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Enhance GVN to do more precise alias queries for non-local memory

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references. For example, this allows gvn to eliminate the load in
this example:

  void foo(int n, int* p, int *q) {
    p[0] = 0;
    p[1] = 1;
    if (n) {
      *q = p[0];
    }
  }

llvm-svn: 118714
This commit is contained in:
Dan Gohman 2010-11-10 20:37:15 +00:00
parent 859cfe5bbd
commit 2b4e8302a6
6 changed files with 129 additions and 27 deletions
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6
include/llvm/Analysis/AliasAnalysis.h
View File

@ -107,6 +107,12 @@ public:
return Copy;
}
Location getWithNewSize(uint64_t NewSize) const {
Location Copy(*this);
Copy.Size = NewSize;
return Copy;
}
Location getWithoutTBAATag() const {
Location Copy(*this);
Copy.TBAATag = 0;

15
include/llvm/Analysis/MemoryDependenceAnalysis.h
View File

@ -227,11 +227,14 @@ namespace llvm {
BBSkipFirstBlockPair Pair;
/// NonLocalDeps - The results of the query for each relevant block.
NonLocalDepInfo NonLocalDeps;
/// Size - The maximum size of the dereferences of the
/// pointer. May be UnknownSize if the sizes are unknown.
uint64_t Size;
/// TBAATag - The TBAA tag associated with dereferences of the
/// pointer. May be null if there are no tags or conflicting tags.
MDNode *TBAATag;
const MDNode *TBAATag;
NonLocalPointerInfo() : TBAATag(0) {}
NonLocalPointerInfo() : Size(0), TBAATag(0) {}
};
/// CachedNonLocalPointerInfo - This map stores the cached results of doing
@ -315,14 +318,6 @@ namespace llvm {
bool isLoad, BasicBlock *BB,
SmallVectorImpl<NonLocalDepResult> &Result);
/// getNonLocalPointerDependence - A convenience wrapper.
void getNonLocalPointerDependency(Value *Pointer, bool isLoad,
BasicBlock *BB,
SmallVectorImpl<NonLocalDepResult> &Result){
return getNonLocalPointerDependency(AliasAnalysis::Location(Pointer),
isLoad, BB, Result);
}
/// removeInstruction - Remove an instruction from the dependence analysis,
/// updating the dependence of instructions that previously depended on it.
void removeInstruction(Instruction *InstToRemove);

23
lib/Analysis/MemDepPrinter.cpp
View File

@ -11,6 +11,7 @@
//===----------------------------------------------------------------------===//
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/LLVMContext.h"
#include "llvm/Analysis/Passes.h"
#include "llvm/Assembly/Writer.h"
#include "llvm/Support/CallSite.h"
@ -40,7 +41,8 @@ namespace {
void print(raw_ostream &OS, const Module * = 0) const;
virtual void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<MemoryDependenceAnalysis>();
AU.addRequiredTransitive<AliasAnalysis>();
AU.addRequiredTransitive<MemoryDependenceAnalysis>();
AU.setPreservesAll();
}
@ -64,6 +66,7 @@ FunctionPass *llvm::createMemDepPrinter() {
bool MemDepPrinter::runOnFunction(Function &F) {
this->F = &F;
AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
MemoryDependenceAnalysis &MDA = getAnalysis<MemoryDependenceAnalysis>();
// All this code uses non-const interfaces because MemDep is not
@ -99,15 +102,23 @@ bool MemDepPrinter::runOnFunction(Function &F) {
SmallVector<NonLocalDepResult, 4> NLDI;
if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
// FIXME: Volatile is not handled properly here.
MDA.getNonLocalPointerDependency(LI->getPointerOperand(), !LI->isVolatile(),
AliasAnalysis::Location Loc(LI->getPointerOperand(),
AA.getTypeStoreSize(LI->getType()),
LI->getMetadata(LLVMContext::MD_tbaa));
MDA.getNonLocalPointerDependency(Loc, !LI->isVolatile(),
LI->getParent(), NLDI);
} else if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
// FIXME: Volatile is not handled properly here.
MDA.getNonLocalPointerDependency(SI->getPointerOperand(), false,
SI->getParent(), NLDI);
AliasAnalysis::Location Loc(SI->getPointerOperand(),
AA.getTypeStoreSize(SI->getValueOperand()
->getType()),
SI->getMetadata(LLVMContext::MD_tbaa));
MDA.getNonLocalPointerDependency(Loc, false, SI->getParent(), NLDI);
} else if (VAArgInst *VI = dyn_cast<VAArgInst>(Inst)) {
MDA.getNonLocalPointerDependency(VI->getPointerOperand(), false,
VI->getParent(), NLDI);
AliasAnalysis::Location Loc(SI->getPointerOperand(),
AliasAnalysis::UnknownSize,
SI->getMetadata(LLVMContext::MD_tbaa));
MDA.getNonLocalPointerDependency(Loc, false, VI->getParent(), NLDI);
} else {
llvm_unreachable("Unknown memory instruction!");
}

47
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -741,16 +741,40 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
// Look up the cached info for Pointer.
ValueIsLoadPair CacheKey(Pointer.getAddr(), isLoad);
NonLocalPointerInfo *CacheInfo = &NonLocalPointerDeps[CacheKey];
// If this query's TBAATag is inconsistent with the cached one, discard the
// tag and restart the query.
if (CacheInfo->TBAATag != Loc.TBAATag) {
CacheInfo->TBAATag = 0;
NonLocalPointerDeps.erase(CacheKey);
return getNonLocalPointerDepFromBB(Pointer, Loc.getWithoutTBAATag(),
isLoad, StartBB, Result, Visited,
SkipFirstBlock);
// Set up a temporary NLPI value. If the map doesn't yet have an entry for
// CacheKey, this value will be inserted as the associated value. Otherwise,
// it'll be ignored, and we'll have to check to see if the cached size and
// tbaa tag are consistent with the current query.
NonLocalPointerInfo InitialNLPI;
InitialNLPI.Size = Loc.Size;
InitialNLPI.TBAATag = Loc.TBAATag;
// Get the NLPI for CacheKey, inserting one into the map if it doesn't
// already have one.
std::pair<CachedNonLocalPointerInfo::iterator, bool> Pair =
NonLocalPointerDeps.insert(std::make_pair(CacheKey, InitialNLPI));
NonLocalPointerInfo *CacheInfo = &Pair.first->second;
if (!Pair.second) {
// If this query's Size is inconsistent with the cached one, take the
// maximum size and restart the query.
if (CacheInfo->Size != Loc.Size) {
CacheInfo->Size = std::max(CacheInfo->Size, Loc.Size);
return getNonLocalPointerDepFromBB(Pointer,
Loc.getWithNewSize(CacheInfo->Size),
isLoad, StartBB, Result, Visited,
SkipFirstBlock);
}
// If this query's TBAATag is inconsistent with the cached one, discard the
// tag and restart the query.
if (CacheInfo->TBAATag != Loc.TBAATag) {
CacheInfo->TBAATag = 0;
return getNonLocalPointerDepFromBB(Pointer, Loc.getWithoutTBAATag(),
isLoad, StartBB, Result, Visited,
SkipFirstBlock);
}
}
NonLocalDepInfo *Cache = &CacheInfo->NonLocalDeps;
@ -796,6 +820,7 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
CacheInfo->Pair = BBSkipFirstBlockPair(StartBB, SkipFirstBlock);
else {
CacheInfo->Pair = BBSkipFirstBlockPair();
CacheInfo->Size = 0;
CacheInfo->TBAATag = 0;
}
@ -921,6 +946,7 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
// cached value to do more work but not miss the phi trans failure.
NonLocalPointerInfo &NLPI = NonLocalPointerDeps[CacheKey];
NLPI.Pair = BBSkipFirstBlockPair();
NLPI.Size = 0;
NLPI.TBAATag = 0;
continue;
}
@ -949,6 +975,7 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
// specific block queries) but we can't do the fastpath "return all
// results from the set" Clear out the indicator for this.
CacheInfo->Pair = BBSkipFirstBlockPair();
CacheInfo->Size = 0;
CacheInfo->TBAATag = 0;
SkipFirstBlock = false;
continue;
@ -967,6 +994,7 @@ getNonLocalPointerDepFromBB(const PHITransAddr &Pointer,
// specific block queries) but we can't do the fastpath "return all
// results from the set". Clear out the indicator for this.
CacheInfo->Pair = BBSkipFirstBlockPair();
CacheInfo->Size = 0;
CacheInfo->TBAATag = 0;
// If *nothing* works, mark the pointer as being clobbered by the first
@ -1184,6 +1212,7 @@ void MemoryDependenceAnalysis::removeInstruction(Instruction *RemInst) {
// The cache is not valid for any specific block anymore.
NonLocalPointerDeps[P].Pair = BBSkipFirstBlockPair();
NonLocalPointerDeps[P].Size = 0;
NonLocalPointerDeps[P].TBAATag = 0;
// Update any entries for RemInst to use the instruction after it.

6
lib/Transforms/Scalar/GVN.cpp
View File

@ -1351,8 +1351,10 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
SmallVectorImpl<Instruction*> &toErase) {
// Find the non-local dependencies of the load.
SmallVector<NonLocalDepResult, 64> Deps;
MD->getNonLocalPointerDependency(LI->getPointerOperand(), true,
LI->getParent(),
AliasAnalysis::Location Loc(LI->getPointerOperand(),
VN.getAliasAnalysis()->getTypeStoreSize(LI->getType()),
LI->getMetadata(LLVMContext::MD_tbaa));
MD->getNonLocalPointerDependency(Loc, true, LI->getParent(),
Deps);
//DEBUG(dbgs() << "INVESTIGATING NONLOCAL LOAD: "
// << Deps.size() << *LI << '\n');

59
test/Transforms/GVN/non-local-offset.ll Normal file
View File

@ -0,0 +1,59 @@
; RUN: opt -basicaa -gvn -S < %s | FileCheck %s
target datalayout = "e-p:64:64:64"
; GVN should ignore the store to p[1] to see that the load from p[0] is
; fully redundant.
; CHECK: @yes
; CHECK: if.then:
; CHECK-NEXT: store i32 0, i32* %q
; CHECK-NEXT: ret void
define void @yes(i1 %c, i32* %p, i32* %q) nounwind {
entry:
store i32 0, i32* %p
%p1 = getelementptr inbounds i32* %p, i64 1
store i32 1, i32* %p1
br i1 %c, label %if.else, label %if.then
if.then:
%t = load i32* %p
store i32 %t, i32* %q
ret void
if.else:
ret void
}
; GVN should ignore the store to p[1] to see that the first load from p[0] is
; fully redundant. However, the second load is larger, so it's not a simple
; redundancy.
; CHECK: @watch_out_for_size_change
; CHECK: if.then:
; CHECK-NEXT: store i32 0, i32* %q
; CHECK-NEXT: ret void
; CHECK: if.else:
; CHECK: load i64* %pc
; CHECK: store i64
define void @watch_out_for_size_change(i1 %c, i32* %p, i32* %q) nounwind {
entry:
store i32 0, i32* %p
%p1 = getelementptr inbounds i32* %p, i64 1
store i32 1, i32* %p1
br i1 %c, label %if.else, label %if.then
if.then:
%t = load i32* %p
store i32 %t, i32* %q
ret void
if.else:
%pc = bitcast i32* %p to i64*
%qc = bitcast i32* %q to i64*
%t64 = load i64* %pc
store i64 %t64, i64* %qc
ret void
}