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Clarify comments on helpers used by LFTR [NFC]

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I'm slowly wrapping my head around this code, and am making comment improvements where I can.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@360968 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Philip Reames 2019-05-17 01:12:02 +00:00
parent 71efe3e09f
commit 3daa450af0
1 changed files with 15 additions and 16 deletions
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31
lib/Transforms/Scalar/IndVarSimplify.cpp
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@ -1993,23 +1993,17 @@ bool IndVarSimplify::simplifyAndExtend(Loop *L,
/// Return true if this loop's backedge taken count expression can be safely and
/// cheaply expanded into an instruction sequence that can be used by
/// linearFunctionTestReplace.
///
/// TODO: This fails for pointer-type loop counters with greater than one byte
/// strides, consequently preventing LFTR from running. For the purpose of LFTR
/// we could skip this check in the case that the LFTR loop counter (chosen by
/// FindLoopCounter) is also pointer type. Instead, we could directly convert
/// the loop test to an inequality test by checking the target data's alignment
/// of element types (given that the initial pointer value originates from or is
/// used by ABI constrained operation, as opposed to inttoptr/ptrtoint).
/// However, we don't yet have a strong motivation for converting loop tests
/// into inequality tests.
static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE,
SCEVExpander &Rewriter) {
const SCEV *BackedgeTakenCount = SE->getBackedgeTakenCount(L);
if (isa<SCEVCouldNotCompute>(BackedgeTakenCount) ||
BackedgeTakenCount->isZero())
if (isa<SCEVCouldNotCompute>(BackedgeTakenCount))
return false;
// Better to break the backedge
if (BackedgeTakenCount->isZero())
return false;
// Loops with multiple exits are not currently suported by lftr
if (!L->getExitingBlock())
return false;
@ -2023,7 +2017,9 @@ static bool canExpandBackedgeTakenCount(Loop *L, ScalarEvolution *SE,
return true;
}
/// Return the loop header phi IFF IncV adds a loop invariant value to the phi.
/// Given an Value which is hoped to be part of an add recurance in the given
/// loop, return the associated Phi node if so. Otherwise, return null. Note
/// that this is less general than SCEVs AddRec checking.
static PHINode *getLoopPhiForCounter(Value *IncV, Loop *L, DominatorTree *DT) {
Instruction *IncI = dyn_cast<Instruction>(IncV);
if (!IncI)
@ -2060,7 +2056,9 @@ static PHINode *getLoopPhiForCounter(Value *IncV, Loop *L, DominatorTree *DT) {
return nullptr;
}
/// Return the compare guarding the loop latch, or NULL for unrecognized tests.
/// Given a loop with one backedge and one exit, return the ICmpInst
/// controlling the sole loop exit. There is no guarantee that the exiting
/// block is also the latch.
static ICmpInst *getLoopTest(Loop *L) {
assert(L->getExitingBlock() && "expected loop exit");
@ -2264,8 +2262,9 @@ static PHINode *FindLoopCounter(Loop *L, const SCEV *BECount,
return BestPhi;
}
/// Help linearFunctionTestReplace by generating a value that holds the RHS of
/// the new loop test.
/// Insert an IR expression which computes the value held by the IV IndVar
/// (which must be an simple addrec w/unit stride) after the backedge of loop L
/// is taken IVCount times.
static Value *genLoopLimit(PHINode *IndVar, const SCEV *IVCount, Loop *L,
SCEVExpander &Rewriter, ScalarEvolution *SE) {
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(IndVar));