BasicAA: Fix value equality and phi cycles
When there are cycles in the value graph we have to be careful interpreting
"Value*" identity as "value" equivalence. We interpret the value of a phi node
as the value of its operands.
When we check for value equivalence now we make sure that the "Value*" dominates
all cycles (phis).
%0 = phi [%noaliasval, %addr2]
%l = load %ptr
%addr1 = gep @a, 0, %l
%addr2 = gep @a, 0, (%l + 1)
store %ptr ...
Before this patch we would return NoAlias for (%0, %addr1) which is wrong
because the value of the load is from different iterations of the loop.
Tested on x86_64 -mavx at O3 and O3 -flto with no performance or compile time
regressions.
PR18068
radar://15653794
llvm-svn: 198290
diff --git a/llvm/lib/Analysis/BasicAliasAnalysis.cpp b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
index b2c2011..14268ec 100644
--- a/llvm/lib/Analysis/BasicAliasAnalysis.cpp
+++ b/llvm/lib/Analysis/BasicAliasAnalysis.cpp
@@ -18,6 +18,7 @@
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemoryBuiltins.h"
#include "llvm/Analysis/ValueTracking.h"
@@ -38,6 +39,12 @@
#include <algorithm>
using namespace llvm;
+/// Cutoff after which to stop analysing a set of phi nodes potentially involved
+/// in a cycle. Because we are analysing 'through' phi nodes we need to be
+/// careful with value equivalence. We use dominance to make sure a value cannot
+/// be involved in a cycle.
+const unsigned MaxNumPhiBBsValueDominanceCheck = 20;
+
//===----------------------------------------------------------------------===//
// Useful predicates
//===----------------------------------------------------------------------===//
@@ -403,42 +410,6 @@
return V;
}
-/// GetIndexDifference - Dest and Src are the variable indices from two
-/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
-/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
-/// difference between the two pointers.
-static void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
- const SmallVectorImpl<VariableGEPIndex> &Src) {
- if (Src.empty()) return;
-
- for (unsigned i = 0, e = Src.size(); i != e; ++i) {
- const Value *V = Src[i].V;
- ExtensionKind Extension = Src[i].Extension;
- int64_t Scale = Src[i].Scale;
-
- // Find V in Dest. This is N^2, but pointer indices almost never have more
- // than a few variable indexes.
- for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
- if (Dest[j].V != V || Dest[j].Extension != Extension) continue;
-
- // If we found it, subtract off Scale V's from the entry in Dest. If it
- // goes to zero, remove the entry.
- if (Dest[j].Scale != Scale)
- Dest[j].Scale -= Scale;
- else
- Dest.erase(Dest.begin()+j);
- Scale = 0;
- break;
- }
-
- // If we didn't consume this entry, add it to the end of the Dest list.
- if (Scale) {
- VariableGEPIndex Entry = { V, Extension, -Scale };
- Dest.push_back(Entry);
- }
- }
-}
-
//===----------------------------------------------------------------------===//
// BasicAliasAnalysis Pass
//===----------------------------------------------------------------------===//
@@ -482,6 +453,7 @@
virtual AliasResult alias(const Location &LocA,
const Location &LocB) {
+ DT = 0;
assert(AliasCache.empty() && "AliasCache must be cleared after use!");
assert(notDifferentParent(LocA.Ptr, LocB.Ptr) &&
"BasicAliasAnalysis doesn't support interprocedural queries.");
@@ -492,6 +464,7 @@
// SmallDenseMap if it ever grows larger.
// FIXME: This should really be shrink_to_inline_capacity_and_clear().
AliasCache.shrink_and_clear();
+ VisitedPhiBBs.clear();
return Alias;
}
@@ -532,9 +505,41 @@
typedef SmallDenseMap<LocPair, AliasResult, 8> AliasCacheTy;
AliasCacheTy AliasCache;
+ /// \brief Track phi nodes we have visited. When interpret "Value" pointer
+ /// equality as value equality we need to make sure that the "Value" is not
+ /// part of a cycle. Otherwise, two uses could come from different
+ /// "iterations" of a cycle and see different values for the same "Value"
+ /// pointer.
+ /// The following example shows the problem:
+ /// %p = phi(%alloca1, %addr2)
+ /// %l = load %ptr
+ /// %addr1 = gep, %alloca2, 0, %l
+ /// %addr2 = gep %alloca2, 0, (%l + 1)
+ /// alias(%p, %addr1) -> MayAlias !
+ /// store %l, ...
+ SmallPtrSet<const BasicBlock*, 8> VisitedPhiBBs;
+
// Visited - Track instructions visited by pointsToConstantMemory.
SmallPtrSet<const Value*, 16> Visited;
+ // We use the dominator tree to check values can't be part of a cycle.
+ DominatorTree *DT;
+
+ /// \brief Check whether two Values can be considered equivalent.
+ ///
+ /// In addition to pointer equivalence of \p V1 and \p V2 this checks
+ /// whether they can not be part of a cycle in the value graph by looking at
+ /// all visited phi nodes an making sure that the value dominates all of
+ /// them.
+ bool isValueEqual(const Value *V1, const Value *V2);
+
+ /// \brief Dest and Src are the variable indices from two decomposed
+ /// GetElementPtr instructions GEP1 and GEP2 which have common base
+ /// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
+ /// difference between the two pointers.
+ void GetIndexDifference(SmallVectorImpl<VariableGEPIndex> &Dest,
+ const SmallVectorImpl<VariableGEPIndex> &Src);
+
// aliasGEP - Provide a bunch of ad-hoc rules to disambiguate a GEP
// instruction against another.
AliasResult aliasGEP(const GEPOperator *V1, uint64_t V1Size,
@@ -1094,6 +1099,10 @@
const MDNode *PNTBAAInfo,
const Value *V2, uint64_t V2Size,
const MDNode *V2TBAAInfo) {
+ // Track phi nodes we have visited. We use this information when we determine
+ // value equivalence.
+ VisitedPhiBBs.insert(PN->getParent());
+
// 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.
@@ -1187,7 +1196,7 @@
V2 = V2->stripPointerCasts();
// Are we checking for alias of the same value?
- if (V1 == V2) return MustAlias;
+ if (isValueEqual(V1, V2)) return MustAlias;
if (!V1->getType()->isPointerTy() || !V2->getType()->isPointerTy())
return NoAlias; // Scalars cannot alias each other
@@ -1307,3 +1316,69 @@
Location(V2, V2Size, V2TBAAInfo));
return AliasCache[Locs] = Result;
}
+
+bool BasicAliasAnalysis::isValueEqual(const Value *V, const Value *V2) {
+ if (V != V2)
+ return false;
+
+ const Instruction *Inst = dyn_cast<Instruction>(V);
+ if (!Inst)
+ return true;
+
+ // Use the dominance if available.
+ DT = getAnalysisIfAvailable<DominatorTree>();
+ if (DT) {
+ if (VisitedPhiBBs.size() > MaxNumPhiBBsValueDominanceCheck)
+ return false;
+
+ // Make sure that the visited phis are dominated by the Value.
+ for (SmallPtrSet<const BasicBlock *, 8>::iterator
+ PI = VisitedPhiBBs.begin(),
+ PE = VisitedPhiBBs.end();
+ PI != PE; ++PI)
+ if (!DT->dominates(Inst, *PI))
+ return false;
+ return true;
+ }
+
+ return false;
+}
+
+/// GetIndexDifference - Dest and Src are the variable indices from two
+/// decomposed GetElementPtr instructions GEP1 and GEP2 which have common base
+/// pointers. Subtract the GEP2 indices from GEP1 to find the symbolic
+/// difference between the two pointers.
+void BasicAliasAnalysis::GetIndexDifference(
+ SmallVectorImpl<VariableGEPIndex> &Dest,
+ const SmallVectorImpl<VariableGEPIndex> &Src) {
+ if (Src.empty())
+ return;
+
+ for (unsigned i = 0, e = Src.size(); i != e; ++i) {
+ const Value *V = Src[i].V;
+ ExtensionKind Extension = Src[i].Extension;
+ int64_t Scale = Src[i].Scale;
+
+ // Find V in Dest. This is N^2, but pointer indices almost never have more
+ // than a few variable indexes.
+ for (unsigned j = 0, e = Dest.size(); j != e; ++j) {
+ if (!isValueEqual(Dest[j].V, V) || Dest[j].Extension != Extension)
+ continue;
+
+ // If we found it, subtract off Scale V's from the entry in Dest. If it
+ // goes to zero, remove the entry.
+ if (Dest[j].Scale != Scale)
+ Dest[j].Scale -= Scale;
+ else
+ Dest.erase(Dest.begin() + j);
+ Scale = 0;
+ break;
+ }
+
+ // If we didn't consume this entry, add it to the end of the Dest list.
+ if (Scale) {
+ VariableGEPIndex Entry = { V, Extension, -Scale };
+ Dest.push_back(Entry);
+ }
+ }
+}