[llvm] r235634 - [getUnderlyingOjbects] Analyze loop PHIs further to remove false positives
Adam Nemet
anemet at apple.com
Thu Apr 23 13:09:20 PDT 2015
Author: anemet
Date: Thu Apr 23 15:09:20 2015
New Revision: 235634
URL: http://llvm.org/viewvc/llvm-project?rev=235634&view=rev
Log:
[getUnderlyingOjbects] Analyze loop PHIs further to remove false positives
Specifically, if a pointer accesses different underlying objects in each
iteration, don't look through the phi node defining the pointer.
The motivating case is the underlyling-objects-2.ll testcase. Consider
the loop nest:
int **A;
for (i)
for (j)
A[i][j] = A[i-1][j] * B[j]
This loop is transformed by Load-PRE to stash away A[i] for the next
iteration of the outer loop:
Curr = A[0]; // Prev_0
for (i: 1..N) {
Prev = Curr; // Prev = PHI (Prev_0, Curr)
Curr = A[i];
for (j: 0..N)
Curr[j] = Prev[j] * B[j]
}
Since A[i] and A[i-1] are likely to be independent pointers,
getUnderlyingObjects should not assume that Curr and Prev share the same
underlying object in the inner loop.
If it did we would try to dependence-analyze Curr and Prev and the
analysis of the corresponding SCEVs would fail with non-constant
distance.
To fix this, the getUnderlyingObjects API is extended with an optional
LoopInfo parameter. This is effectively what controls whether we want
the above behavior or the original. Currently, I only changed to use
this approach for LoopAccessAnalysis.
The other testcase is to guard the opposite case where we do want to
look through the loop PHI. If we step through an array by incrementing
a pointer, the underlying object is the incoming value of the phi as the
loop is entered.
Fixes rdar://problem/19566729
Added:
llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-1.ll
llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-2.ll
Modified:
llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h
llvm/trunk/include/llvm/Analysis/ValueTracking.h
llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp
llvm/trunk/lib/Analysis/ValueTracking.cpp
Modified: llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h?rev=235634&r1=235633&r2=235634&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h (original)
+++ llvm/trunk/include/llvm/Analysis/LoopAccessAnalysis.h Thu Apr 23 15:09:20 2015
@@ -370,7 +370,8 @@ public:
LoopAccessInfo(Loop *L, ScalarEvolution *SE, const DataLayout &DL,
const TargetLibraryInfo *TLI, AliasAnalysis *AA,
- DominatorTree *DT, const ValueToValueMap &Strides);
+ DominatorTree *DT, LoopInfo *LI,
+ const ValueToValueMap &Strides);
/// Return true we can analyze the memory accesses in the loop and there are
/// no memory dependence cycles.
@@ -467,6 +468,7 @@ private:
const TargetLibraryInfo *TLI;
AliasAnalysis *AA;
DominatorTree *DT;
+ LoopInfo *LI;
unsigned NumLoads;
unsigned NumStores;
@@ -541,6 +543,7 @@ private:
const TargetLibraryInfo *TLI;
AliasAnalysis *AA;
DominatorTree *DT;
+ LoopInfo *LI;
};
} // End llvm namespace
Modified: llvm/trunk/include/llvm/Analysis/ValueTracking.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/ValueTracking.h?rev=235634&r1=235633&r2=235634&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/ValueTracking.h (original)
+++ llvm/trunk/include/llvm/Analysis/ValueTracking.h Thu Apr 23 15:09:20 2015
@@ -28,6 +28,7 @@ namespace llvm {
class AssumptionCache;
class DominatorTree;
class TargetLibraryInfo;
+ class LoopInfo;
/// Determine which bits of V are known to be either zero or one and return
/// them in the KnownZero/KnownOne bit sets.
@@ -176,11 +177,37 @@ namespace llvm {
return GetUnderlyingObject(const_cast<Value *>(V), DL, MaxLookup);
}
- /// GetUnderlyingObjects - This method is similar to GetUnderlyingObject
- /// except that it can look through phi and select instructions and return
- /// multiple objects.
+ /// \brief This method is similar to GetUnderlyingObject except that it can
+ /// look through phi and select instructions and return multiple objects.
+ ///
+ /// If LoopInfo is passed, loop phis are further analyzed. If a pointer
+ /// accesses different objects in each iteration, we don't look through the
+ /// phi node. E.g. consider this loop nest:
+ ///
+ /// int **A;
+ /// for (i)
+ /// for (j) {
+ /// A[i][j] = A[i-1][j] * B[j]
+ /// }
+ ///
+ /// This is transformed by Load-PRE to stash away A[i] for the next iteration
+ /// of the outer loop:
+ ///
+ /// Curr = A[0]; // Prev_0
+ /// for (i: 1..N) {
+ /// Prev = Curr; // Prev = PHI (Prev_0, Curr)
+ /// Curr = A[i];
+ /// for (j: 0..N) {
+ /// Curr[j] = Prev[j] * B[j]
+ /// }
+ /// }
+ ///
+ /// Since A[i] and A[i-1] are independent pointers, getUnderlyingObjects
+ /// should not assume that Curr and Prev share the same underlying object thus
+ /// it shouldn't look through the phi above.
void GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
- const DataLayout &DL, unsigned MaxLookup = 6);
+ const DataLayout &DL, LoopInfo *LI = nullptr,
+ unsigned MaxLookup = 6);
/// onlyUsedByLifetimeMarkers - Return true if the only users of this pointer
/// are lifetime markers.
Modified: llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp?rev=235634&r1=235633&r2=235634&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/LoopAccessAnalysis.cpp Thu Apr 23 15:09:20 2015
@@ -199,9 +199,9 @@ public:
typedef PointerIntPair<Value *, 1, bool> MemAccessInfo;
typedef SmallPtrSet<MemAccessInfo, 8> MemAccessInfoSet;
- AccessAnalysis(const DataLayout &Dl, AliasAnalysis *AA,
+ AccessAnalysis(const DataLayout &Dl, AliasAnalysis *AA, LoopInfo *LI,
MemoryDepChecker::DepCandidates &DA)
- : DL(Dl), AST(*AA), DepCands(DA), IsRTCheckNeeded(false) {}
+ : DL(Dl), AST(*AA), LI(LI), DepCands(DA), IsRTCheckNeeded(false) {}
/// \brief Register a load and whether it is only read from.
void addLoad(AliasAnalysis::Location &Loc, bool IsReadOnly) {
@@ -261,6 +261,8 @@ private:
//intrinsic property (such as TBAA metadata).
AliasSetTracker AST;
+ LoopInfo *LI;
+
/// Sets of potentially dependent accesses - members of one set share an
/// underlying pointer. The set "CheckDeps" identfies which sets really need a
/// dependence check.
@@ -477,7 +479,9 @@ void AccessAnalysis::processMemAccesses(
// underlying object.
typedef SmallVector<Value *, 16> ValueVector;
ValueVector TempObjects;
- GetUnderlyingObjects(Ptr, TempObjects, DL);
+
+ GetUnderlyingObjects(Ptr, TempObjects, DL, LI);
+ DEBUG(dbgs() << "Underlying objects for pointer " << *Ptr << "\n");
for (Value *UnderlyingObj : TempObjects) {
UnderlyingObjToAccessMap::iterator Prev =
ObjToLastAccess.find(UnderlyingObj);
@@ -485,6 +489,7 @@ void AccessAnalysis::processMemAccesses(
DepCands.unionSets(Access, Prev->second);
ObjToLastAccess[UnderlyingObj] = Access;
+ DEBUG(dbgs() << " " << *UnderlyingObj << "\n");
}
}
}
@@ -1035,7 +1040,7 @@ void LoopAccessInfo::analyzeLoop(const V
MemoryDepChecker::DepCandidates DependentAccesses;
AccessAnalysis Accesses(TheLoop->getHeader()->getModule()->getDataLayout(),
- AA, DependentAccesses);
+ AA, LI, DependentAccesses);
// Holds the analyzed pointers. We don't want to call GetUnderlyingObjects
// multiple times on the same object. If the ptr is accessed twice, once
@@ -1306,10 +1311,10 @@ std::pair<Instruction *, Instruction *>
LoopAccessInfo::LoopAccessInfo(Loop *L, ScalarEvolution *SE,
const DataLayout &DL,
const TargetLibraryInfo *TLI, AliasAnalysis *AA,
- DominatorTree *DT,
+ DominatorTree *DT, LoopInfo *LI,
const ValueToValueMap &Strides)
: DepChecker(SE, L), NumComparisons(0), TheLoop(L), SE(SE), DL(DL),
- TLI(TLI), AA(AA), DT(DT), NumLoads(0), NumStores(0),
+ TLI(TLI), AA(AA), DT(DT), LI(LI), NumLoads(0), NumStores(0),
MaxSafeDepDistBytes(-1U), CanVecMem(false),
StoreToLoopInvariantAddress(false) {
if (canAnalyzeLoop())
@@ -1356,7 +1361,8 @@ LoopAccessAnalysis::getInfo(Loop *L, con
if (!LAI) {
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
- LAI = llvm::make_unique<LoopAccessInfo>(L, SE, DL, TLI, AA, DT, Strides);
+ LAI = llvm::make_unique<LoopAccessInfo>(L, SE, DL, TLI, AA, DT, LI,
+ Strides);
#ifndef NDEBUG
LAI->NumSymbolicStrides = Strides.size();
#endif
@@ -1367,7 +1373,6 @@ LoopAccessAnalysis::getInfo(Loop *L, con
void LoopAccessAnalysis::print(raw_ostream &OS, const Module *M) const {
LoopAccessAnalysis &LAA = *const_cast<LoopAccessAnalysis *>(this);
- LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
ValueToValueMap NoSymbolicStrides;
for (Loop *TopLevelLoop : *LI)
@@ -1384,6 +1389,7 @@ bool LoopAccessAnalysis::runOnFunction(F
TLI = TLIP ? &TLIP->getTLI() : nullptr;
AA = &getAnalysis<AliasAnalysis>();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
return false;
}
Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=235634&r1=235633&r2=235634&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Thu Apr 23 15:09:20 2015
@@ -17,6 +17,7 @@
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/MemoryBuiltins.h"
+#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/ConstantRange.h"
#include "llvm/IR/Constants.h"
@@ -2737,6 +2738,32 @@ uint64_t llvm::GetStringLength(Value *V)
return Len == ~0ULL ? 1 : Len;
}
+/// \brief \p PN defines a loop-variant pointer to an object. Check if the
+/// previous iteration of the loop was referring to the same object as \p PN.
+static bool isSameUnderlyingObjectInLoop(PHINode *PN, LoopInfo *LI) {
+ // Find the loop-defined value.
+ Loop *L = LI->getLoopFor(PN->getParent());
+ if (PN->getNumIncomingValues() != 2)
+ return true;
+
+ // Find the value from previous iteration.
+ auto *PrevValue = dyn_cast<Instruction>(PN->getIncomingValue(0));
+ if (!PrevValue || LI->getLoopFor(PrevValue->getParent()) != L)
+ PrevValue = dyn_cast<Instruction>(PN->getIncomingValue(1));
+ if (!PrevValue || LI->getLoopFor(PrevValue->getParent()) != L)
+ return true;
+
+ // If a new pointer is loaded in the loop, the pointer references a different
+ // object in every iteration. E.g.:
+ // for (i)
+ // int *p = a[i];
+ // ...
+ if (auto *Load = dyn_cast<LoadInst>(PrevValue))
+ if (!L->isLoopInvariant(Load->getPointerOperand()))
+ return false;
+ return true;
+}
+
Value *llvm::GetUnderlyingObject(Value *V, const DataLayout &DL,
unsigned MaxLookup) {
if (!V->getType()->isPointerTy())
@@ -2768,7 +2795,8 @@ Value *llvm::GetUnderlyingObject(Value *
}
void llvm::GetUnderlyingObjects(Value *V, SmallVectorImpl<Value *> &Objects,
- const DataLayout &DL, unsigned MaxLookup) {
+ const DataLayout &DL, LoopInfo *LI,
+ unsigned MaxLookup) {
SmallPtrSet<Value *, 4> Visited;
SmallVector<Value *, 4> Worklist;
Worklist.push_back(V);
@@ -2786,8 +2814,20 @@ void llvm::GetUnderlyingObjects(Value *V
}
if (PHINode *PN = dyn_cast<PHINode>(P)) {
- for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
- Worklist.push_back(PN->getIncomingValue(i));
+ // If this PHI changes the underlying object in every iteration of the
+ // loop, don't look through it. Consider:
+ // int **A;
+ // for (i) {
+ // Prev = Curr; // Prev = PHI (Prev_0, Curr)
+ // Curr = A[i];
+ // *Prev, *Curr;
+ //
+ // Prev is tracking Curr one iteration behind so they refer to different
+ // underlying objects.
+ if (!LI || !LI->isLoopHeader(PN->getParent()) ||
+ isSameUnderlyingObjectInLoop(PN, LI))
+ for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+ Worklist.push_back(PN->getIncomingValue(i));
continue;
}
Added: llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-1.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-1.ll?rev=235634&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-1.ll (added)
+++ llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-1.ll Thu Apr 23 15:09:20 2015
@@ -0,0 +1,47 @@
+; RUN: opt -basicaa -loop-accesses -analyze < %s | FileCheck %s
+
+; In:
+;
+; store_ptr = A;
+; load_ptr = &A[2];
+; for (i = 0; i < n; i++)
+; *store_ptr++ = *load_ptr++ *10; // A[i] = A[i+2] * 10
+;
+; make sure, we look through the PHI to conclude that store_ptr and load_ptr
+; both have A as their underlying object. The dependence is safe for
+; vectorization requiring no memchecks.
+;
+; Otherwise we would try to prove independence with a memcheck that is going
+; to always fail.
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.10.0"
+
+; CHECK: Memory dependences are safe{{$}}
+
+define void @f(i8* noalias %A, i64 %width) {
+for.body.preheader:
+ %A_ahead = getelementptr inbounds i8, i8* %A, i64 2
+ br label %for.body
+
+for.body:
+ %i = phi i64 [ %i.1, %for.body ], [ 0, %for.body.preheader ]
+ %load_ptr = phi i8* [ %load_ptr.1, %for.body ], [ %A_ahead, %for.body.preheader ]
+ %store_ptr = phi i8* [ %store_ptr.1, %for.body ], [ %A, %for.body.preheader ]
+
+ %loadA = load i8, i8* %load_ptr, align 1
+
+ %mul = mul i8 %loadA, 10
+
+ store i8 %mul, i8* %store_ptr, align 1
+
+ %load_ptr.1 = getelementptr inbounds i8, i8* %load_ptr, i64 1
+ %store_ptr.1 = getelementptr inbounds i8, i8* %store_ptr, i64 1
+ %i.1 = add nuw i64 %i, 1
+
+ %exitcond = icmp eq i64 %i.1, %width
+ br i1 %exitcond, label %for.end, label %for.body
+
+for.end:
+ ret void
+}
Added: llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-2.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-2.ll?rev=235634&view=auto
==============================================================================
--- llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-2.ll (added)
+++ llvm/trunk/test/Analysis/LoopAccessAnalysis/underlying-objects-2.ll Thu Apr 23 15:09:20 2015
@@ -0,0 +1,91 @@
+; RUN: opt -basicaa -loop-accesses -analyze < %s | FileCheck %s
+
+; This loop:
+;
+; int **A;
+; for (i)
+; for (j) {
+; A[i][j] = A[i-1][j] * B[j]
+; B[j+1] = 2 // backward dep between this and the previous
+; }
+;
+; is transformed by Load-PRE to stash away A[i] for the next iteration of the
+; outer loop:
+;
+; Curr = A[0]; // Prev_0
+; for (i: 1..N) {
+; Prev = Curr; // Prev = PHI (Prev_0, Curr)
+; Curr = A[i];
+; for (j: 0..N) {
+; Curr[j] = Prev[j] * B[j]
+; B[j+1] = 2 // backward dep between this and the previous
+; }
+; }
+;
+; Since A[i] and A[i-1] are likely to be independent, getUnderlyingObjects
+; should not assume that Curr and Prev share the same underlying object.
+;
+; If it did we would try to dependence-analyze Curr and Prev and the analysis
+; would fail with non-constant distance.
+;
+; To illustrate one of the negative consequences of this, if the loop has a
+; backward dependence we won't detect this but instead fully fall back on
+; memchecks (that is what LAA does after encountering a case of non-constant
+; distance).
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-apple-macosx10.10.0"
+
+; CHECK: for_j.body:
+; CHECK-NEXT: Store to invariant address was not found in loop
+; CHECK-NEXT: Report: unsafe dependent memory operations in loop
+; CHECK-NEXT: Interesting Dependences:
+; CHECK-NEXT: Backward:
+; CHECK-NEXT: %loadB = load i8, i8* %gepB, align 1 ->
+; CHECK-NEXT: store i8 2, i8* %gepB_plus_one, align 1
+
+define void @f(i8** noalias %A, i8* noalias %B, i64 %N) {
+for_i.preheader:
+ %prev_0 = load i8*, i8** %A, align 8
+ br label %for_i.body
+
+for_i.body:
+ %i = phi i64 [1, %for_i.preheader], [%i.1, %for_j.end]
+ %prev = phi i8* [%prev_0, %for_i.preheader], [%curr, %for_j.end]
+ %gep = getelementptr inbounds i8*, i8** %A, i64 %i
+ %curr = load i8*, i8** %gep, align 8
+ br label %for_j.preheader
+
+for_j.preheader:
+ br label %for_j.body
+
+for_j.body:
+ %j = phi i64 [0, %for_j.preheader], [%j.1, %for_j.body]
+
+ %gepPrev = getelementptr inbounds i8, i8* %prev, i64 %j
+ %gepCurr = getelementptr inbounds i8, i8* %curr, i64 %j
+ %gepB = getelementptr inbounds i8, i8* %B, i64 %j
+
+ %loadPrev = load i8, i8* %gepPrev, align 1
+ %loadB = load i8, i8* %gepB, align 1
+
+ %mul = mul i8 %loadPrev, %loadB
+
+ store i8 %mul, i8* %gepCurr, align 1
+
+ %gepB_plus_one = getelementptr inbounds i8, i8* %gepB, i64 1
+ store i8 2, i8* %gepB_plus_one, align 1
+
+ %j.1 = add nuw i64 %j, 1
+ %exitcondj = icmp eq i64 %j.1, %N
+ br i1 %exitcondj, label %for_j.end, label %for_j.body
+
+for_j.end:
+
+ %i.1 = add nuw i64 %i, 1
+ %exitcond = icmp eq i64 %i.1, %N
+ br i1 %exitcond, label %for_i.end, label %for_i.body
+
+for_i.end:
+ ret void
+}
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