|
|
} else
|
|
return false;
|
|
|
|
Instruction *PTI2 = SecondEntryBlock->getTerminator();
|
|
Instruction *PBI2 = &SecondEntryBlock->front();
|
|
|
|
// 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 = cast<BranchInst>(FirstEntryBlock->getTerminator());
|
|
assert(PBI->getCondition() == CInst2);
|
|
BasicBlock *SaveInsertBB = Builder.GetInsertBlock();
|
|
BasicBlock::iterator SaveInsertPt = Builder.GetInsertPoint();
|
|
Builder.SetInsertPoint(PBI);
|
|
if (InvertCond2) {
|
|
// If this is a "cmp" instruction, only used for branching (and nowhere
|
|
// else), then we can simply invert the predicate.
|
|
auto Cmp2 = dyn_cast<CmpInst>(CInst2);
|
|
if (Cmp2 && Cmp2->hasOneUse())
|
|
Cmp2->setPredicate(Cmp2->getInversePredicate());
|
|
else
|
|
CInst2 = cast<Instruction>(Builder.CreateNot(CInst2));
|
|
PBI->swapSuccessors();
|
|
}
|
|
Value *NC = Builder.CreateBinOp(CombineOp, CInst1, CInst2);
|
|
PBI->replaceUsesOfWith(CInst2, NC);
|
|
Builder.SetInsertPoint(SaveInsertBB, SaveInsertPt);
|
|
|
|
// Handle PHI node to replace its predecessors to FirstEntryBlock.
|
|
for (BasicBlock *Succ : successors(PBI)) {
|
|
for (PHINode &Phi : Succ->phis()) {
|
|
for (unsigned i = 0, e = Phi.getNumIncomingValues(); i != e; ++i) {
|
|
if (Phi.getIncomingBlock(i) == SecondEntryBlock)
|
|
Phi.setIncomingBlock(i, FirstEntryBlock);
|
|
}
|
|
}
|
|
}
|
|
|
|
// 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();
|
|
LLVM_DEBUG(dbgs() << "If conditions merged into:\n" << *FirstEntryBlock);
|
|
return true;
|
|
}
|
|
|
|
bool FlattenCFGOpt::run(BasicBlock *BB) {
|
|
assert(BB && BB->getParent() && "Block not embedded in function!");
|
|
assert(BB->getTerminator() && "Degenerate basic block encountered!");
|
|
|
|
IRBuilder<> Builder(BB);
|
|
|
|
if (FlattenParallelAndOr(BB, Builder) || MergeIfRegion(BB, Builder))
|
|
return true;
|
|
return false;
|
|
}
|
|
|
|
/// FlattenCFG - This function is used to flatten a CFG. For
|
|
/// example, it uses parallel-and and parallel-or mode to collapse
|
|
/// if-conditions and merge if-regions with identical statements.
|
|
bool llvm::FlattenCFG(BasicBlock *BB, AAResults *AA) {
|
|
return FlattenCFGOpt(AA).run(BB);
|
|
}
|