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[LoopCacheAnalysis]: Add support for negative stride

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LoopCacheAnalysis currently assumes the loop will be iterated over in
a forward direction. This patch addresses the issue by using the
absolute value of the stride when iterating backwards.

Note: this patch will treat negative and positive array access the
same, resulting in the same cost being calculated for single and
bi-directional access patterns. This should be improved in a
subsequent patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D73064
This commit is contained in:
Rachel Craik 2020-02-10 13:14:59 -05:00
parent e2027b37b2
commit e80d68f22a
2 changed files with 156 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

37
lib/Analysis/LoopCacheAnalysis.cpp
View File

@ -90,7 +90,11 @@ static bool isOneDimensionalArray(const SCEV &AccessFn, const SCEV &ElemSize,
if (!SE.isLoopInvariant(Start, &L) || !SE.isLoopInvariant(Step, &L))
return false;
return AR->getStepRecurrence(SE) == &ElemSize;
const SCEV *StepRec = AR->getStepRecurrence(SE);
if (StepRec && SE.isKnownNegative(StepRec))
StepRec = SE.getNegativeSCEV(StepRec);
return StepRec == &ElemSize;
}
/// Compute the trip count for the given loop \p L. Return the SCEV expression
@ -285,10 +289,13 @@ CacheCostTy IndexedReference::computeRefCost(const Loop &L,
const SCEV *Stride = SE.getMulExpr(Coeff, ElemSize);
const SCEV *CacheLineSize = SE.getConstant(Stride->getType(), CLS);
Type *WiderType = SE.getWiderType(Stride->getType(), TripCount->getType());
Stride = SE.getNoopOrSignExtend(Stride, WiderType);
if (SE.isKnownNegative(Stride))
Stride = SE.getNegativeSCEV(Stride);
Stride = SE.getNoopOrAnyExtend(Stride, WiderType);
TripCount = SE.getNoopOrAnyExtend(TripCount, WiderType);
const SCEV *Numerator = SE.getMulExpr(Stride, TripCount);
RefCost = SE.getUDivExpr(Numerator, CacheLineSize);
LLVM_DEBUG(dbgs().indent(4)
<< "Access is consecutive: RefCost=(TripCount*Stride)/CLS="
<< *RefCost << "\n");
@ -349,6 +356,19 @@ bool IndexedReference::delinearize(const LoopInfo &LI) {
return false;
}
// The array may be accessed in reverse, for example:
// for (i = N; i > 0; i--)
// A[i] = 0;
// In this case, reconstruct the access function using the absolute value
// of the step recurrence.
const SCEVAddRecExpr *AccessFnAR = dyn_cast<SCEVAddRecExpr>(AccessFn);
const SCEV *StepRec = AccessFnAR ? AccessFnAR->getStepRecurrence(SE) : nullptr;
if (StepRec && SE.isKnownNegative(StepRec))
AccessFn = SE.getAddRecExpr(AccessFnAR->getStart(),
SE.getNegativeSCEV(StepRec),
AccessFnAR->getLoop(),
AccessFnAR->getNoWrapFlags());
const SCEV *Div = SE.getUDivExactExpr(AccessFn, ElemSize);
Subscripts.push_back(Div);
Sizes.push_back(ElemSize);
@ -396,6 +416,7 @@ bool IndexedReference::isConsecutive(const Loop &L, unsigned CLS) const {
const SCEV *Stride = SE.getMulExpr(Coeff, ElemSize);
const SCEV *CacheLineSize = SE.getConstant(Stride->getType(), CLS);
Stride = SE.isKnownNegative(Stride) ? SE.getNegativeSCEV(Stride) : Stride;
return SE.isKnownPredicate(ICmpInst::ICMP_ULT, Stride, CacheLineSize);
}
@ -537,6 +558,18 @@ bool CacheCost::populateReferenceGroups(ReferenceGroupsTy &RefGroups) const {
dbgs().indent(2) << Representative << "\n";
});
// FIXME: Both positive and negative access functions will be placed
// into the same reference group, resulting in a bi-directional array
// access such as:
// for (i = N; i > 0; i--)
// A[i] = A[N - i];
// having the same cost calculation as a single dimention access pattern
// for (i = 0; i < N; i++)
// A[i] = A[i];
// when in actuality, depending on the array size, the first example
// should have a cost closer to 2x the second due to the two cache
// access per iteration from opposite ends of the array
Optional<bool> HasTemporalReuse =
R->hasTemporalReuse(Representative, *TRT, *InnerMostLoop, DI, AA);
Optional<bool> HasSpacialReuse =

122
test/Analysis/LoopCacheAnalysis/PowerPC/compute-cost.ll
View File

@ -31,5 +31,125 @@ for.body: ; preds = %for.cond
; Exit blocks
for.end: ; preds = %for.cond
ret void
}
; Check IndexedReference::computeRefCost can handle negative stride
; CHECK: Loop 'for.neg.cond' has cost = 64
define void @handle_to_ptr_neg_stride(%struct._Handleitem** %blocks) {
; Preheader:
entry:
br label %for.neg.cond
; Loop:
for.neg.cond: ; preds = %for.neg.body, %entry
%i.0 = phi i32 [ 1023, %entry ], [ %dec, %for.neg.body ]
%cmp = icmp sgt i32 %i.0, 0
br i1 %cmp, label %for.neg.body, label %for.neg.end
for.neg.body: ; preds = %for.neg.cond
%idxprom = zext i32 %i.0 to i64
%arrayidx = getelementptr inbounds %struct._Handleitem*, %struct._Handleitem** %blocks, i64 %idxprom
store %struct._Handleitem* null, %struct._Handleitem** %arrayidx, align 8
%dec = add nsw i32 %i.0, -1
br label %for.neg.cond
; Exit blocks
for.neg.end: ; preds = %for.neg.cond
ret void
}
; for (int i = 40960; i > 0; i--)
; B[i] = B[40960 - i];
; FIXME: Currently negative access functions are treated the same as positive
; access functions. When this is fixed this testcase should have a cost
; approximately 2x higher.
; CHECK: Loop 'for.cond2' has cost = 2560
define void @Test2(double* %B) {
entry:
br label %for.cond2
for.cond2: ; preds = %for.body, %entry
%i.0 = phi i32 [ 40960, %entry ], [ %dec, %for.body ]
%cmp = icmp sgt i32 %i.0, 0
br i1 %cmp, label %for.body, label %for.end
for.body: ; preds = %for.cond
%sub = sub nsw i32 40960, %i.0
%idxprom = sext i32 %sub to i64
%arrayidx = getelementptr inbounds double, double* %B, i64 %idxprom
%0 = load double, double* %arrayidx, align 8
%idxprom1 = sext i32 %i.0 to i64
%arrayidx2 = getelementptr inbounds double, double* %B, i64 %idxprom1
store double %0, double* %arrayidx2, align 8
%dec = add nsw i32 %i.0, -1
br label %for.cond2
for.end: ; preds = %for.cond
ret void
}
; for (i = 40960; i > 0; i--)
; C[i] = C[i];
; CHECK: Loop 'for.cond3' has cost = 2560
define void @Test3(double** %C) {
entry:
br label %for.cond3
for.cond3: ; preds = %for.body, %entry
%i.0 = phi i32 [ 40960, %entry ], [ %dec, %for.body ]
%cmp = icmp sgt i32 %i.0, 0
br i1 %cmp, label %for.body, label %for.end
for.body: ; preds = %for.cond
%idxprom = sext i32 %i.0 to i64
%arrayidx = getelementptr inbounds double*, double** %C, i64 %idxprom
%0 = load double*, double** %arrayidx, align 8
%idxprom1 = sext i32 %i.0 to i64
%arrayidx2 = getelementptr inbounds double*, double** %C, i64 %idxprom1
store double* %0, double** %arrayidx2, align 8
%dec = add nsw i32 %i.0, -1
br label %for.cond3
for.end: ; preds = %for.cond
ret void
}
; for (i = 0; i < 40960; i++)
; D[i] = D[i];
; CHECK: Loop 'for.cond4' has cost = 2560
define void @Test4(double** %D) {
entry:
br label %for.cond4
for.cond4: ; preds = %for.body, %entry
%i.0 = phi i32 [ 0, %entry ], [ %inc, %for.body ]
%cmp = icmp slt i32 %i.0, 40960
br i1 %cmp, label %for.body, label %for.end
for.body: ; preds = %for.cond
%idxprom = sext i32 %i.0 to i64
%arrayidx = getelementptr inbounds double*, double** %D, i64 %idxprom
%0 = load double*, double** %arrayidx, align 8
%idxprom1 = sext i32 %i.0 to i64
%arrayidx2 = getelementptr inbounds double*, double** %D, i64 %idxprom1
store double* %0, double** %arrayidx2, align 8
%inc = add nsw i32 %i.0, 1
br label %for.cond4
for.end: ; preds = %for.cond
ret void
}