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add more comments around the delinearization of arrays

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@194612 91177308-0d34-0410-b5e6-96231b3b80d8
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2013-11-13 22:37:58 +00:00
parent c9024c6ebc
commit f44941d81d
3 changed files with 88 additions and 17 deletions
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21
lib/Analysis/DependenceAnalysis.cpp
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@ -24,11 +24,11 @@
// Both of these are conservative weaknesses;
// that is, not a source of correctness problems.
//
// The implementation depends on the GEP instruction to
// differentiate subscripts. Since Clang linearizes subscripts
// for most arrays, we give up some precision (though the existing MIV tests
// will help). We trust that the GEP instruction will eventually be extended.
// In the meantime, we should explore Maslov's ideas about delinearization.
// The implementation depends on the GEP instruction to differentiate
// subscripts. Since Clang linearizes some array subscripts, the dependence
// analysis is using SCEV->delinearize to recover the representation of multiple
// subscripts, and thus avoid the more expensive and less precise MIV tests. The
// delinearization is controlled by the flag -da-delinearize.
//
// We should pay some careful attention to the possibility of integer overflow
// in the implementation of the various tests. This could happen with Add,
@ -3206,10 +3206,21 @@ DependenceAnalysis::tryDelinearize(const SCEV *SrcSCEV, const SCEV *DstSCEV,
DEBUG(errs() << *DstSubscripts[i]);
#endif
// The delinearization transforms a single-subscript MIV dependence test into
// a multi-subscript SIV dependence test that is easier to compute. So we
// resize Pair to contain as many pairs of subscripts as the delinearization
// has found, and then initialize the pairs following the delinearization.
Pair.resize(size);
for (int i = 0; i < size; ++i) {
Pair[i].Src = SrcSubscripts[i];
Pair[i].Dst = DstSubscripts[i];
// FIXME: we should record the bounds SrcSizes[i] and DstSizes[i] that the
// delinearization has found, and add these constraints to the dependence
// check to avoid memory accesses overflow from one dimension into another.
// This is related to the problem of determining the existence of data
// dependences in array accesses using a different number of subscripts: in
// C one can access an array A[100][100]; as A[0][9999], *A[9999], etc.
}
return true;