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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@697 91177308-0d34-0410-b5e6-96231b3b80d8
405 lines
16 KiB
C++
405 lines
16 KiB
C++
//===- InductionVars.cpp - Induction Variable Cannonicalization code --------=//
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//
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// This file implements induction variable cannonicalization of loops.
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//
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// Specifically, after this executes, the following is true:
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// - There is a single induction variable for each loop (at least loops that
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// used to contain at least one induction variable)
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// * This induction variable starts at 0 and steps by 1 per iteration
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// * This induction variable is represented by the first PHI node in the
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// Header block, allowing it to be found easily.
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// - All other preexisting induction variables are adjusted to operate in
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// terms of this primary induction variable
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// - Induction variables with a step size of 0 have been eliminated.
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//
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// This code assumes the following is true to perform its full job:
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// - The CFG has been simplified to not have multiple entrances into an
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// interval header. Interval headers should only have two predecessors,
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// one from inside of the loop and one from outside of the loop.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Optimizations/InductionVars.h"
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#include "llvm/ConstPoolVals.h"
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#include "llvm/Analysis/IntervalPartition.h"
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#include "llvm/Assembly/Writer.h"
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#include "llvm/Support/STLExtras.h"
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#include "llvm/SymbolTable.h"
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#include "llvm/iOther.h"
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#include <algorithm>
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#include "llvm/Analysis/LoopDepth.h"
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using namespace opt;
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// isLoopInvariant - Return true if the specified value/basic block source is
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// an interval invariant computation.
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//
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static bool isLoopInvariant(cfg::Interval *Int, Value *V) {
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assert(isa<ConstPoolVal>(V) || isa<Instruction>(V) || isa<MethodArgument>(V));
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if (!isa<Instruction>(V))
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return true; // Constants and arguments are always loop invariant
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BasicBlock *ValueBlock = cast<Instruction>(V)->getParent();
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assert(ValueBlock && "Instruction not embedded in basic block!");
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// For now, only consider values from outside of the interval, regardless of
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// whether the expression could be lifted out of the loop by some LICM.
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//
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// TODO: invoke LICM library if we find out it would be useful.
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//
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return !Int->contains(ValueBlock);
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}
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// isLinearInductionVariableH - Return isLIV if the expression V is a linear
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// expression defined in terms of loop invariant computations, and a single
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// instance of the PHI node PN. Return isLIC if the expression V is a loop
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// invariant computation. Return isNLIV if the expression is a negated linear
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// induction variable. Return isOther if it is neither.
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//
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// Currently allowed operators are: ADD, SUB, NEG
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// TODO: This should allow casts!
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//
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enum LIVType { isLIV, isLIC, isNLIV, isOther };
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//
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// neg - Negate the sign of a LIV expression.
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inline LIVType neg(LIVType T) {
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assert(T == isLIV || T == isNLIV && "Negate Only works on LIV expressions");
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return T == isLIV ? isNLIV : isLIV;
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}
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//
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static LIVType isLinearInductionVariableH(cfg::Interval *Int, Value *V,
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PHINode *PN) {
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if (V == PN) { return isLIV; } // PHI node references are (0+PHI)
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if (isLoopInvariant(Int, V)) return isLIC;
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// loop variant computations must be instructions!
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Instruction *I = cast<Instruction>(V);
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switch (I->getOpcode()) { // Handle each instruction seperately
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case Instruction::Add:
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case Instruction::Sub: {
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Value *SubV1 = cast<BinaryOperator>(I)->getOperand(0);
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Value *SubV2 = cast<BinaryOperator>(I)->getOperand(1);
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LIVType SubLIVType1 = isLinearInductionVariableH(Int, SubV1, PN);
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if (SubLIVType1 == isOther) return isOther; // Early bailout
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LIVType SubLIVType2 = isLinearInductionVariableH(Int, SubV2, PN);
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switch (SubLIVType2) {
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case isOther: return isOther; // Unknown subexpression type
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case isLIC: return SubLIVType1; // Constant offset, return type #1
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case isLIV:
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case isNLIV:
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// So now we know that we have a linear induction variable on the RHS of
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// the ADD or SUB instruction. SubLIVType1 cannot be isOther, so it is
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// either a Loop Invariant computation, or a LIV type.
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if (SubLIVType1 == isLIC) {
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// Loop invariant computation, we know this is a LIV then.
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return (I->getOpcode() == Instruction::Add) ?
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SubLIVType2 : neg(SubLIVType2);
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}
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// If the LHS is also a LIV Expression, we cannot add two LIVs together
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if (I->getOpcode() == Instruction::Add) return isOther;
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// We can only subtract two LIVs if they are the same type, which yields
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// a LIC, because the LIVs cancel each other out.
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return (SubLIVType1 == SubLIVType2) ? isLIC : isOther;
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}
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// NOT REACHED
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}
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default: // Any other instruction is not a LINEAR induction var
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return isOther;
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}
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}
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// isLinearInductionVariable - Return true if the specified expression is a
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// "linear induction variable", which is an expression involving a single
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// instance of the PHI node and a loop invariant value that is added or
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// subtracted to the PHI node. This is calculated by walking the SSA graph
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//
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static inline bool isLinearInductionVariable(cfg::Interval *Int, Value *V,
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PHINode *PN) {
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return isLinearInductionVariableH(Int, V, PN) == isLIV;
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}
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// isSimpleInductionVar - Return true iff the cannonical induction variable PN
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// has an initializer of the constant value 0, and has a step size of constant
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// 1.
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static inline bool isSimpleInductionVar(PHINode *PN) {
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assert(PN->getNumIncomingValues() == 2 && "Must have cannonical PHI node!");
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Value *Initializer = PN->getIncomingValue(0);
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if (!isa<ConstPoolVal>(Initializer)) return false;
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if (Initializer->getType()->isSigned()) { // Signed constant value...
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if (((ConstPoolSInt*)Initializer)->getValue() != 0) return false;
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} else if (Initializer->getType()->isUnsigned()) { // Unsigned constant value
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if (((ConstPoolUInt*)Initializer)->getValue() != 0) return false;
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} else {
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return false; // Not signed or unsigned? Must be FP type or something
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}
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Value *StepExpr = PN->getIncomingValue(1);
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if (!isa<Instruction>(StepExpr) ||
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cast<Instruction>(StepExpr)->getOpcode() != Instruction::Add)
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return false;
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BinaryOperator *I = cast<BinaryOperator>(StepExpr);
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assert(isa<PHINode>(I->getOperand(0)) &&
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"PHI node should be first operand of ADD instruction!");
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// Get the right hand side of the ADD node. See if it is a constant 1.
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Value *StepSize = I->getOperand(1);
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if (!isa<ConstPoolVal>(StepSize)) return false;
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if (StepSize->getType()->isSigned()) { // Signed constant value...
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if (((ConstPoolSInt*)StepSize)->getValue() != 1) return false;
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} else if (StepSize->getType()->isUnsigned()) { // Unsigned constant value
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if (((ConstPoolUInt*)StepSize)->getValue() != 1) return false;
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} else {
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return false; // Not signed or unsigned? Must be FP type or something
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}
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// At this point, we know the initializer is a constant value 0 and the step
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// size is a constant value 1. This is our simple induction variable!
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return true;
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}
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// InjectSimpleInductionVariable - Insert a cannonical induction variable into
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// the interval header Header. This assumes that the flow graph is in
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// simplified form (so we know that the header block has exactly 2 predecessors)
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//
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// TODO: This should inherit the largest type that is being used by the already
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// present induction variables (instead of always using uint)
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//
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static PHINode *InjectSimpleInductionVariable(cfg::Interval *Int) {
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string PHIName, AddName;
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BasicBlock *Header = Int->getHeaderNode();
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Method *M = Header->getParent();
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if (M->hasSymbolTable()) {
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// Only name the induction variable if the method isn't stripped.
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PHIName = M->getSymbolTable()->getUniqueName(Type::UIntTy, "ind_var");
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AddName = M->getSymbolTable()->getUniqueName(Type::UIntTy, "ind_var_next");
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}
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// Create the neccesary instructions...
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PHINode *PN = new PHINode(Type::UIntTy, PHIName);
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ConstPoolVal *One = ConstPoolUInt::get(Type::UIntTy, 1);
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ConstPoolVal *Zero = ConstPoolUInt::get(Type::UIntTy, 0);
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BinaryOperator *AddNode = BinaryOperator::create(Instruction::Add,
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PN, One, AddName);
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// Figure out which predecessors I have to play with... there should be
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// exactly two... one of which is a loop predecessor, and one of which is not.
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//
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BasicBlock::pred_iterator PI = Header->pred_begin();
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assert(PI != Header->pred_end() && "Header node should have 2 preds!");
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BasicBlock *Pred1 = *PI; ++PI;
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assert(PI != Header->pred_end() && "Header node should have 2 preds!");
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BasicBlock *Pred2 = *PI;
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assert(++PI == Header->pred_end() && "Header node should have 2 preds!");
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// Make Pred1 be the loop entrance predecessor, Pred2 be the Loop predecessor
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if (Int->contains(Pred1)) swap(Pred1, Pred2);
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assert(!Int->contains(Pred1) && "Pred1 should be loop entrance!");
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assert( Int->contains(Pred2) && "Pred2 should be looping edge!");
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// Link the instructions into the PHI node...
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PN->addIncoming(Zero, Pred1); // The initializer is first argument
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PN->addIncoming(AddNode, Pred2); // The step size is second PHI argument
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// Insert the PHI node into the Header of the loop. It shall be the first
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// instruction, because the "Simple" Induction Variable must be first in the
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// block.
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//
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BasicBlock::InstListType &IL = Header->getInstList();
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IL.push_front(PN);
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// Insert the Add instruction as the first (non-phi) instruction in the
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// header node's basic block.
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BasicBlock::iterator I = IL.begin();
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while (isa<PHINode>(*I)) ++I;
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IL.insert(I, AddNode);
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return PN;
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}
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// ProcessInterval - This function is invoked once for each interval in the
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// IntervalPartition of the program. It looks for auxilliary induction
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// variables in loops. If it finds one, it:
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// * Cannonicalizes the induction variable. This consists of:
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// A. Making the first element of the PHI node be the loop invariant
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// computation, and the second element be the linear induction portion.
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// B. Changing the first element of the linear induction portion of the PHI
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// node to be of the form ADD(PHI, <loop invariant expr>).
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// * Add the induction variable PHI to a list of induction variables found.
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//
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// After this, a list of cannonical induction variables is known. This list
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// is searched to see if there is an induction variable that counts from
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// constant 0 with a step size of constant 1. If there is not one, one is
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// injected into the loop. Thus a "simple" induction variable is always known
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//
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// One a simple induction variable is known, all other induction variables are
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// modified to refer to the "simple" induction variable.
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//
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static bool ProcessInterval(cfg::Interval *Int) {
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if (!Int->isLoop()) return false; // Not a loop? Ignore it!
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vector<PHINode *> InductionVars;
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BasicBlock *Header = Int->getHeaderNode();
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// Loop over all of the PHI nodes in the interval header...
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for (BasicBlock::iterator I = Header->begin(), E = Header->end();
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I != E && isa<PHINode>(*I); ++I) {
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PHINode *PN = cast<PHINode>(*I);
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if (PN->getNumIncomingValues() != 2) { // These should be eliminated by now.
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cerr << "Found interval header with more than 2 predecessors! Ignoring\n";
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return false; // Todo, make an assertion.
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}
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// For this to be an induction variable, one of the arguments must be a
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// loop invariant expression, and the other must be an expression involving
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// the PHI node, along with possible additions and subtractions of loop
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// invariant values.
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//
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BasicBlock *BB1 = PN->getIncomingBlock(0);
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Value *V1 = PN->getIncomingValue(0);
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BasicBlock *BB2 = PN->getIncomingBlock(1);
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Value *V2 = PN->getIncomingValue(1);
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// Figure out which computation is loop invariant...
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if (!isLoopInvariant(Int, V1)) {
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// V1 is *not* loop invariant. Check to see if V2 is:
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if (isLoopInvariant(Int, V2)) {
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// They *are* loop invariant. Exchange BB1/BB2 and V1/V2 so that
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// V1 is always the loop invariant computation.
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swap(V1, V2); swap(BB1, BB2);
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} else {
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// Neither value is loop invariant. Must not be an induction variable.
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// This case can happen if there is an unreachable loop in the CFG that
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// has two tail loops in it that was not split by the cleanup phase
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// before.
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continue;
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}
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}
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// At this point, we know that BB1/V1 are loop invariant. We don't know
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// anything about BB2/V2. Check now to see if V2 is a linear induction
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// variable.
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//
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cerr << "Found loop invariant computation: " << V1 << endl;
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if (!isLinearInductionVariable(Int, V2, PN))
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continue; // No, it is not a linear ind var, ignore the PHI node.
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cerr << "Found linear induction variable: " << V2;
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// TODO: Cannonicalize V2
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// Add this PHI node to the list of induction variables found...
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InductionVars.push_back(PN);
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}
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// No induction variables found?
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if (InductionVars.empty()) return false;
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// Search to see if there is already a "simple" induction variable.
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vector<PHINode*>::iterator It =
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find_if(InductionVars.begin(), InductionVars.end(), isSimpleInductionVar);
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PHINode *PrimaryIndVar;
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// A simple induction variable was not found, inject one now...
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if (It == InductionVars.end()) {
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PrimaryIndVar = InjectSimpleInductionVariable(Int);
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} else {
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// Move the PHI node for this induction variable to the start of the PHI
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// list in HeaderNode... we do not need to do this for the inserted case
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// because the inserted node will always be placed at the beginning of
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// HeaderNode.
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//
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PrimaryIndVar = *It;
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BasicBlock::iterator i =
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find(Header->begin(), Header->end(), PrimaryIndVar);
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assert(i != Header->end() &&
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"How could Primary IndVar not be in the header!?!!?");
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if (i != Header->begin())
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iter_swap(i, Header->begin());
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}
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// Now we know that there is a simple induction variable PrimaryIndVar.
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// Simplify all of the other induction variables to use this induction
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// variable as their counter, and destroy the PHI nodes that correspond to
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// the old indvars.
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//
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// TODO
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cerr << "Found Interval Header with indvars (primary indvar should be first "
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<< "phi): \n" << Header << "\nPrimaryIndVar: " << PrimaryIndVar;
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return false; // TODO: true;
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}
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// ProcessIntervalPartition - This function loops over the interval partition
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// processing each interval with ProcessInterval
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//
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static bool ProcessIntervalPartition(cfg::IntervalPartition &IP) {
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// This currently just prints out information about the interval structure
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// of the method...
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#if 0
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static unsigned N = 0;
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cerr << "\n***********Interval Partition #" << (++N) << "************\n\n";
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copy(IP.begin(), IP.end(), ostream_iterator<cfg::Interval*>(cerr, "\n"));
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cerr << "\n*********** PERFORMING WORK ************\n\n";
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#endif
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// Loop over all of the intervals in the partition and look for induction
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// variables in intervals that represent loops.
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//
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return reduce_apply(IP.begin(), IP.end(), bitwise_or<bool>(), false,
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ptr_fun(ProcessInterval));
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}
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// DoInductionVariableCannonicalize - Simplify induction variables in loops.
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// This function loops over an interval partition of a program, reducing it
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// until the graph is gone.
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//
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bool opt::DoInductionVariableCannonicalize(Method *M) {
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// TODO: REMOVE
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if (0) { // Print basic blocks with their depth
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LoopDepthCalculator LDC(M);
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for (Method::iterator I = M->begin(); I != M->end(); ++I) {
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cerr << "Basic Block Depth: " << LDC.getLoopDepth(*I) << *I;
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}
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}
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cfg::IntervalPartition *IP = new cfg::IntervalPartition(M);
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bool Changed = false;
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while (!IP->isDegeneratePartition()) {
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Changed |= ProcessIntervalPartition(*IP);
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// Calculate the reduced version of this graph until we get to an
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// irreducible graph or a degenerate graph...
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//
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cfg::IntervalPartition *NewIP = new cfg::IntervalPartition(*IP, false);
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if (NewIP->size() == IP->size()) {
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cerr << "IRREDUCIBLE GRAPH FOUND!!!\n";
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return Changed;
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}
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delete IP;
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IP = NewIP;
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}
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delete IP;
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return Changed;
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}
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