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Enhance MemDep: When alias analysis returns a partial alias result,

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return it as a clobber.  This allows GVN to do smart things.

Enhance GVN to be smart about the case when a small load is clobbered
by a larger overlapping load.  In this case, forward the value.  This
allows us to compile stuff like this:

int test(void *P) {
  int tmp = *(unsigned int*)P;
  return tmp+*((unsigned char*)P+1);
}

into:

_test:                                  ## @test
	movl	(%rdi), %ecx
	movzbl	%ch, %eax
	addl	%ecx, %eax
	ret

which has one load.  We already handled the case where the smaller
load was from a must-aliased base pointer.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@130180 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2011-04-26 01:21:15 +00:00
parent 0c99861836
commit 1f821512fc
4 changed files with 125 additions and 29 deletions
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5
include/llvm/Analysis/MemoryDependenceAnalysis.h
View File

@ -48,6 +48,11 @@ namespace llvm {
/// this occurs when we see a may-aliased store to the memory location we
/// care about.
///
/// There are several cases that may be interesting here:
/// 1. Loads are clobbered by may-alias stores.
/// 2. Loads are considered clobbered by partially-aliased loads. The
/// client may choose to analyze deeper into these cases.
///
/// A dependence query on the first instruction of the entry block will
/// return a clobber(self) result.
Clobber,

20
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -291,16 +291,26 @@ getPointerDependencyFrom(const AliasAnalysis::Location &MemLoc, bool isLoad,
if (R == AliasAnalysis::NoAlias)
continue;
// May-alias loads don't depend on each other without a dependence.
if (isLoad && R != AliasAnalysis::MustAlias)
if (isLoad) {
// Must aliased loads are defs of each other.
if (R == AliasAnalysis::MustAlias)
return MemDepResult::getDef(Inst);
// If we have a partial alias, then return this as a clobber for the
// client to handle.
if (R == AliasAnalysis::PartialAlias)
return MemDepResult::getClobber(Inst);
// Random may-alias loads don't depend on each other without a
// dependence.
continue;
}
// Stores don't alias loads from read-only memory.
if (!isLoad && AA->pointsToConstantMemory(LoadLoc))
if (AA->pointsToConstantMemory(LoadLoc))
continue;
// Stores depend on may and must aliased loads, loads depend on must-alias
// loads.
// Stores depend on may/must aliased loads.
return MemDepResult::getDef(Inst);
}

102
lib/Transforms/Scalar/GVN.cpp
View File

@ -636,10 +636,9 @@ static Value *CoerceAvailableValueToLoadType(Value *StoredVal,
// If the store and reload are the same size, we can always reuse it.
if (StoreSize == LoadSize) {
if (StoredValTy->isPointerTy() && LoadedTy->isPointerTy()) {
// Pointer to Pointer -> use bitcast.
// Pointer to Pointer -> use bitcast.
if (StoredValTy->isPointerTy() && LoadedTy->isPointerTy())
return new BitCastInst(StoredVal, LoadedTy, "", InsertPt);
}
// Convert source pointers to integers, which can be bitcast.
if (StoredValTy->isPointerTy()) {
@ -796,6 +795,22 @@ static int AnalyzeLoadFromClobberingStore(const Type *LoadTy, Value *LoadPtr,
StorePtr, StoreSize, TD);
}
/// AnalyzeLoadFromClobberingLoad - This function is called when we have a
/// memdep query of a load that ends up being clobbered by another load. See if
/// the other load can feed into the second load.
static int AnalyzeLoadFromClobberingLoad(const Type *LoadTy, Value *LoadPtr,
LoadInst *DepLI, const TargetData &TD){
// Cannot handle reading from store of first-class aggregate yet.
if (DepLI->getType()->isStructTy() || DepLI->getType()->isArrayTy())
return -1;
Value *DepPtr = DepLI->getPointerOperand();
uint64_t DepSize = TD.getTypeSizeInBits(DepLI->getType());
return AnalyzeLoadFromClobberingWrite(LoadTy, LoadPtr, DepPtr, DepSize, TD);
}
static int AnalyzeLoadFromClobberingMemInst(const Type *LoadTy, Value *LoadPtr,
MemIntrinsic *MI,
const TargetData &TD) {
@ -1129,6 +1144,26 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
}
}
}
// Check to see if we have something like this:
// load i32* P
// load i8* (P+1)
// if we have this, replace the later with an extraction from the former.
if (LoadInst *DepLI = dyn_cast<LoadInst>(DepInfo.getInst())) {
// If this is a clobber and L is the first instruction in its block, then
// we have the first instruction in the entry block.
if (DepLI != LI && Address && TD) {
int Offset = AnalyzeLoadFromClobberingLoad(LI->getType(),
LI->getPointerOperand(),
DepLI, *TD);
if (Offset != -1) {
ValuesPerBlock.push_back(AvailableValueInBlock::get(DepBB, DepLI,
Offset));
continue;
}
}
}
// If the clobbering value is a memset/memcpy/memmove, see if we can
// forward a value on from it.
@ -1454,8 +1489,9 @@ bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
// ... to a pointer that has been loaded from before...
MemDepResult Dep = MD->getDependency(L);
// If the value isn't available, don't do anything!
if (Dep.isClobber()) {
// If we have a clobber and target data is around, see if this is a clobber
// that we can fix up through code synthesis.
if (Dep.isClobber() && TD) {
// Check to see if we have something like this:
// store i32 123, i32* %P
// %A = bitcast i32* %P to i8*
@ -1467,26 +1503,40 @@ bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
// completely covers this load. This sort of thing can happen in bitfield
// access code.
Value *AvailVal = 0;
if (StoreInst *DepSI = dyn_cast<StoreInst>(Dep.getInst()))
if (TD) {
int Offset = AnalyzeLoadFromClobberingStore(L->getType(),
L->getPointerOperand(),
DepSI, *TD);
if (Offset != -1)
AvailVal = GetStoreValueForLoad(DepSI->getValueOperand(), Offset,
L->getType(), L, *TD);
}
if (StoreInst *DepSI = dyn_cast<StoreInst>(Dep.getInst())) {
int Offset = AnalyzeLoadFromClobberingStore(L->getType(),
L->getPointerOperand(),
DepSI, *TD);
if (Offset != -1)
AvailVal = GetStoreValueForLoad(DepSI->getValueOperand(), Offset,
L->getType(), L, *TD);
}
// Check to see if we have something like this:
// load i32* P
// load i8* (P+1)
// if we have this, replace the later with an extraction from the former.
if (LoadInst *DepLI = dyn_cast<LoadInst>(Dep.getInst())) {
// If this is a clobber and L is the first instruction in its block, then
// we have the first instruction in the entry block.
if (DepLI == L)
return false;
int Offset = AnalyzeLoadFromClobberingLoad(L->getType(),
L->getPointerOperand(),
DepLI, *TD);
if (Offset != -1)
AvailVal = GetStoreValueForLoad(DepLI, Offset, L->getType(), L, *TD);
}
// If the clobbering value is a memset/memcpy/memmove, see if we can forward
// a value on from it.
if (MemIntrinsic *DepMI = dyn_cast<MemIntrinsic>(Dep.getInst())) {
if (TD) {
int Offset = AnalyzeLoadFromClobberingMemInst(L->getType(),
L->getPointerOperand(),
DepMI, *TD);
if (Offset != -1)
AvailVal = GetMemInstValueForLoad(DepMI, Offset, L->getType(), L,*TD);
}
int Offset = AnalyzeLoadFromClobberingMemInst(L->getType(),
L->getPointerOperand(),
DepMI, *TD);
if (Offset != -1)
AvailVal = GetMemInstValueForLoad(DepMI, Offset, L->getType(), L, *TD);
}
if (AvailVal) {
@ -1502,9 +1552,12 @@ bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
++NumGVNLoad;
return true;
}
}
// If the value isn't available, don't do anything!
if (Dep.isClobber()) {
DEBUG(
// fast print dep, using operator<< on instruction would be too slow
// fast print dep, using operator<< on instruction is too slow.
dbgs() << "GVN: load ";
WriteAsOperand(dbgs(), L);
Instruction *I = Dep.getInst();
@ -1556,7 +1609,8 @@ bool GVN::processLoad(LoadInst *L, SmallVectorImpl<Instruction*> &toErase) {
// (depending on its type).
if (DepLI->getType() != L->getType()) {
if (TD) {
AvailableVal = CoerceAvailableValueToLoadType(DepLI, L->getType(), L,*TD);
AvailableVal = CoerceAvailableValueToLoadType(DepLI, L->getType(),
L, *TD);
if (AvailableVal == 0)
return false;

27
test/Transforms/GVN/rle.ll
View File

@ -544,3 +544,30 @@ entry:
; CHECK: ret i32 0
}
;;===----------------------------------------------------------------------===;;
;; Load -> Load forwarding in partial alias case.
;;===----------------------------------------------------------------------===;;
define i32 @load_load_partial_alias(i8* %P) nounwind ssp {
entry:
%0 = bitcast i8* %P to i32*
%tmp2 = load i32* %0
%add.ptr = getelementptr inbounds i8* %P, i64 1
%tmp5 = load i8* %add.ptr
%conv = zext i8 %tmp5 to i32
%add = add nsw i32 %tmp2, %conv
ret i32 %add
; CHECK: @load_load_partial_alias
; CHECK: load i32*
; CHECK-NOT: load
; CHECK: lshr i32 {{.*}}, 8
; CHECK-NOT: load
; CHECK: trunc i32 {{.*}} to i8
; CHECK-NOT: load
; CHECK: ret i32
}