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split delinearization pass in 3 steps

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To compute the dimensions of the array in a unique way, we split the
delinearization analysis in three steps:

- find parametric terms in all memory access functions
- compute the array dimensions from the set of terms
- compute the delinearized access functions for each dimension

The first step is executed on all the memory access functions such that we
gather all the patterns in which an array is accessed. The second step reduces
all this information in a unique description of the sizes of the array. The
third step is delinearizing each memory access function following the common
description of the shape of the array computed in step 2.

This rewrite of the delinearization pass also solves a problem we had with the
previous implementation: because the previous algorithm was by induction on the
structure of the SCEV, it would not correctly recognize the shape of the array
when the memory access was not following the nesting of the loops: for example,
see polly/test/ScopInfo/multidim_only_ivs_3d_reverse.ll

; void foo(long n, long m, long o, double A[n][m][o]) {
;
;   for (long i = 0; i < n; i++)
;     for (long j = 0; j < m; j++)
;       for (long k = 0; k < o; k++)
;         A[i][k][j] = 1.0;

Starting with this patch we no longer delinearize access functions that do not
contain parameters, for example in test/Analysis/DependenceAnalysis/GCD.ll

;;  for (long int i = 0; i < 100; i++)
;;    for (long int j = 0; j < 100; j++) {
;;      A[2*i - 4*j] = i;
;;      *B++ = A[6*i + 8*j];

these accesses will not be delinearized as the upper bound of the loops are
constants, and their access functions do not contain SCEVUnknown parameters.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@208232 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Sebastian Pop
2014-05-07 18:01:20 +00:00
parent 905e33545c
commit 5026b2cc8b
16 changed files with 594 additions and 513 deletions
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58
lib/Analysis/DependenceAnalysis.cpp
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@@ -3188,51 +3188,41 @@ DependenceAnalysis::tryDelinearize(const SCEV *SrcSCEV, const SCEV *DstSCEV,
if (!SrcAR || !DstAR || !SrcAR->isAffine() || !DstAR->isAffine())
return false;
SmallVector<const SCEV *, 4> SrcSubscripts, DstSubscripts, SrcSizes, DstSizes;
const SCEV *RemainderS = SrcAR->delinearize(*SE, SrcSubscripts, SrcSizes);
const SCEV *RemainderD = DstAR->delinearize(*SE, DstSubscripts, DstSizes);
// First step: collect parametric terms in both array references.
SmallVector<const SCEV *, 4> Terms;
SrcAR->collectParametricTerms(*SE, Terms);
DstAR->collectParametricTerms(*SE, Terms);
// Second step: find subscript sizes.
SmallVector<const SCEV *, 4> Sizes;
SrcAR->findArrayDimensions(*SE, Terms, Sizes);
// Third step: compute the access functions for each subscript.
SmallVector<const SCEV *, 4> SrcSubscripts, DstSubscripts;
const SCEV *RemainderS = SrcAR->computeAccessFunctions(*SE, SrcSubscripts, Sizes);
const SCEV *RemainderD = DstAR->computeAccessFunctions(*SE, DstSubscripts, Sizes);
int size = SrcSubscripts.size();
// Fail when there is only a subscript: that's a linearized access function.
if (size < 2)
if (SrcSubscripts.size() < 2 || DstSubscripts.size() < 2 ||
SrcSubscripts.size() != DstSubscripts.size())
return false;
int dstSize = DstSubscripts.size();
// Fail when the number of subscripts in Src and Dst differ.
if (size != dstSize)
return false;
// Fail when the size of any of the subscripts in Src and Dst differs: the
// dependence analysis assumes that elements in the same array have same size.
// SCEV delinearization does not have a context based on which it would decide
// globally the size of subscripts that would best fit all the array accesses.
for (int i = 0; i < size; ++i)
if (SrcSizes[i] != DstSizes[i])
return false;
// When the difference in remainders is different than a constant it might be
// that the base address of the arrays is not the same.
const SCEV *DiffRemainders = SE->getMinusSCEV(RemainderS, RemainderD);
if (!isa<SCEVConstant>(DiffRemainders))
return false;
// Normalize the last dimension: integrate the size of the "scalar dimension"
// and the remainder of the delinearization.
DstSubscripts[size-1] = SE->getMulExpr(DstSubscripts[size-1],
DstSizes[size-1]);
SrcSubscripts[size-1] = SE->getMulExpr(SrcSubscripts[size-1],
SrcSizes[size-1]);
SrcSubscripts[size-1] = SE->getAddExpr(SrcSubscripts[size-1], RemainderS);
DstSubscripts[size-1] = SE->getAddExpr(DstSubscripts[size-1], RemainderD);
int size = SrcSubscripts.size();
#ifndef NDEBUG
DEBUG(errs() << "\nSrcSubscripts: ");
for (int i = 0; i < size; i++)
DEBUG(errs() << *SrcSubscripts[i]);
DEBUG(errs() << "\nDstSubscripts: ");
for (int i = 0; i < size; i++)
DEBUG(errs() << *DstSubscripts[i]);
#endif
DEBUG({
dbgs() << "\nSrcSubscripts: ";
for (int i = 0; i < size; i++)
dbgs() << *SrcSubscripts[i];
dbgs() << "\nDstSubscripts: ";
for (int i = 0; i < size; i++)
dbgs() << *DstSubscripts[i];
});
// The delinearization transforms a single-subscript MIV dependence test into
// a multi-subscript SIV dependence test that is easier to compute. So we