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Fix BasicAA's recursion detection so that it doesn't pessimize

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queries in the case of a DAG, where a query reaches a node
visited earlier, but it's not on a cycle. This avoids
MayAlias results in cases where BasicAA is expected to
return MustAlias or PartialAlias in order to protect TBAA.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@132609 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Dan Gohman 2011-06-04 00:31:50 +00:00
parent 865f09334f
commit 1fc18d71de
3 changed files with 95 additions and 37 deletions
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27
include/llvm/Analysis/AliasAnalysis.h
View File

@ -38,6 +38,7 @@
#define LLVM_ANALYSIS_ALIAS_ANALYSIS_H
#include "llvm/Support/CallSite.h"
#include "llvm/ADT/DenseMap.h"
namespace llvm {
@ -488,6 +489,32 @@ public:
}
};
// Specialize DenseMapInfo for Location.
template<>
struct DenseMapInfo<AliasAnalysis::Location> {
static inline AliasAnalysis::Location getEmptyKey() {
return
AliasAnalysis::Location(DenseMapInfo<const Value *>::getEmptyKey(),
0, 0);
}
static inline AliasAnalysis::Location getTombstoneKey() {
return
AliasAnalysis::Location(DenseMapInfo<const Value *>::getTombstoneKey(),
0, 0);
}
static unsigned getHashValue(const AliasAnalysis::Location &Val) {
return DenseMapInfo<const Value *>::getHashValue(Val.Ptr) ^
DenseMapInfo<uint64_t>::getHashValue(Val.Size) ^
DenseMapInfo<const MDNode *>::getHashValue(Val.TBAATag);
}
static bool isEqual(const AliasAnalysis::Location &LHS,
const AliasAnalysis::Location &RHS) {
return LHS.Ptr == RHS.Ptr &&
LHS.Size == RHS.Size &&
LHS.TBAATag == RHS.TBAATag;
}
};
/// isNoAliasCall - Return true if this pointer is returned by a noalias
/// function.
bool isNoAliasCall(const Value *V);

64
lib/Analysis/BasicAliasAnalysis.cpp
View File

@ -465,12 +465,12 @@ namespace {
virtual AliasResult alias(const Location &LocA,
const Location &LocB) {
assert(Visited.empty() && "Visited must be cleared after use!");
assert(AliasCache.empty() && "AliasCache must be cleared after use!");
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
AliasResult Alias = aliasCheck(LocA.Ptr, LocA.Size, LocA.TBAATag,
LocB.Ptr, LocB.Size, LocB.TBAATag);
Visited.clear();
AliasCache.clear();
return Alias;
}
@ -506,7 +506,12 @@ namespace {
}
private:
// Visited - Track instructions visited by a aliasPHI, aliasSelect(), and aliasGEP().
// AliasCache - Track alias queries to guard against recursion.
typedef std::pair<Location, Location> LocPair;
typedef DenseMap<LocPair, AliasResult> AliasCacheTy;
AliasCacheTy AliasCache;
// Visited - Track instructions visited by pointsToConstantMemory.
SmallPtrSet<const Value*, 16> Visited;
// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
@ -822,13 +827,6 @@ BasicAliasAnalysis::aliasGEP(const GEPOperator *GEP1, uint64_t V1Size,
const MDNode *V2TBAAInfo,
const Value *UnderlyingV1,
const Value *UnderlyingV2) {
// If this GEP has been visited before, we're on a use-def cycle.
// Such cycles are only valid when PHI nodes are involved or in unreachable
// code. The visitPHI function catches cycles containing PHIs, but there
// could still be a cycle without PHIs in unreachable code.
if (!Visited.insert(GEP1))
return MayAlias;
int64_t GEP1BaseOffset;
SmallVector<VariableGEPIndex, 4> GEP1VariableIndices;
@ -969,13 +967,6 @@ BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
const MDNode *SITBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
// If this select has been visited before, we're on a use-def cycle.
// Such cycles are only valid when PHI nodes are involved or in unreachable
// code. The visitPHI function catches cycles containing PHIs, but there
// could still be a cycle without PHIs in unreachable code.
if (!Visited.insert(SI))
return MayAlias;
// If the values are Selects with the same condition, we can do a more precise
// check: just check for aliases between the values on corresponding arms.
if (const SelectInst *SI2 = dyn_cast<SelectInst>(V2))
@ -998,11 +989,6 @@ BasicAliasAnalysis::aliasSelect(const SelectInst *SI, uint64_t SISize,
if (Alias == MayAlias)
return MayAlias;
// If V2 is visited, the recursive case will have been caught in the
// above aliasCheck call, so these subsequent calls to aliasCheck
// don't need to assume that V2 is being visited recursively.
Visited.erase(V2);
AliasResult ThisAlias =
aliasCheck(V2, V2Size, V2TBAAInfo, SI->getFalseValue(), SISize, SITBAAInfo);
return MergeAliasResults(ThisAlias, Alias);
@ -1015,10 +1001,6 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
const MDNode *PNTBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
// The PHI node has already been visited, avoid recursion any further.
if (!Visited.insert(PN))
return MayAlias;
// If the values are PHIs in the same block, we can do a more precise
// as well as efficient check: just check for aliases between the values
// on corresponding edges.
@ -1068,11 +1050,6 @@ BasicAliasAnalysis::aliasPHI(const PHINode *PN, uint64_t PNSize,
for (unsigned i = 1, e = V1Srcs.size(); i != e; ++i) {
Value *V = V1Srcs[i];
// If V2 is visited, the recursive case will have been caught in the
// above aliasCheck call, so these subsequent calls to aliasCheck
// don't need to assume that V2 is being visited recursively.
Visited.erase(V2);
AliasResult ThisAlias = aliasCheck(V2, V2Size, V2TBAAInfo,
V, PNSize, PNTBAAInfo);
Alias = MergeAliasResults(ThisAlias, Alias);
@ -1162,6 +1139,17 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
(V2Size != UnknownSize && isObjectSmallerThan(O1, V2Size, *TD)))
return NoAlias;
// Check the cache before climbing up use-def chains. This also terminates
// otherwise infinitely recursive queries.
LocPair Locs(Location(V1, V1Size, V1TBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
if (V1 > V2)
std::swap(Locs.first, Locs.second);
std::pair<AliasCacheTy::iterator, bool> Pair =
AliasCache.insert(std::make_pair(Locs, MayAlias));
if (!Pair.second)
return Pair.first->second;
// FIXME: This isn't aggressively handling alias(GEP, PHI) for example: if the
// GEP can't simplify, we don't even look at the PHI cases.
if (!isa<GEPOperator>(V1) && isa<GEPOperator>(V2)) {
@ -1171,7 +1159,7 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
}
if (const GEPOperator *GV1 = dyn_cast<GEPOperator>(V1)) {
AliasResult Result = aliasGEP(GV1, V1Size, V2, V2Size, V2TBAAInfo, O1, O2);
if (Result != MayAlias) return Result;
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<PHINode>(V2) && !isa<PHINode>(V1)) {
@ -1181,7 +1169,7 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
if (const PHINode *PN = dyn_cast<PHINode>(V1)) {
AliasResult Result = aliasPHI(PN, V1Size, V1TBAAInfo,
V2, V2Size, V2TBAAInfo);
if (Result != MayAlias) return Result;
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
if (isa<SelectInst>(V2) && !isa<SelectInst>(V1)) {
@ -1191,7 +1179,7 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
if (const SelectInst *S1 = dyn_cast<SelectInst>(V1)) {
AliasResult Result = aliasSelect(S1, V1Size, V1TBAAInfo,
V2, V2Size, V2TBAAInfo);
if (Result != MayAlias) return Result;
if (Result != MayAlias) return AliasCache[Locs] = Result;
}
// If both pointers are pointing into the same object and one of them
@ -1200,8 +1188,10 @@ BasicAliasAnalysis::aliasCheck(const Value *V1, uint64_t V1Size,
if (TD && O1 == O2)
if ((V1Size != UnknownSize && isObjectSize(O1, V1Size, *TD)) ||
(V2Size != UnknownSize && isObjectSize(O2, V2Size, *TD)))
return PartialAlias;
return AliasCache[Locs] = PartialAlias;
return AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
AliasResult Result =
AliasAnalysis::alias(Location(V1, V1Size, V1TBAAInfo),
Location(V2, V2Size, V2TBAAInfo));
return AliasCache[Locs] = Result;
}

41
test/Analysis/BasicAA/dag.ll Normal file
View File

@ -0,0 +1,41 @@
; RUN: opt < %s -basicaa -aa-eval -print-all-alias-modref-info |& FileCheck %s
target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64"
; BasicAA's guard against use-def cycles shouldn't prevent it from
; analyzing use-def dags.
; CHECK: MustAlias: i8* %base, i8* %phi
; CHECK: MustAlias: i8* %phi, i8* %wwa
; CHECK: MustAlias: i8* %phi, i8* %wwb
; CHECK: MustAlias: i16* %bigbase, i8* %phi
define i8 @foo(i8* %base, i1 %x, i1 %w) {
entry:
br i1 %w, label %wa, label %wb
wa:
%wwa = bitcast i8* %base to i8*
br label %wc
wb:
%wwb = bitcast i8* %base to i8*
br label %wc
wc:
%first = phi i8* [ %wwa, %wa ], [ %wwb, %wb ]
%fc = bitcast i8* %first to i8*
br i1 %x, label %xa, label %xb
xa:
%xxa = bitcast i8* %fc to i8*
br label %xc
xb:
%xxb = bitcast i8* %fc to i8*
br label %xc
xc:
%phi = phi i8* [ %xxa, %xa ], [ %xxb, %xb ]
store i8 0, i8* %phi
%bigbase = bitcast i8* %base to i16*
store i16 -1, i16* %bigbase
%loaded = load i8* %phi
ret i8 %loaded
}