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Fix the logic in BasicAliasAnalysis::aliasGEP for comparing GEP's with variable differences so that it actually does something sane. Fixes PR10881.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139276 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Eli Friedman
2011-09-08 02:23:31 +00:00
parent e64fb28da1
commit 81ac8ddc67
2 changed files with 65 additions and 33 deletions
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lib/Analysis/BasicAliasAnalysis.cpp
+33 -33
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@@ -955,43 +955,43 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
return MustAlias;
// If there is a difference between the pointers, but the difference is
// less than the size of the associated memory object, then we know
// that the objects are partially overlapping.
// If there is a constant difference between the pointers, but the difference
// is less than the size of the associated memory object, then we know
// that the objects are partially overlapping. If the difference is
// greater, we know they do not overlap.
if (GEP1BaseOffset != 0 && GEP1VariableIndices.empty()) {
if (GEP1BaseOffset >= 0 ?
(V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) :
(V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size &&
GEP1BaseOffset != INT64_MIN))
return PartialAlias;
if (GEP1BaseOffset >= 0) {
if (V2Size != UnknownSize) {
if ((uint64_t)GEP1BaseOffset < V2Size)
return PartialAlias;
return NoAlias;
}
} else {
if (V1Size != UnknownSize) {
if (-(uint64_t)GEP1BaseOffset < V1Size)
return PartialAlias;
return NoAlias;
}
}
}
// If we have a known constant offset, see if this offset is larger than the
// access size being queried. If so, and if no variable indices can remove
// pieces of this constant, then we know we have a no-alias. For example,
// &A[100] != &A.
// In order to handle cases like &A[100][i] where i is an out of range
// subscript, we have to ignore all constant offset pieces that are a multiple
// of a scaled index. Do this by removing constant offsets that are a
// multiple of any of our variable indices. This allows us to transform
// things like &A[i][1] because i has a stride of (e.g.) 8 bytes but the 1
// provides an offset of 4 bytes (assuming a <= 4 byte access).
// Try to distinguish something like &A[i][1] against &A[42][0].
// Grab the least significant bit set in any of the scales.
uint64_t Modulo = 0;
for (unsigned i = 0, e = GEP1VariableIndices.size();
i != e && GEP1BaseOffset;++i)
if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale)
GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale;
// If our known offset is bigger than the access size, we know we don't have
// an alias.
if (GEP1BaseOffset) {
if (GEP1BaseOffset >= 0 ?
(V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) :
(V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size &&
GEP1BaseOffset != INT64_MIN))
return NoAlias;
}
i != e; ++i)
Modulo |= (uint64_t)GEP1VariableIndices[0].Scale;
Modulo = Modulo ^ (Modulo & (Modulo - 1));
// We can compute the difference between the two addresses
// mod Modulo. Check whether that difference guarantees that the
// two locations do not alias.
uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
if (V1Size != UnknownSize && V2Size != UnknownSize &&
ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
return NoAlias;
// Statically, we can see that the base objects are the same, but the
// pointers have dynamic offsets which we can't resolve. And none of our
// little tricks above worked.