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[DA] control compile-time spent by MIV tests 

Function exploreDirections() in DependenceAnalysis implements a recursive
algorithm for refining direction vectors. This algorithm has worst-case
complexity of O(3^(n+1)) where n is the number of common loop levels.
In this patch I'm adding a threshold to control the amount of time we
spend in doing MIV tests (which most of the time end up resulting in over
pessimistic direction vectors anyway).

Reviewed By: Meinersbur

Differential Revision: https://reviews.llvm.org/D107159
This commit is contained in:
Bardia Mahjour 2021-08-05 09:45:53 -04:00
parent 8d08a84745
commit 0e08891ec1
2 changed files with 104 additions and 0 deletions
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18
llvm/lib/Analysis/DependenceAnalysis.cpp
View File

@ -119,6 +119,11 @@ static cl::opt<bool> DisableDelinearizationChecks(
"dependence vectors for languages that allow the subscript of one "
"dimension to underflow or overflow into another dimension."));
static cl::opt<unsigned> MIVMaxLevelThreshold(
"da-miv-max-level-threshold", cl::init(7), cl::Hidden, cl::ZeroOrMore,
cl::desc("Maximum depth allowed for the recursive algorithm used to "
"explore MIV direction vectors."));
//===----------------------------------------------------------------------===//
// basics
@ -2602,6 +2607,19 @@ unsigned DependenceInfo::exploreDirections(unsigned Level, CoefficientInfo *A,
const SmallBitVector &Loops,
unsigned &DepthExpanded,
const SCEV *Delta) const {
// This algorithm has worst case complexity of O(3^n), where 'n' is the number
// of common loop levels. To avoid excessive compile-time, pessimize all the
// results and immediately return when the number of common levels is beyond
// the given threshold.
if (CommonLevels > MIVMaxLevelThreshold) {
LLVM_DEBUG(dbgs() << "Number of common levels exceeded the threshold. MIV "
"direction exploration is terminated.\n");
for (unsigned K = 1; K <= CommonLevels; ++K)
if (Loops[K])
Bound[K].DirSet = Dependence::DVEntry::ALL;
return 1;
}
if (Level > CommonLevels) {
// record result
LLVM_DEBUG(dbgs() << "\t[");

86
llvm/test/Analysis/DependenceAnalysis/MIVMaxLevelThreshold.ll Normal file
View File

@ -0,0 +1,86 @@
; RUN: opt < %s -aa-pipeline=default -passes='print<da>' -da-miv-max-level-threshold=2
;; Check to make sure when MIV tests reach a maximum depth level
;; threshold, the resulting dependence is conservatively correct.
;; for (int i = 0; i < n; i++)
;; for (int j = 0; j < m; j++)
;; for (int k = 0; k < o; k++)
;; A[i+j+k] = A[i-j+k] + B[i+j+k];
; CHECK: Src: %2 = load float, float* %arrayidx, align 4 --> Dst: %5 = load float, float* %arrayidx12, align 4
; CHECK-NEXT: da analyze - none!
; CHECK: Src: %2 = load float, float* %arrayidx, align 4 --> Dst: store float %add13, float* %arrayidx17, align 4
; CHECK-NEXT: da analyze - anti [* * *|<]!
; CHECK: Src: %5 = load float, float* %arrayidx12, align 4 --> Dst: store float %add13, float* %arrayidx17, align 4
; CHECK-NEXT: da analyze - none!
define void @foo(float* noalias %A, float* noalias %B, i32 signext %m, i32 signext %n, i32 signext %o) {
entry:
%cmp5 = icmp sgt i32 %n, 0
br i1 %cmp5, label %for.body.preheader, label %for.end23
for.body.preheader: ; preds = %entry
%wide.trip.count21 = zext i32 %n to i64
br label %for.body
for.body: ; preds = %for.body.preheader, %for.inc21
%indvars.iv19 = phi i64 [ 0, %for.body.preheader ], [ %indvars.iv.next20, %for.inc21 ]
%cmp23 = icmp sgt i32 %m, 0
br i1 %cmp23, label %for.body4.preheader, label %for.inc21
for.body4.preheader: ; preds = %for.body
%wide.trip.count17 = zext i32 %m to i64
br label %for.body4
for.body4: ; preds = %for.body4.preheader, %for.inc18
%indvars.iv12 = phi i64 [ 0, %for.body4.preheader ], [ %indvars.iv.next13, %for.inc18 ]
%cmp61 = icmp sgt i32 %o, 0
br i1 %cmp61, label %for.body8.preheader, label %for.inc18
for.body8.preheader: ; preds = %for.body4
%wide.trip.count = zext i32 %o to i64
br label %for.body8
for.body8: ; preds = %for.body8.preheader, %for.body8
%indvars.iv = phi i64 [ 0, %for.body8.preheader ], [ %indvars.iv.next, %for.body8 ]
%0 = sub nsw i64 %indvars.iv19, %indvars.iv12
%1 = add nsw i64 %0, %indvars.iv
%arrayidx = getelementptr inbounds float, float* %A, i64 %1
%2 = load float, float* %arrayidx, align 4
%3 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
%4 = add nuw nsw i64 %3, %indvars.iv
%arrayidx12 = getelementptr inbounds float, float* %B, i64 %4
%5 = load float, float* %arrayidx12, align 4
%add13 = fadd fast float %2, %5
%6 = add nuw nsw i64 %indvars.iv19, %indvars.iv12
%7 = add nuw nsw i64 %6, %indvars.iv
%arrayidx17 = getelementptr inbounds float, float* %A, i64 %7
store float %add13, float* %arrayidx17, align 4
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%exitcond = icmp ne i64 %indvars.iv.next, %wide.trip.count
br i1 %exitcond, label %for.body8, label %for.inc18.loopexit
for.inc18.loopexit: ; preds = %for.body8
br label %for.inc18
for.inc18: ; preds = %for.inc18.loopexit, %for.body4
%indvars.iv.next13 = add nuw nsw i64 %indvars.iv12, 1
%exitcond18 = icmp ne i64 %indvars.iv.next13, %wide.trip.count17
br i1 %exitcond18, label %for.body4, label %for.inc21.loopexit
for.inc21.loopexit: ; preds = %for.inc18
br label %for.inc21
for.inc21: ; preds = %for.inc21.loopexit, %for.body
%indvars.iv.next20 = add nuw nsw i64 %indvars.iv19, 1
%exitcond22 = icmp ne i64 %indvars.iv.next20, %wide.trip.count21
br i1 %exitcond22, label %for.body, label %for.end23.loopexit
for.end23.loopexit: ; preds = %for.inc21
br label %for.end23
for.end23: ; preds = %for.end23.loopexit, %entry
ret void
}