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This requires a number of steps. 1) Move value_use_iterator into the Value class as an implementation detail 2) Change it to actually be a *Use* iterator rather than a *User* iterator. 3) Add an adaptor which is a User iterator that always looks through the Use to the User. 4) Wrap these in Value::use_iterator and Value::user_iterator typedefs. 5) Add the range adaptors as Value::uses() and Value::users(). 6) Update *all* of the callers to correctly distinguish between whether they wanted a use_iterator (and to explicitly dig out the User when needed), or a user_iterator which makes the Use itself totally opaque. Because #6 requires churning essentially everything that walked the Use-Def chains, I went ahead and added all of the range adaptors and switched them to range-based loops where appropriate. Also because the renaming requires at least churning every line of code, it didn't make any sense to split these up into multiple commits -- all of which would touch all of the same lies of code. The result is still not quite optimal. The Value::use_iterator is a nice regular iterator, but Value::user_iterator is an iterator over User*s rather than over the User objects themselves. As a consequence, it fits a bit awkwardly into the range-based world and it has the weird extra-dereferencing 'operator->' that so many of our iterators have. I think this could be fixed by providing something which transforms a range of T&s into a range of T*s, but that *can* be separated into another patch, and it isn't yet 100% clear whether this is the right move. However, this change gets us most of the benefit and cleans up a substantial amount of code around Use and User. =] git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203364 91177308-0d34-0410-b5e6-96231b3b80d8
315 lines
11 KiB
C++
315 lines
11 KiB
C++
//===-- LCSSA.cpp - Convert loops into loop-closed SSA form ---------------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This pass transforms loops by placing phi nodes at the end of the loops for
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// all values that are live across the loop boundary. For example, it turns
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// the left into the right code:
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//
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// for (...) for (...)
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// if (c) if (c)
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// X1 = ... X1 = ...
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// else else
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// X2 = ... X2 = ...
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// X3 = phi(X1, X2) X3 = phi(X1, X2)
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// ... = X3 + 4 X4 = phi(X3)
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// ... = X4 + 4
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//
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// This is still valid LLVM; the extra phi nodes are purely redundant, and will
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// be trivially eliminated by InstCombine. The major benefit of this
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// transformation is that it makes many other loop optimizations, such as
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// LoopUnswitching, simpler.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "lcssa"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/STLExtras.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/LoopPass.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/PredIteratorCache.h"
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#include "llvm/Pass.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/SSAUpdater.h"
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using namespace llvm;
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STATISTIC(NumLCSSA, "Number of live out of a loop variables");
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/// Return true if the specified block is in the list.
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static bool isExitBlock(BasicBlock *BB,
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const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (ExitBlocks[i] == BB)
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return true;
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return false;
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}
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/// Given an instruction in the loop, check to see if it has any uses that are
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/// outside the current loop. If so, insert LCSSA PHI nodes and rewrite the
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/// uses.
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static bool processInstruction(Loop &L, Instruction &Inst, DominatorTree &DT,
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const SmallVectorImpl<BasicBlock *> &ExitBlocks,
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PredIteratorCache &PredCache) {
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SmallVector<Use *, 16> UsesToRewrite;
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BasicBlock *InstBB = Inst.getParent();
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for (Use &U : Inst.uses()) {
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Instruction *User = cast<Instruction>(U.getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(U);
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if (InstBB != UserBB && !L.contains(UserBB))
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UsesToRewrite.push_back(&U);
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}
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// If there are no uses outside the loop, exit with no change.
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if (UsesToRewrite.empty())
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return false;
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++NumLCSSA; // We are applying the transformation
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// Invoke instructions are special in that their result value is not available
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// along their unwind edge. The code below tests to see whether DomBB
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// dominates
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// the value, so adjust DomBB to the normal destination block, which is
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// effectively where the value is first usable.
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BasicBlock *DomBB = Inst.getParent();
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if (InvokeInst *Inv = dyn_cast<InvokeInst>(&Inst))
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DomBB = Inv->getNormalDest();
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DomTreeNode *DomNode = DT.getNode(DomBB);
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SmallVector<PHINode *, 16> AddedPHIs;
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SSAUpdater SSAUpdate;
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SSAUpdate.Initialize(Inst.getType(), Inst.getName());
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// Insert the LCSSA phi's into all of the exit blocks dominated by the
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// value, and add them to the Phi's map.
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for (SmallVectorImpl<BasicBlock *>::const_iterator BBI = ExitBlocks.begin(),
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BBE = ExitBlocks.end();
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BBI != BBE; ++BBI) {
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BasicBlock *ExitBB = *BBI;
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if (!DT.dominates(DomNode, DT.getNode(ExitBB)))
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continue;
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// If we already inserted something for this BB, don't reprocess it.
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if (SSAUpdate.HasValueForBlock(ExitBB))
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continue;
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PHINode *PN = PHINode::Create(Inst.getType(), PredCache.GetNumPreds(ExitBB),
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Inst.getName() + ".lcssa", ExitBB->begin());
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// Add inputs from inside the loop for this PHI.
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for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
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PN->addIncoming(&Inst, *PI);
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// If the exit block has a predecessor not within the loop, arrange for
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// the incoming value use corresponding to that predecessor to be
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// rewritten in terms of a different LCSSA PHI.
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if (!L.contains(*PI))
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UsesToRewrite.push_back(
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&PN->getOperandUse(PN->getOperandNumForIncomingValue(
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PN->getNumIncomingValues() - 1)));
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}
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AddedPHIs.push_back(PN);
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// Remember that this phi makes the value alive in this block.
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SSAUpdate.AddAvailableValue(ExitBB, PN);
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}
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// Rewrite all uses outside the loop in terms of the new PHIs we just
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// inserted.
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for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
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// If this use is in an exit block, rewrite to use the newly inserted PHI.
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// This is required for correctness because SSAUpdate doesn't handle uses in
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// the same block. It assumes the PHI we inserted is at the end of the
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// block.
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Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
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BasicBlock *UserBB = User->getParent();
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if (PHINode *PN = dyn_cast<PHINode>(User))
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UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);
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if (isa<PHINode>(UserBB->begin()) && isExitBlock(UserBB, ExitBlocks)) {
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// Tell the VHs that the uses changed. This updates SCEV's caches.
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if (UsesToRewrite[i]->get()->hasValueHandle())
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ValueHandleBase::ValueIsRAUWd(*UsesToRewrite[i], UserBB->begin());
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UsesToRewrite[i]->set(UserBB->begin());
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continue;
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}
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// Otherwise, do full PHI insertion.
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SSAUpdate.RewriteUse(*UsesToRewrite[i]);
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}
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// Remove PHI nodes that did not have any uses rewritten.
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for (unsigned i = 0, e = AddedPHIs.size(); i != e; ++i) {
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if (AddedPHIs[i]->use_empty())
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AddedPHIs[i]->eraseFromParent();
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}
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return true;
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}
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/// Return true if the specified block dominates at least
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/// one of the blocks in the specified list.
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static bool
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blockDominatesAnExit(BasicBlock *BB,
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DominatorTree &DT,
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const SmallVectorImpl<BasicBlock *> &ExitBlocks) {
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DomTreeNode *DomNode = DT.getNode(BB);
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for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
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if (DT.dominates(DomNode, DT.getNode(ExitBlocks[i])))
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return true;
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return false;
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}
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bool llvm::formLCSSA(Loop &L, DominatorTree &DT, ScalarEvolution *SE) {
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bool Changed = false;
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// Get the set of exiting blocks.
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SmallVector<BasicBlock *, 8> ExitBlocks;
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L.getExitBlocks(ExitBlocks);
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if (ExitBlocks.empty())
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return false;
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PredIteratorCache PredCache;
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// Look at all the instructions in the loop, checking to see if they have uses
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// outside the loop. If so, rewrite those uses.
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for (Loop::block_iterator BBI = L.block_begin(), BBE = L.block_end();
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BBI != BBE; ++BBI) {
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BasicBlock *BB = *BBI;
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// For large loops, avoid use-scanning by using dominance information: In
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// particular, if a block does not dominate any of the loop exits, then none
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// of the values defined in the block could be used outside the loop.
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if (!blockDominatesAnExit(BB, DT, ExitBlocks))
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continue;
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for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
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// Reject two common cases fast: instructions with no uses (like stores)
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// and instructions with one use that is in the same block as this.
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if (I->use_empty() ||
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(I->hasOneUse() && I->user_back()->getParent() == BB &&
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!isa<PHINode>(I->user_back())))
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continue;
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Changed |= processInstruction(L, *I, DT, ExitBlocks, PredCache);
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}
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}
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// If we modified the code, remove any caches about the loop from SCEV to
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// avoid dangling entries.
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// FIXME: This is a big hammer, can we clear the cache more selectively?
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if (SE && Changed)
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SE->forgetLoop(&L);
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assert(L.isLCSSAForm(DT));
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return Changed;
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}
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/// Process a loop nest depth first.
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bool llvm::formLCSSARecursively(Loop &L, DominatorTree &DT,
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ScalarEvolution *SE) {
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bool Changed = false;
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// Recurse depth-first through inner loops.
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for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
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Changed |= formLCSSARecursively(**LI, DT, SE);
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Changed |= formLCSSA(L, DT, SE);
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return Changed;
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}
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namespace {
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struct LCSSA : public FunctionPass {
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static char ID; // Pass identification, replacement for typeid
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LCSSA() : FunctionPass(ID) {
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initializeLCSSAPass(*PassRegistry::getPassRegistry());
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}
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// Cached analysis information for the current function.
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DominatorTree *DT;
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LoopInfo *LI;
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ScalarEvolution *SE;
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bool runOnFunction(Function &F) override;
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG. It maintains both of these,
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/// as well as the CFG. It also requires dominator information.
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.setPreservesCFG();
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AU.addRequired<DominatorTreeWrapperPass>();
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AU.addRequired<LoopInfo>();
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AU.addPreservedID(LoopSimplifyID);
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AU.addPreserved<AliasAnalysis>();
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AU.addPreserved<ScalarEvolution>();
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}
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private:
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bool processLoop(Loop &L);
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void verifyAnalysis() const override;
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};
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}
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char LCSSA::ID = 0;
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INITIALIZE_PASS_BEGIN(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
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INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
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INITIALIZE_PASS_DEPENDENCY(LoopInfo)
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INITIALIZE_PASS_END(LCSSA, "lcssa", "Loop-Closed SSA Form Pass", false, false)
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Pass *llvm::createLCSSAPass() { return new LCSSA(); }
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char &llvm::LCSSAID = LCSSA::ID;
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/// Process all loops in the function, inner-most out.
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bool LCSSA::runOnFunction(Function &F) {
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bool Changed = false;
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LI = &getAnalysis<LoopInfo>();
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DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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SE = getAnalysisIfAvailable<ScalarEvolution>();
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// Simplify each loop nest in the function.
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for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
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Changed |= formLCSSARecursively(**I, *DT, SE);
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return Changed;
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}
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static void verifyLoop(Loop &L, DominatorTree &DT) {
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// Recurse depth-first through inner loops.
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for (Loop::iterator LI = L.begin(), LE = L.end(); LI != LE; ++LI)
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verifyLoop(**LI, DT);
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// Check the special guarantees that LCSSA makes.
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//assert(L.isLCSSAForm(DT) && "LCSSA form not preserved!");
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}
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void LCSSA::verifyAnalysis() const {
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// Verify each loop nest in the function, assuming LI still points at that
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// function's loop info.
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for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
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verifyLoop(**I, *DT);
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}
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