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IRCE: generalize InductiveRangeCheck::computeSafeIterationSpace to

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work with a non-canonical induction variable.

This is currently a non-functional change because we only ever call
computeSafeIterationSpace on a canonical induction variable; but the
generalization will be useful in a later commit.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@230151 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sanjoy Das 2015-02-21 22:20:22 +00:00
parent c5e1132ac2
commit b9b88bd77b
1 changed files with 57 additions and 32 deletions
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89
lib/Transforms/Scalar/InductiveRangeCheckElimination.cpp
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@ -162,9 +162,11 @@ public:
/// branch to take the hot successor (see (1) above).
bool getPassingDirection() { return true; }
/// Computes a range for the induction variable in which the range check is
/// redundant and can be constant-folded away.
/// Computes a range for the induction variable (IndVar) in which the range
/// check is redundant and can be constant-folded away. The induction
/// variable is not required to be the canonical {0,+,1} induction variable.
Optional<Range> computeSafeIterationSpace(ScalarEvolution &SE,
const SCEVAddRecExpr *IndVar,
IRBuilder<> &B) const;
/// Create an inductive range check out of BI if possible, else return
@ -617,10 +619,7 @@ bool LoopConstrainer::recognizeLoop(LoopStructure &LoopStructureOut,
}
PHINode *CIV = OriginalLoop.getCanonicalInductionVariable();
if (!CIV) {
FailureReason = "no CIV";
return false;
}
assert(CIV && "precondition");
BasicBlock *Header = OriginalLoop.getHeader();
BasicBlock *Preheader = OriginalLoop.getLoopPreheader();
@ -1082,43 +1081,62 @@ bool LoopConstrainer::run() {
return true;
}
/// Computes and returns a range of values for the induction variable in which
/// the range check can be safely elided. If it cannot compute such a range,
/// returns None.
/// Computes and returns a range of values for the induction variable (IndVar)
/// in which the range check can be safely elided. If it cannot compute such a
/// range, returns None.
Optional<InductiveRangeCheck::Range>
InductiveRangeCheck::computeSafeIterationSpace(ScalarEvolution &SE,
IRBuilder<> &B) const {
// Currently we support inequalities of the form:
const SCEVAddRecExpr *IndVar,
IRBuilder<> &) const {
// IndVar is of the form "A + B * I" (where "I" is the canonical induction
// variable, that may or may not exist as a real llvm::Value in the loop) and
// this inductive range check is a range check on the "C + D * I" ("C" is
// getOffset() and "D" is getScale()). We rewrite the value being range
// checked to "M + N * IndVar" where "N" = "D * B^(-1)" and "M" = "C - NA".
// Currently we support this only for "B" = "D" = { 1 or -1 }, but the code
// can be generalized as needed.
//
// 0 <= Offset + 1 * CIV < L given L >= 0
// The actual inequalities we solve are of the form
//
// The inequality is satisfied by -Offset <= CIV < (L - Offset) [^1]. All
// additions and subtractions are twos-complement wrapping and comparisons are
// signed.
// 0 <= M + 1 * IndVar < L given L >= 0 (i.e. N == 1)
//
// The inequality is satisfied by -M <= IndVar < (L - M) [^1]. All additions
// and subtractions are twos-complement wrapping and comparisons are signed.
//
// Proof:
//
// If there exists CIV such that -Offset <= CIV < (L - Offset) then it
// follows that -Offset <= (-Offset + L) [== Eq. 1]. Since L >= 0, if
// (-Offset + L) sign-overflows then (-Offset + L) < (-Offset). Hence by
// [Eq. 1], (-Offset + L) could not have overflown.
// If there exists IndVar such that -M <= IndVar < (L - M) then it follows
// that -M <= (-M + L) [== Eq. 1]. Since L >= 0, if (-M + L) sign-overflows
// then (-M + L) < (-M). Hence by [Eq. 1], (-M + L) could not have
// overflown.
//
// This means CIV = t + (-Offset) for t in [0, L). Hence (CIV + Offset) =
// t. Hence 0 <= (CIV + Offset) < L
// This means IndVar = t + (-M) for t in [0, L). Hence (IndVar + M) = t.
// Hence 0 <= (IndVar + M) < L
// [^1]: Note that the solution does _not_ apply if L < 0; consider values
// Offset = 127, CIV = 126 and L = -2 in an i8 world.
// [^1]: Note that the solution does _not_ apply if L < 0; consider values M =
// 127, IndVar = 126 and L = -2 in an i8 world.
const SCEVConstant *ScaleC = dyn_cast<SCEVConstant>(getScale());
if (!(ScaleC && ScaleC->getValue()->getValue() == 1)) {
DEBUG(dbgs() << "irce: could not compute safe iteration space for:\n";
print(dbgs()));
if (!IndVar->isAffine())
return None;
}
const SCEV *Begin = SE.getNegativeSCEV(getOffset());
const SCEV *End = SE.getMinusSCEV(SE.getSCEV(getLength()), getOffset());
const SCEV *A = IndVar->getStart();
const SCEVConstant *B = dyn_cast<SCEVConstant>(IndVar->getStepRecurrence(SE));
if (!B)
return None;
const SCEV *C = getOffset();
const SCEVConstant *D = dyn_cast<SCEVConstant>(getScale());
if (D != B)
return None;
ConstantInt *ConstD = D->getValue();
if (!(ConstD->isMinusOne() || ConstD->isOne()))
return None;
const SCEV *M = SE.getMinusSCEV(C, A);
const SCEV *Begin = SE.getNegativeSCEV(M);
const SCEV *End = SE.getMinusSCEV(SE.getSCEV(getLength()), M);
return InductiveRangeCheck::Range(Begin, End);
}
@ -1160,6 +1178,13 @@ bool InductiveRangeCheckElimination::runOnLoop(Loop *L, LPPassManager &LPM) {
ScalarEvolution &SE = getAnalysis<ScalarEvolution>();
BranchProbabilityInfo &BPI = getAnalysis<BranchProbabilityInfo>();
PHINode *CIV = L->getCanonicalInductionVariable();
if (!CIV) {
DEBUG(dbgs() << "irce: loop has no canonical induction variable\n");
return false;
}
const SCEVAddRecExpr *IndVar = cast<SCEVAddRecExpr>(SE.getSCEV(CIV));
for (auto BBI : L->getBlocks())
if (BranchInst *TBI = dyn_cast<BranchInst>(BBI->getTerminator()))
if (InductiveRangeCheck *IRC =
@ -1183,7 +1208,7 @@ bool InductiveRangeCheckElimination::runOnLoop(Loop *L, LPPassManager &LPM) {
IRBuilder<> B(ExprInsertPt);
for (InductiveRangeCheck *IRC : RangeChecks) {
auto Result = IRC->computeSafeIterationSpace(SE, B);
auto Result = IRC->computeSafeIterationSpace(SE, IndVar, B);
if (Result.hasValue()) {
auto MaybeSafeIterRange =
IntersectRange(SE, SafeIterRange, Result.getValue(), B);