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Teach ScalarEvolution to consider loop preheaders in the search for

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an if statement that guards a loop, to allow indvars to avoid smax
operations in more situations.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@56232 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman
2008-09-15 22:18:04 +00:00
parent 99500aeb7c
commit fd6edef87b
3 changed files with 272 additions and 8 deletions
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46
lib/Analysis/ScalarEvolution.cpp
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@ -1480,6 +1480,12 @@ namespace {
SCEVHandle HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L,
bool isSigned);
/// getPredecessorWithUniqueSuccessorForBB - Return a predecessor of BB
/// (which may not be an immediate predecessor) which has exactly one
/// successor from which BB is reachable, or null if no such block is
/// found.
BasicBlock* getPredecessorWithUniqueSuccessorForBB(BasicBlock *BB);
/// executesAtLeastOnce - Test whether entry to the loop is protected by
/// a conditional between LHS and RHS.
bool executesAtLeastOnce(const Loop *L, bool isSigned, SCEV *LHS, SCEV *RHS);
@ -1825,6 +1831,11 @@ SCEVHandle ScalarEvolutionsImpl::createSCEV(Value *V) {
case Instruction::Select:
// This could be a smax or umax that was lowered earlier.
// Try to recover it.
//
// FIXME: This doesn't recognize code like this:
// %t = icmp sgt i32 %n, -1
// %max = select i1 %t, i32 %n, i32 0
//
if (ICmpInst *ICI = dyn_cast<ICmpInst>(U->getOperand(0))) {
Value *LHS = ICI->getOperand(0);
Value *RHS = ICI->getOperand(1);
@ -2703,6 +2714,28 @@ SCEVHandle ScalarEvolutionsImpl::HowFarToNonZero(SCEV *V, const Loop *L) {
return UnknownValue;
}
/// getPredecessorWithUniqueSuccessorForBB - Return a predecessor of BB
/// (which may not be an immediate predecessor) which has exactly one
/// successor from which BB is reachable, or null if no such block is
/// found.
///
BasicBlock *
ScalarEvolutionsImpl::getPredecessorWithUniqueSuccessorForBB(BasicBlock *BB) {
// If the block has a unique predecessor, the predecessor must have
// no other successors from which BB is reachable.
if (BasicBlock *Pred = BB->getSinglePredecessor())
return Pred;
// A loop's header is defined to be a block that dominates the loop.
// If the loop has a preheader, it must be a block that has exactly
// one successor that can reach BB. This is slightly more strict
// than necessary, but works if critical edges are split.
if (Loop *L = LI.getLoopFor(BB))
return L->getLoopPreheader();
return 0;
}
/// executesAtLeastOnce - Test whether entry to the loop is protected by
/// a conditional between LHS and RHS.
bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
@ -2711,14 +2744,11 @@ bool ScalarEvolutionsImpl::executesAtLeastOnce(const Loop *L, bool isSigned,
BasicBlock *PreheaderDest = L->getHeader();
// Starting at the preheader, climb up the predecessor chain, as long as
// there are unique predecessors, looking for a conditional branch that
// protects the loop.
//
// This is a conservative apporoximation of a climb of the
// control-dependence predecessors.
for (; Preheader; PreheaderDest = Preheader,
Preheader = Preheader->getSinglePredecessor()) {
// there are predecessors that can be found that have unique successors
// leading to the original header.
for (; Preheader;
PreheaderDest = Preheader,
Preheader = getPredecessorWithUniqueSuccessorForBB(Preheader)) {
BranchInst *LoopEntryPredicate =
dyn_cast<BranchInst>(Preheader->getTerminator());