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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@78948 91177308-0d34-0410-b5e6-96231b3b80d8
885 lines
34 KiB
C++
885 lines
34 KiB
C++
//===-- LICM.cpp - Loop Invariant Code Motion Pass ------------------------===//
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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 performs loop invariant code motion, attempting to remove as much
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// code from the body of a loop as possible. It does this by either hoisting
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// code into the preheader block, or by sinking code to the exit blocks if it is
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// safe. This pass also promotes must-aliased memory locations in the loop to
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// live in registers, thus hoisting and sinking "invariant" loads and stores.
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//
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// This pass uses alias analysis for two purposes:
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//
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// 1. Moving loop invariant loads and calls out of loops. If we can determine
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// that a load or call inside of a loop never aliases anything stored to,
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// we can hoist it or sink it like any other instruction.
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// 2. Scalar Promotion of Memory - If there is a store instruction inside of
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// the loop, we try to move the store to happen AFTER the loop instead of
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// inside of the loop. This can only happen if a few conditions are true:
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// A. The pointer stored through is loop invariant
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// B. There are no stores or loads in the loop which _may_ alias the
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// pointer. There are no calls in the loop which mod/ref the pointer.
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// If these conditions are true, we can promote the loads and stores in the
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// loop of the pointer to use a temporary alloca'd variable. We then use
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// the mem2reg functionality to construct the appropriate SSA form for the
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// variable.
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//
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//===----------------------------------------------------------------------===//
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#define DEBUG_TYPE "licm"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Constants.h"
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#include "llvm/DerivedTypes.h"
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#include "llvm/Instructions.h"
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#include "llvm/LLVMContext.h"
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#include "llvm/Target/TargetData.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/AliasAnalysis.h"
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#include "llvm/Analysis/AliasSetTracker.h"
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#include "llvm/Analysis/Dominators.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Transforms/Utils/PromoteMemToReg.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Support/Compiler.h"
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#include "llvm/Support/CommandLine.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/ADT/Statistic.h"
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#include <algorithm>
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using namespace llvm;
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STATISTIC(NumSunk , "Number of instructions sunk out of loop");
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STATISTIC(NumHoisted , "Number of instructions hoisted out of loop");
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STATISTIC(NumMovedLoads, "Number of load insts hoisted or sunk");
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STATISTIC(NumMovedCalls, "Number of call insts hoisted or sunk");
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STATISTIC(NumPromoted , "Number of memory locations promoted to registers");
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static cl::opt<bool>
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DisablePromotion("disable-licm-promotion", cl::Hidden,
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cl::desc("Disable memory promotion in LICM pass"));
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// This feature is currently disabled by default because CodeGen is not yet
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// capable of rematerializing these constants in PIC mode, so it can lead to
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// degraded performance. Compile test/CodeGen/X86/remat-constant.ll with
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// -relocation-model=pic to see an example of this.
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static cl::opt<bool>
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EnableLICMConstantMotion("enable-licm-constant-variables", cl::Hidden,
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cl::desc("Enable hoisting/sinking of constant "
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"global variables"));
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namespace {
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struct VISIBILITY_HIDDEN LICM : public LoopPass {
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static char ID; // Pass identification, replacement for typeid
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LICM() : LoopPass(&ID) {}
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virtual bool runOnLoop(Loop *L, LPPassManager &LPM);
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/// This transformation requires natural loop information & requires that
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/// loop preheaders be inserted into the CFG...
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///
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virtual void getAnalysisUsage(AnalysisUsage &AU) const {
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AU.setPreservesCFG();
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AU.addRequiredID(LoopSimplifyID);
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AU.addRequired<LoopInfo>();
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AU.addRequired<DominatorTree>();
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AU.addRequired<DominanceFrontier>(); // For scalar promotion (mem2reg)
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AU.addRequired<AliasAnalysis>();
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AU.addPreserved<ScalarEvolution>();
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AU.addPreserved<DominanceFrontier>();
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}
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bool doFinalization() {
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// Free the values stored in the map
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for (std::map<Loop *, AliasSetTracker *>::iterator
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I = LoopToAliasMap.begin(), E = LoopToAliasMap.end(); I != E; ++I)
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delete I->second;
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LoopToAliasMap.clear();
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return false;
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}
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private:
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// Various analyses that we use...
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AliasAnalysis *AA; // Current AliasAnalysis information
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LoopInfo *LI; // Current LoopInfo
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DominatorTree *DT; // Dominator Tree for the current Loop...
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DominanceFrontier *DF; // Current Dominance Frontier
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// State that is updated as we process loops
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bool Changed; // Set to true when we change anything.
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BasicBlock *Preheader; // The preheader block of the current loop...
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Loop *CurLoop; // The current loop we are working on...
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AliasSetTracker *CurAST; // AliasSet information for the current loop...
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std::map<Loop *, AliasSetTracker *> LoopToAliasMap;
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/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
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void cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L);
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/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
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/// set.
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void deleteAnalysisValue(Value *V, Loop *L);
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/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
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/// dominated by the specified block, and that are in the current loop) in
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/// reverse depth first order w.r.t the DominatorTree. This allows us to
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/// visit uses before definitions, allowing us to sink a loop body in one
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/// pass without iteration.
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///
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void SinkRegion(DomTreeNode *N);
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/// HoistRegion - Walk the specified region of the CFG (defined by all
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/// blocks dominated by the specified block, and that are in the current
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/// loop) in depth first order w.r.t the DominatorTree. This allows us to
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/// visit definitions before uses, allowing us to hoist a loop body in one
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/// pass without iteration.
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///
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void HoistRegion(DomTreeNode *N);
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/// inSubLoop - Little predicate that returns true if the specified basic
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/// block is in a subloop of the current one, not the current one itself.
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///
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bool inSubLoop(BasicBlock *BB) {
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assert(CurLoop->contains(BB) && "Only valid if BB is IN the loop");
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for (Loop::iterator I = CurLoop->begin(), E = CurLoop->end(); I != E; ++I)
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if ((*I)->contains(BB))
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return true; // A subloop actually contains this block!
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return false;
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}
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/// isExitBlockDominatedByBlockInLoop - This method checks to see if the
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/// specified exit block of the loop is dominated by the specified block
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/// that is in the body of the loop. We use these constraints to
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/// dramatically limit the amount of the dominator tree that needs to be
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/// searched.
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bool isExitBlockDominatedByBlockInLoop(BasicBlock *ExitBlock,
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BasicBlock *BlockInLoop) const {
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// If the block in the loop is the loop header, it must be dominated!
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BasicBlock *LoopHeader = CurLoop->getHeader();
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if (BlockInLoop == LoopHeader)
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return true;
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DomTreeNode *BlockInLoopNode = DT->getNode(BlockInLoop);
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DomTreeNode *IDom = DT->getNode(ExitBlock);
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// Because the exit block is not in the loop, we know we have to get _at
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// least_ its immediate dominator.
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do {
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// Get next Immediate Dominator.
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IDom = IDom->getIDom();
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// If we have got to the header of the loop, then the instructions block
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// did not dominate the exit node, so we can't hoist it.
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if (IDom->getBlock() == LoopHeader)
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return false;
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} while (IDom != BlockInLoopNode);
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return true;
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}
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/// sink - When an instruction is found to only be used outside of the loop,
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/// this function moves it to the exit blocks and patches up SSA form as
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/// needed.
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///
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void sink(Instruction &I);
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/// hoist - When an instruction is found to only use loop invariant operands
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/// that is safe to hoist, this instruction is called to do the dirty work.
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///
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void hoist(Instruction &I);
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/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it
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/// is not a trapping instruction or if it is a trapping instruction and is
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/// guaranteed to execute.
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///
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bool isSafeToExecuteUnconditionally(Instruction &I);
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/// pointerInvalidatedByLoop - Return true if the body of this loop may
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/// store into the memory location pointed to by V.
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///
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bool pointerInvalidatedByLoop(Value *V, unsigned Size) {
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// Check to see if any of the basic blocks in CurLoop invalidate *V.
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return CurAST->getAliasSetForPointer(V, Size).isMod();
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}
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bool canSinkOrHoistInst(Instruction &I);
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bool isLoopInvariantInst(Instruction &I);
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bool isNotUsedInLoop(Instruction &I);
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/// PromoteValuesInLoop - Look at the stores in the loop and promote as many
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/// to scalars as we can.
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///
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void PromoteValuesInLoop();
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/// FindPromotableValuesInLoop - Check the current loop for stores to
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/// definite pointers, which are not loaded and stored through may aliases.
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/// If these are found, create an alloca for the value, add it to the
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/// PromotedValues list, and keep track of the mapping from value to
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/// alloca...
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///
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void FindPromotableValuesInLoop(
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std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
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std::map<Value*, AllocaInst*> &Val2AlMap);
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};
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}
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char LICM::ID = 0;
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static RegisterPass<LICM> X("licm", "Loop Invariant Code Motion");
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Pass *llvm::createLICMPass() { return new LICM(); }
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/// Hoist expressions out of the specified loop. Note, alias info for inner
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/// loop is not preserved so it is not a good idea to run LICM multiple
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/// times on one loop.
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///
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bool LICM::runOnLoop(Loop *L, LPPassManager &LPM) {
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Changed = false;
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// Get our Loop and Alias Analysis information...
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LI = &getAnalysis<LoopInfo>();
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AA = &getAnalysis<AliasAnalysis>();
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DF = &getAnalysis<DominanceFrontier>();
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DT = &getAnalysis<DominatorTree>();
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CurAST = new AliasSetTracker(*AA);
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// Collect Alias info from subloops
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for (Loop::iterator LoopItr = L->begin(), LoopItrE = L->end();
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LoopItr != LoopItrE; ++LoopItr) {
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Loop *InnerL = *LoopItr;
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AliasSetTracker *InnerAST = LoopToAliasMap[InnerL];
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assert (InnerAST && "Where is my AST?");
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// What if InnerLoop was modified by other passes ?
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CurAST->add(*InnerAST);
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}
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CurLoop = L;
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// Get the preheader block to move instructions into...
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Preheader = L->getLoopPreheader();
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assert(Preheader&&"Preheader insertion pass guarantees we have a preheader!");
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// Loop over the body of this loop, looking for calls, invokes, and stores.
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// Because subloops have already been incorporated into AST, we skip blocks in
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// subloops.
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//
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for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
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I != E; ++I) {
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BasicBlock *BB = *I;
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if (LI->getLoopFor(BB) == L) // Ignore blocks in subloops...
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CurAST->add(*BB); // Incorporate the specified basic block
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}
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// We want to visit all of the instructions in this loop... that are not parts
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// of our subloops (they have already had their invariants hoisted out of
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// their loop, into this loop, so there is no need to process the BODIES of
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// the subloops).
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//
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// Traverse the body of the loop in depth first order on the dominator tree so
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// that we are guaranteed to see definitions before we see uses. This allows
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// us to sink instructions in one pass, without iteration. After sinking
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// instructions, we perform another pass to hoist them out of the loop.
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//
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SinkRegion(DT->getNode(L->getHeader()));
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HoistRegion(DT->getNode(L->getHeader()));
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// Now that all loop invariants have been removed from the loop, promote any
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// memory references to scalars that we can...
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if (!DisablePromotion)
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PromoteValuesInLoop();
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// Clear out loops state information for the next iteration
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CurLoop = 0;
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Preheader = 0;
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LoopToAliasMap[L] = CurAST;
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return Changed;
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}
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/// SinkRegion - Walk the specified region of the CFG (defined by all blocks
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/// dominated by the specified block, and that are in the current loop) in
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/// reverse depth first order w.r.t the DominatorTree. This allows us to visit
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/// uses before definitions, allowing us to sink a loop body in one pass without
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/// iteration.
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///
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void LICM::SinkRegion(DomTreeNode *N) {
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assert(N != 0 && "Null dominator tree node?");
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BasicBlock *BB = N->getBlock();
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// If this subregion is not in the top level loop at all, exit.
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if (!CurLoop->contains(BB)) return;
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// We are processing blocks in reverse dfo, so process children first...
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const std::vector<DomTreeNode*> &Children = N->getChildren();
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for (unsigned i = 0, e = Children.size(); i != e; ++i)
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SinkRegion(Children[i]);
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// Only need to process the contents of this block if it is not part of a
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// subloop (which would already have been processed).
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if (inSubLoop(BB)) return;
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for (BasicBlock::iterator II = BB->end(); II != BB->begin(); ) {
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Instruction &I = *--II;
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// Check to see if we can sink this instruction to the exit blocks
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// of the loop. We can do this if the all users of the instruction are
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// outside of the loop. In this case, it doesn't even matter if the
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// operands of the instruction are loop invariant.
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//
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if (isNotUsedInLoop(I) && canSinkOrHoistInst(I)) {
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++II;
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sink(I);
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}
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}
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}
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/// HoistRegion - Walk the specified region of the CFG (defined by all blocks
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/// dominated by the specified block, and that are in the current loop) in depth
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/// first order w.r.t the DominatorTree. This allows us to visit definitions
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/// before uses, allowing us to hoist a loop body in one pass without iteration.
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///
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void LICM::HoistRegion(DomTreeNode *N) {
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assert(N != 0 && "Null dominator tree node?");
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BasicBlock *BB = N->getBlock();
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// If this subregion is not in the top level loop at all, exit.
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if (!CurLoop->contains(BB)) return;
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// Only need to process the contents of this block if it is not part of a
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// subloop (which would already have been processed).
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if (!inSubLoop(BB))
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for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ) {
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Instruction &I = *II++;
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// Try hoisting the instruction out to the preheader. We can only do this
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// if all of the operands of the instruction are loop invariant and if it
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// is safe to hoist the instruction.
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//
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if (isLoopInvariantInst(I) && canSinkOrHoistInst(I) &&
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isSafeToExecuteUnconditionally(I))
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hoist(I);
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}
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const std::vector<DomTreeNode*> &Children = N->getChildren();
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for (unsigned i = 0, e = Children.size(); i != e; ++i)
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HoistRegion(Children[i]);
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}
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/// canSinkOrHoistInst - Return true if the hoister and sinker can handle this
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/// instruction.
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///
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bool LICM::canSinkOrHoistInst(Instruction &I) {
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// Loads have extra constraints we have to verify before we can hoist them.
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if (LoadInst *LI = dyn_cast<LoadInst>(&I)) {
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if (LI->isVolatile())
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return false; // Don't hoist volatile loads!
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// Loads from constant memory are always safe to move, even if they end up
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// in the same alias set as something that ends up being modified.
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if (EnableLICMConstantMotion &&
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AA->pointsToConstantMemory(LI->getOperand(0)))
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return true;
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// Don't hoist loads which have may-aliased stores in loop.
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unsigned Size = 0;
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if (LI->getType()->isSized())
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Size = AA->getTypeStoreSize(LI->getType());
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return !pointerInvalidatedByLoop(LI->getOperand(0), Size);
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} else if (CallInst *CI = dyn_cast<CallInst>(&I)) {
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// Handle obvious cases efficiently.
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AliasAnalysis::ModRefBehavior Behavior = AA->getModRefBehavior(CI);
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if (Behavior == AliasAnalysis::DoesNotAccessMemory)
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return true;
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else if (Behavior == AliasAnalysis::OnlyReadsMemory) {
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// If this call only reads from memory and there are no writes to memory
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// in the loop, we can hoist or sink the call as appropriate.
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bool FoundMod = false;
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for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
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I != E; ++I) {
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AliasSet &AS = *I;
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if (!AS.isForwardingAliasSet() && AS.isMod()) {
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FoundMod = true;
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break;
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}
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}
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if (!FoundMod) return true;
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}
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// FIXME: This should use mod/ref information to see if we can hoist or sink
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// the call.
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return false;
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}
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// Otherwise these instructions are hoistable/sinkable
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return isa<BinaryOperator>(I) || isa<CastInst>(I) ||
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isa<SelectInst>(I) || isa<GetElementPtrInst>(I) || isa<CmpInst>(I) ||
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isa<InsertElementInst>(I) || isa<ExtractElementInst>(I) ||
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isa<ShuffleVectorInst>(I);
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}
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/// isNotUsedInLoop - Return true if the only users of this instruction are
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/// outside of the loop. If this is true, we can sink the instruction to the
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/// exit blocks of the loop.
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///
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bool LICM::isNotUsedInLoop(Instruction &I) {
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for (Value::use_iterator UI = I.use_begin(), E = I.use_end(); UI != E; ++UI) {
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Instruction *User = cast<Instruction>(*UI);
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if (PHINode *PN = dyn_cast<PHINode>(User)) {
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// PHI node uses occur in predecessor blocks!
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for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
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if (PN->getIncomingValue(i) == &I)
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if (CurLoop->contains(PN->getIncomingBlock(i)))
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return false;
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} else if (CurLoop->contains(User->getParent())) {
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return false;
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}
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}
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return true;
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}
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/// isLoopInvariantInst - Return true if all operands of this instruction are
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/// loop invariant. We also filter out non-hoistable instructions here just for
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/// efficiency.
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///
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bool LICM::isLoopInvariantInst(Instruction &I) {
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// The instruction is loop invariant if all of its operands are loop-invariant
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for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i)
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if (!CurLoop->isLoopInvariant(I.getOperand(i)))
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return false;
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// If we got this far, the instruction is loop invariant!
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return true;
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}
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|
/// sink - When an instruction is found to only be used outside of the loop,
|
|
/// this function moves it to the exit blocks and patches up SSA form as needed.
|
|
/// This method is guaranteed to remove the original instruction from its
|
|
/// position, and may either delete it or move it to outside of the loop.
|
|
///
|
|
void LICM::sink(Instruction &I) {
|
|
DOUT << "LICM sinking instruction: " << I;
|
|
|
|
SmallVector<BasicBlock*, 8> ExitBlocks;
|
|
CurLoop->getExitBlocks(ExitBlocks);
|
|
|
|
if (isa<LoadInst>(I)) ++NumMovedLoads;
|
|
else if (isa<CallInst>(I)) ++NumMovedCalls;
|
|
++NumSunk;
|
|
Changed = true;
|
|
|
|
LLVMContext &Context = I.getContext();
|
|
|
|
// The case where there is only a single exit node of this loop is common
|
|
// enough that we handle it as a special (more efficient) case. It is more
|
|
// efficient to handle because there are no PHI nodes that need to be placed.
|
|
if (ExitBlocks.size() == 1) {
|
|
if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[0], I.getParent())) {
|
|
// Instruction is not used, just delete it.
|
|
CurAST->deleteValue(&I);
|
|
if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
|
|
I.replaceAllUsesWith(UndefValue::get(I.getType()));
|
|
I.eraseFromParent();
|
|
} else {
|
|
// Move the instruction to the start of the exit block, after any PHI
|
|
// nodes in it.
|
|
I.removeFromParent();
|
|
|
|
BasicBlock::iterator InsertPt = ExitBlocks[0]->getFirstNonPHI();
|
|
ExitBlocks[0]->getInstList().insert(InsertPt, &I);
|
|
}
|
|
} else if (ExitBlocks.empty()) {
|
|
// The instruction is actually dead if there ARE NO exit blocks.
|
|
CurAST->deleteValue(&I);
|
|
if (!I.use_empty()) // If I has users in unreachable blocks, eliminate.
|
|
I.replaceAllUsesWith(UndefValue::get(I.getType()));
|
|
I.eraseFromParent();
|
|
} else {
|
|
// Otherwise, if we have multiple exits, use the PromoteMem2Reg function to
|
|
// do all of the hard work of inserting PHI nodes as necessary. We convert
|
|
// the value into a stack object to get it to do this.
|
|
|
|
// Firstly, we create a stack object to hold the value...
|
|
AllocaInst *AI = 0;
|
|
|
|
if (I.getType() != Type::getVoidTy(I.getContext())) {
|
|
AI = new AllocaInst(I.getType(), 0, I.getName(),
|
|
I.getParent()->getParent()->getEntryBlock().begin());
|
|
CurAST->add(AI);
|
|
}
|
|
|
|
// Secondly, insert load instructions for each use of the instruction
|
|
// outside of the loop.
|
|
while (!I.use_empty()) {
|
|
Instruction *U = cast<Instruction>(I.use_back());
|
|
|
|
// If the user is a PHI Node, we actually have to insert load instructions
|
|
// in all predecessor blocks, not in the PHI block itself!
|
|
if (PHINode *UPN = dyn_cast<PHINode>(U)) {
|
|
// Only insert into each predecessor once, so that we don't have
|
|
// different incoming values from the same block!
|
|
std::map<BasicBlock*, Value*> InsertedBlocks;
|
|
for (unsigned i = 0, e = UPN->getNumIncomingValues(); i != e; ++i)
|
|
if (UPN->getIncomingValue(i) == &I) {
|
|
BasicBlock *Pred = UPN->getIncomingBlock(i);
|
|
Value *&PredVal = InsertedBlocks[Pred];
|
|
if (!PredVal) {
|
|
// Insert a new load instruction right before the terminator in
|
|
// the predecessor block.
|
|
PredVal = new LoadInst(AI, "", Pred->getTerminator());
|
|
CurAST->add(cast<LoadInst>(PredVal));
|
|
}
|
|
|
|
UPN->setIncomingValue(i, PredVal);
|
|
}
|
|
|
|
} else {
|
|
LoadInst *L = new LoadInst(AI, "", U);
|
|
U->replaceUsesOfWith(&I, L);
|
|
CurAST->add(L);
|
|
}
|
|
}
|
|
|
|
// Thirdly, insert a copy of the instruction in each exit block of the loop
|
|
// that is dominated by the instruction, storing the result into the memory
|
|
// location. Be careful not to insert the instruction into any particular
|
|
// basic block more than once.
|
|
std::set<BasicBlock*> InsertedBlocks;
|
|
BasicBlock *InstOrigBB = I.getParent();
|
|
|
|
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
|
|
BasicBlock *ExitBlock = ExitBlocks[i];
|
|
|
|
if (isExitBlockDominatedByBlockInLoop(ExitBlock, InstOrigBB)) {
|
|
// If we haven't already processed this exit block, do so now.
|
|
if (InsertedBlocks.insert(ExitBlock).second) {
|
|
// Insert the code after the last PHI node...
|
|
BasicBlock::iterator InsertPt = ExitBlock->getFirstNonPHI();
|
|
|
|
// If this is the first exit block processed, just move the original
|
|
// instruction, otherwise clone the original instruction and insert
|
|
// the copy.
|
|
Instruction *New;
|
|
if (InsertedBlocks.size() == 1) {
|
|
I.removeFromParent();
|
|
ExitBlock->getInstList().insert(InsertPt, &I);
|
|
New = &I;
|
|
} else {
|
|
New = I.clone(Context);
|
|
CurAST->copyValue(&I, New);
|
|
if (!I.getName().empty())
|
|
New->setName(I.getName()+".le");
|
|
ExitBlock->getInstList().insert(InsertPt, New);
|
|
}
|
|
|
|
// Now that we have inserted the instruction, store it into the alloca
|
|
if (AI) new StoreInst(New, AI, InsertPt);
|
|
}
|
|
}
|
|
}
|
|
|
|
// If the instruction doesn't dominate any exit blocks, it must be dead.
|
|
if (InsertedBlocks.empty()) {
|
|
CurAST->deleteValue(&I);
|
|
I.eraseFromParent();
|
|
}
|
|
|
|
// Finally, promote the fine value to SSA form.
|
|
if (AI) {
|
|
std::vector<AllocaInst*> Allocas;
|
|
Allocas.push_back(AI);
|
|
PromoteMemToReg(Allocas, *DT, *DF, Context, CurAST);
|
|
}
|
|
}
|
|
}
|
|
|
|
/// hoist - When an instruction is found to only use loop invariant operands
|
|
/// that is safe to hoist, this instruction is called to do the dirty work.
|
|
///
|
|
void LICM::hoist(Instruction &I) {
|
|
DEBUG(errs() << "LICM hoisting to " << Preheader->getName() << ": " << I);
|
|
|
|
// Remove the instruction from its current basic block... but don't delete the
|
|
// instruction.
|
|
I.removeFromParent();
|
|
|
|
// Insert the new node in Preheader, before the terminator.
|
|
Preheader->getInstList().insert(Preheader->getTerminator(), &I);
|
|
|
|
if (isa<LoadInst>(I)) ++NumMovedLoads;
|
|
else if (isa<CallInst>(I)) ++NumMovedCalls;
|
|
++NumHoisted;
|
|
Changed = true;
|
|
}
|
|
|
|
/// isSafeToExecuteUnconditionally - Only sink or hoist an instruction if it is
|
|
/// not a trapping instruction or if it is a trapping instruction and is
|
|
/// guaranteed to execute.
|
|
///
|
|
bool LICM::isSafeToExecuteUnconditionally(Instruction &Inst) {
|
|
// If it is not a trapping instruction, it is always safe to hoist.
|
|
if (Inst.isSafeToSpeculativelyExecute())
|
|
return true;
|
|
|
|
// Otherwise we have to check to make sure that the instruction dominates all
|
|
// of the exit blocks. If it doesn't, then there is a path out of the loop
|
|
// which does not execute this instruction, so we can't hoist it.
|
|
|
|
// If the instruction is in the header block for the loop (which is very
|
|
// common), it is always guaranteed to dominate the exit blocks. Since this
|
|
// is a common case, and can save some work, check it now.
|
|
if (Inst.getParent() == CurLoop->getHeader())
|
|
return true;
|
|
|
|
// Get the exit blocks for the current loop.
|
|
SmallVector<BasicBlock*, 8> ExitBlocks;
|
|
CurLoop->getExitBlocks(ExitBlocks);
|
|
|
|
// For each exit block, get the DT node and walk up the DT until the
|
|
// instruction's basic block is found or we exit the loop.
|
|
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
|
|
if (!isExitBlockDominatedByBlockInLoop(ExitBlocks[i], Inst.getParent()))
|
|
return false;
|
|
|
|
return true;
|
|
}
|
|
|
|
|
|
/// PromoteValuesInLoop - Try to promote memory values to scalars by sinking
|
|
/// stores out of the loop and moving loads to before the loop. We do this by
|
|
/// looping over the stores in the loop, looking for stores to Must pointers
|
|
/// which are loop invariant. We promote these memory locations to use allocas
|
|
/// instead. These allocas can easily be raised to register values by the
|
|
/// PromoteMem2Reg functionality.
|
|
///
|
|
void LICM::PromoteValuesInLoop() {
|
|
// PromotedValues - List of values that are promoted out of the loop. Each
|
|
// value has an alloca instruction for it, and a canonical version of the
|
|
// pointer.
|
|
std::vector<std::pair<AllocaInst*, Value*> > PromotedValues;
|
|
std::map<Value*, AllocaInst*> ValueToAllocaMap; // Map of ptr to alloca
|
|
|
|
FindPromotableValuesInLoop(PromotedValues, ValueToAllocaMap);
|
|
if (ValueToAllocaMap.empty()) return; // If there are values to promote.
|
|
|
|
Changed = true;
|
|
NumPromoted += PromotedValues.size();
|
|
|
|
std::vector<Value*> PointerValueNumbers;
|
|
|
|
// Emit a copy from the value into the alloca'd value in the loop preheader
|
|
TerminatorInst *LoopPredInst = Preheader->getTerminator();
|
|
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
|
|
Value *Ptr = PromotedValues[i].second;
|
|
|
|
// If we are promoting a pointer value, update alias information for the
|
|
// inserted load.
|
|
Value *LoadValue = 0;
|
|
if (isa<PointerType>(cast<PointerType>(Ptr->getType())->getElementType())) {
|
|
// Locate a load or store through the pointer, and assign the same value
|
|
// to LI as we are loading or storing. Since we know that the value is
|
|
// stored in this loop, this will always succeed.
|
|
for (Value::use_iterator UI = Ptr->use_begin(), E = Ptr->use_end();
|
|
UI != E; ++UI)
|
|
if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
|
|
LoadValue = LI;
|
|
break;
|
|
} else if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
|
|
if (SI->getOperand(1) == Ptr) {
|
|
LoadValue = SI->getOperand(0);
|
|
break;
|
|
}
|
|
}
|
|
assert(LoadValue && "No store through the pointer found!");
|
|
PointerValueNumbers.push_back(LoadValue); // Remember this for later.
|
|
}
|
|
|
|
// Load from the memory we are promoting.
|
|
LoadInst *LI = new LoadInst(Ptr, Ptr->getName()+".promoted", LoopPredInst);
|
|
|
|
if (LoadValue) CurAST->copyValue(LoadValue, LI);
|
|
|
|
// Store into the temporary alloca.
|
|
new StoreInst(LI, PromotedValues[i].first, LoopPredInst);
|
|
}
|
|
|
|
// Scan the basic blocks in the loop, replacing uses of our pointers with
|
|
// uses of the allocas in question.
|
|
//
|
|
for (Loop::block_iterator I = CurLoop->block_begin(),
|
|
E = CurLoop->block_end(); I != E; ++I) {
|
|
BasicBlock *BB = *I;
|
|
// Rewrite all loads and stores in the block of the pointer...
|
|
for (BasicBlock::iterator II = BB->begin(), E = BB->end(); II != E; ++II) {
|
|
if (LoadInst *L = dyn_cast<LoadInst>(II)) {
|
|
std::map<Value*, AllocaInst*>::iterator
|
|
I = ValueToAllocaMap.find(L->getOperand(0));
|
|
if (I != ValueToAllocaMap.end())
|
|
L->setOperand(0, I->second); // Rewrite load instruction...
|
|
} else if (StoreInst *S = dyn_cast<StoreInst>(II)) {
|
|
std::map<Value*, AllocaInst*>::iterator
|
|
I = ValueToAllocaMap.find(S->getOperand(1));
|
|
if (I != ValueToAllocaMap.end())
|
|
S->setOperand(1, I->second); // Rewrite store instruction...
|
|
}
|
|
}
|
|
}
|
|
|
|
// Now that the body of the loop uses the allocas instead of the original
|
|
// memory locations, insert code to copy the alloca value back into the
|
|
// original memory location on all exits from the loop. Note that we only
|
|
// want to insert one copy of the code in each exit block, though the loop may
|
|
// exit to the same block more than once.
|
|
//
|
|
SmallPtrSet<BasicBlock*, 16> ProcessedBlocks;
|
|
|
|
SmallVector<BasicBlock*, 8> ExitBlocks;
|
|
CurLoop->getExitBlocks(ExitBlocks);
|
|
for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
|
|
if (!ProcessedBlocks.insert(ExitBlocks[i]))
|
|
continue;
|
|
|
|
// Copy all of the allocas into their memory locations.
|
|
BasicBlock::iterator BI = ExitBlocks[i]->getFirstNonPHI();
|
|
Instruction *InsertPos = BI;
|
|
unsigned PVN = 0;
|
|
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i) {
|
|
// Load from the alloca.
|
|
LoadInst *LI = new LoadInst(PromotedValues[i].first, "", InsertPos);
|
|
|
|
// If this is a pointer type, update alias info appropriately.
|
|
if (isa<PointerType>(LI->getType()))
|
|
CurAST->copyValue(PointerValueNumbers[PVN++], LI);
|
|
|
|
// Store into the memory we promoted.
|
|
new StoreInst(LI, PromotedValues[i].second, InsertPos);
|
|
}
|
|
}
|
|
|
|
// Now that we have done the deed, use the mem2reg functionality to promote
|
|
// all of the new allocas we just created into real SSA registers.
|
|
//
|
|
std::vector<AllocaInst*> PromotedAllocas;
|
|
PromotedAllocas.reserve(PromotedValues.size());
|
|
for (unsigned i = 0, e = PromotedValues.size(); i != e; ++i)
|
|
PromotedAllocas.push_back(PromotedValues[i].first);
|
|
PromoteMemToReg(PromotedAllocas, *DT, *DF, Preheader->getContext(), CurAST);
|
|
}
|
|
|
|
/// FindPromotableValuesInLoop - Check the current loop for stores to definite
|
|
/// pointers, which are not loaded and stored through may aliases and are safe
|
|
/// for promotion. If these are found, create an alloca for the value, add it
|
|
/// to the PromotedValues list, and keep track of the mapping from value to
|
|
/// alloca.
|
|
void LICM::FindPromotableValuesInLoop(
|
|
std::vector<std::pair<AllocaInst*, Value*> > &PromotedValues,
|
|
std::map<Value*, AllocaInst*> &ValueToAllocaMap) {
|
|
Instruction *FnStart = CurLoop->getHeader()->getParent()->begin()->begin();
|
|
|
|
// Loop over all of the alias sets in the tracker object.
|
|
for (AliasSetTracker::iterator I = CurAST->begin(), E = CurAST->end();
|
|
I != E; ++I) {
|
|
AliasSet &AS = *I;
|
|
// We can promote this alias set if it has a store, if it is a "Must" alias
|
|
// set, if the pointer is loop invariant, and if we are not eliminating any
|
|
// volatile loads or stores.
|
|
if (AS.isForwardingAliasSet() || !AS.isMod() || !AS.isMustAlias() ||
|
|
AS.isVolatile() || !CurLoop->isLoopInvariant(AS.begin()->getValue()))
|
|
continue;
|
|
|
|
assert(!AS.empty() &&
|
|
"Must alias set should have at least one pointer element in it!");
|
|
Value *V = AS.begin()->getValue();
|
|
|
|
// Check that all of the pointers in the alias set have the same type. We
|
|
// cannot (yet) promote a memory location that is loaded and stored in
|
|
// different sizes.
|
|
{
|
|
bool PointerOk = true;
|
|
for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
|
|
if (V->getType() != I->getValue()->getType()) {
|
|
PointerOk = false;
|
|
break;
|
|
}
|
|
if (!PointerOk)
|
|
continue;
|
|
}
|
|
|
|
// It isn't safe to promote a load/store from the loop if the load/store is
|
|
// conditional. For example, turning:
|
|
//
|
|
// for () { if (c) *P += 1; }
|
|
//
|
|
// into:
|
|
//
|
|
// tmp = *P; for () { if (c) tmp +=1; } *P = tmp;
|
|
//
|
|
// is not safe, because *P may only be valid to access if 'c' is true.
|
|
//
|
|
// It is safe to promote P if all uses are direct load/stores and if at
|
|
// least one is guaranteed to be executed.
|
|
bool GuaranteedToExecute = false;
|
|
bool InvalidInst = false;
|
|
for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
|
|
UI != UE; ++UI) {
|
|
// Ignore instructions not in this loop.
|
|
Instruction *Use = dyn_cast<Instruction>(*UI);
|
|
if (!Use || !CurLoop->contains(Use->getParent()))
|
|
continue;
|
|
|
|
if (!isa<LoadInst>(Use) && !isa<StoreInst>(Use)) {
|
|
InvalidInst = true;
|
|
break;
|
|
}
|
|
|
|
if (!GuaranteedToExecute)
|
|
GuaranteedToExecute = isSafeToExecuteUnconditionally(*Use);
|
|
}
|
|
|
|
// If there is an non-load/store instruction in the loop, we can't promote
|
|
// it. If there isn't a guaranteed-to-execute instruction, we can't
|
|
// promote.
|
|
if (InvalidInst || !GuaranteedToExecute)
|
|
continue;
|
|
|
|
const Type *Ty = cast<PointerType>(V->getType())->getElementType();
|
|
AllocaInst *AI = new AllocaInst(Ty, 0, V->getName()+".tmp", FnStart);
|
|
PromotedValues.push_back(std::make_pair(AI, V));
|
|
|
|
// Update the AST and alias analysis.
|
|
CurAST->copyValue(V, AI);
|
|
|
|
for (AliasSet::iterator I = AS.begin(), E = AS.end(); I != E; ++I)
|
|
ValueToAllocaMap.insert(std::make_pair(I->getValue(), AI));
|
|
|
|
DOUT << "LICM: Promoting value: " << *V << "\n";
|
|
}
|
|
}
|
|
|
|
/// cloneBasicBlockAnalysis - Simple Analysis hook. Clone alias set info.
|
|
void LICM::cloneBasicBlockAnalysis(BasicBlock *From, BasicBlock *To, Loop *L) {
|
|
AliasSetTracker *AST = LoopToAliasMap[L];
|
|
if (!AST)
|
|
return;
|
|
|
|
AST->copyValue(From, To);
|
|
}
|
|
|
|
/// deleteAnalysisValue - Simple Analysis hook. Delete value V from alias
|
|
/// set.
|
|
void LICM::deleteAnalysisValue(Value *V, Loop *L) {
|
|
AliasSetTracker *AST = LoopToAliasMap[L];
|
|
if (!AST)
|
|
return;
|
|
|
|
AST->deleteValue(V);
|
|
}
|