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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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llvm-svn: 139276
This commit is contained in:
Eli Friedman 2011-09-08 02:23:31 +00:00
parent 5ac3aa158b
commit 6e9cab83b0
2 changed files with 65 additions and 33 deletions
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66
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -955,43 +955,43 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty()) if (GEP1BaseOffset == 0 && GEP1VariableIndices.empty())
return MustAlias; return MustAlias;
// If there is a difference between the pointers, but the difference is // If there is a constant difference between the pointers, but the difference
// less than the size of the associated memory object, then we know // is less than the size of the associated memory object, then we know
// that the objects are partially overlapping. // 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 && GEP1VariableIndices.empty()) {
if (GEP1BaseOffset >= 0 ? if (GEP1BaseOffset >= 0) {
(V2Size != UnknownSize && (uint64_t)GEP1BaseOffset < V2Size) : if (V2Size != UnknownSize) {
(V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset < V1Size && if ((uint64_t)GEP1BaseOffset < V2Size)
GEP1BaseOffset != INT64_MIN)) return PartialAlias;
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 // Try to distinguish something like &A[i][1] against &A[42][0].
// access size being queried. If so, and if no variable indices can remove // Grab the least significant bit set in any of the scales.
// pieces of this constant, then we know we have a no-alias. For example, uint64_t Modulo = 0;
// &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).
for (unsigned i = 0, e = GEP1VariableIndices.size(); for (unsigned i = 0, e = GEP1VariableIndices.size();
i != e && GEP1BaseOffset;++i) i != e; ++i)
if (int64_t RemovedOffset = GEP1BaseOffset/GEP1VariableIndices[i].Scale) Modulo |= (uint64_t)GEP1VariableIndices[0].Scale;
GEP1BaseOffset -= RemovedOffset*GEP1VariableIndices[i].Scale; Modulo = Modulo ^ (Modulo & (Modulo - 1));
// If our known offset is bigger than the access size, we know we don't have // We can compute the difference between the two addresses
// an alias. // mod Modulo. Check whether that difference guarantees that the
if (GEP1BaseOffset) { // two locations do not alias.
if (GEP1BaseOffset >= 0 ? uint64_t ModOffset = (uint64_t)GEP1BaseOffset & (Modulo - 1);
(V2Size != UnknownSize && (uint64_t)GEP1BaseOffset >= V2Size) : if (V1Size != UnknownSize && V2Size != UnknownSize &&
(V1Size != UnknownSize && -(uint64_t)GEP1BaseOffset >= V1Size && ModOffset >= V2Size && V1Size <= Modulo - ModOffset)
GEP1BaseOffset != INT64_MIN)) return NoAlias;
return NoAlias;
}
// Statically, we can see that the base objects are the same, but the // 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 // pointers have dynamic offsets which we can't resolve. And none of our
// little tricks above worked. // little tricks above worked.

32
test/Analysis/BasicAA/gep-alias.ll
View File

@ -169,3 +169,35 @@ define i8 @test10([4 x i8] *%P, i32 %i) {
; CHECK: @test10 ; CHECK: @test10
; CHECK: ret i8 0 ; CHECK: ret i8 0
} }
; (This was a miscompilation.)
define float @test11(i32 %indvar, [4 x [2 x float]]* %q) nounwind ssp {
%tmp = mul i32 %indvar, -1
%dec = add i32 %tmp, 3
%scevgep = getelementptr [4 x [2 x float]]* %q, i32 0, i32 %dec
%scevgep35 = bitcast [2 x float]* %scevgep to i64*
%arrayidx28 = getelementptr inbounds [4 x [2 x float]]* %q, i32 0, i32 0
%y29 = getelementptr inbounds [2 x float]* %arrayidx28, i32 0, i32 1
store float 1.0, float* %y29, align 4
store i64 0, i64* %scevgep35, align 4
%tmp30 = load float* %y29, align 4
ret float %tmp30
; CHECK: @test11
; CHECK: ret float %tmp30
}
; (This was a miscompilation.)
define i32 @test12(i32 %x, i32 %y, i8* %p) nounwind {
%a = bitcast i8* %p to [13 x i8]*
%b = getelementptr [13 x i8]* %a, i32 %x
%c = bitcast [13 x i8]* %b to [15 x i8]*
%d = getelementptr [15 x i8]* %c, i32 %y, i32 8
%castd = bitcast i8* %d to i32*
%castp = bitcast i8* %p to i32*
store i32 1, i32* %castp
store i32 0, i32* %castd
%r = load i32* %castp
ret i32 %r
; CHECK: @test12
; CHECK: ret i32 %r
}