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DeadStoreElimination can now trim the size of a store if the end of the store is dead.

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Currently checks alignment and killing stores on a power of 2 boundary as this is likely
to trim the size of the earlier store without breaking large vector stores into scalar ones.

Fixes <rdar://problem/10140300>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@144239 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Pete Cooper 2011-11-09 23:07:35 +00:00
parent ffab7d0c4f
commit 5ccb0825ed
2 changed files with 187 additions and 32 deletions
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141
lib/Transforms/Scalar/DeadStoreElimination.cpp
View File

@ -241,6 +241,24 @@ static bool isRemovable(Instruction *I) {
}
}
/// isShortenable - Returns true if this instruction can be safely shortened in
/// length.
static bool isShortenable(Instruction *I) {
// Don't shorten stores for now
if (isa<StoreInst>(I))
return false;
IntrinsicInst *II = cast<IntrinsicInst>(I);
switch (II->getIntrinsicID()) {
default: return false;
case Intrinsic::memset:
case Intrinsic::memcpy:
// Do shorten memory intrinsics.
return true;
}
}
/// getStoredPointerOperand - Return the pointer that is being written to.
static Value *getStoredPointerOperand(Instruction *I) {
if (StoreInst *SI = dyn_cast<StoreInst>(I))
@ -295,11 +313,24 @@ static bool isObjectPointerWithTrustworthySize(const Value *V) {
return false;
}
/// isCompleteOverwrite - Return true if a store to the 'Later' location
namespace {
enum OverwriteResult
{
OverwriteComplete,
OverwriteEnd,
OverwriteUnknown
};
}
/// isOverwrite - Return 'OverwriteComplete' if a store to the 'Later' location
/// completely overwrites a store to the 'Earlier' location.
static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
const AliasAnalysis::Location &Earlier,
AliasAnalysis &AA) {
/// 'OverwriteEnd' if the end of the 'Earlier' location is completely
/// overwritten by 'Later', or 'OverwriteUnknown' if nothing can be determined
static OverwriteResult isOverwrite(const AliasAnalysis::Location &Later,
const AliasAnalysis::Location &Earlier,
AliasAnalysis &AA,
int64_t& EarlierOff,
int64_t& LaterOff) {
const Value *P1 = Earlier.Ptr->stripPointerCasts();
const Value *P2 = Later.Ptr->stripPointerCasts();
@ -313,23 +344,24 @@ static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
// If we have no TargetData information around, then the size of the store
// is inferrable from the pointee type. If they are the same type, then
// we know that the store is safe.
if (AA.getTargetData() == 0)
return Later.Ptr->getType() == Earlier.Ptr->getType();
return false;
if (AA.getTargetData() == 0 &&
Later.Ptr->getType() == Earlier.Ptr->getType())
return OverwriteComplete;
return OverwriteUnknown;
}
// Make sure that the Later size is >= the Earlier size.
if (Later.Size < Earlier.Size)
return false;
return true;
if (Later.Size >= Earlier.Size)
return OverwriteComplete;
}
// Otherwise, we have to have size information, and the later store has to be
// larger than the earlier one.
if (Later.Size == AliasAnalysis::UnknownSize ||
Earlier.Size == AliasAnalysis::UnknownSize ||
Later.Size <= Earlier.Size || AA.getTargetData() == 0)
return false;
AA.getTargetData() == 0)
return OverwriteUnknown;
// Check to see if the later store is to the entire object (either a global,
// an alloca, or a byval argument). If so, then it clearly overwrites any
@ -342,26 +374,27 @@ static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
// If we can't resolve the same pointers to the same object, then we can't
// analyze them at all.
if (UO1 != UO2)
return false;
return OverwriteUnknown;
// If the "Later" store is to a recognizable object, get its size.
if (isObjectPointerWithTrustworthySize(UO2)) {
uint64_t ObjectSize =
TD.getTypeAllocSize(cast<PointerType>(UO2->getType())->getElementType());
if (ObjectSize == Later.Size)
return true;
return OverwriteComplete;
}
// Okay, we have stores to two completely different pointers. Try to
// decompose the pointer into a "base + constant_offset" form. If the base
// pointers are equal, then we can reason about the two stores.
int64_t EarlierOff = 0, LaterOff = 0;
EarlierOff = 0;
LaterOff = 0;
const Value *BP1 = GetPointerBaseWithConstantOffset(P1, EarlierOff, TD);
const Value *BP2 = GetPointerBaseWithConstantOffset(P2, LaterOff, TD);
// If the base pointers still differ, we have two completely different stores.
if (BP1 != BP2)
return false;
return OverwriteUnknown;
// The later store completely overlaps the earlier store if:
//
@ -379,11 +412,25 @@ static bool isCompleteOverwrite(const AliasAnalysis::Location &Later,
//
// We have to be careful here as *Off is signed while *.Size is unsigned.
if (EarlierOff >= LaterOff &&
Later.Size > Earlier.Size &&
uint64_t(EarlierOff - LaterOff) + Earlier.Size <= Later.Size)
return true;
return OverwriteComplete;
// The other interesting case is if the later store overwrites the end of
// the earlier store
//
// |--earlier--|
// |-- later --|
//
// In this case we may want to trim the size of earlier to avoid generating
// writes to addresses which will definitely be overwritten later
if (LaterOff > EarlierOff &&
LaterOff < int64_t(EarlierOff + Earlier.Size) &&
LaterOff + Later.Size >= EarlierOff + Earlier.Size)
return OverwriteEnd;
// Otherwise, they don't completely overlap.
return false;
return OverwriteUnknown;
}
/// isPossibleSelfRead - If 'Inst' might be a self read (i.e. a noop copy of a
@ -507,22 +554,52 @@ bool DSE::runOnBasicBlock(BasicBlock &BB) {
// If we find a write that is a) removable (i.e., non-volatile), b) is
// completely obliterated by the store to 'Loc', and c) which we know that
// 'Inst' doesn't load from, then we can remove it.
if (isRemovable(DepWrite) && isCompleteOverwrite(Loc, DepLoc, *AA) &&
if (isRemovable(DepWrite) &&
!isPossibleSelfRead(Inst, Loc, DepWrite, *AA)) {
DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
<< *DepWrite << "\n KILLER: " << *Inst << '\n');
int64_t InstWriteOffset, DepWriteOffset;
OverwriteResult OR = isOverwrite(Loc, DepLoc, *AA,
DepWriteOffset, InstWriteOffset);
if (OR == OverwriteComplete) {
DEBUG(dbgs() << "DSE: Remove Dead Store:\n DEAD: "
<< *DepWrite << "\n KILLER: " << *Inst << '\n');
// Delete the store and now-dead instructions that feed it.
DeleteDeadInstruction(DepWrite, *MD);
++NumFastStores;
MadeChange = true;
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
--BBI;
break;
// Delete the store and now-dead instructions that feed it.
DeleteDeadInstruction(DepWrite, *MD);
++NumFastStores;
MadeChange = true;
// DeleteDeadInstruction can delete the current instruction in loop
// cases, reset BBI.
BBI = Inst;
if (BBI != BB.begin())
--BBI;
break;
} else if (OR == OverwriteEnd && isShortenable(DepWrite)) {
// TODO: base this on the target vector size so that if the earlier
// store was too small to get vector writes anyway then its likely
// a good idea to shorten it
// Power of 2 vector writes are probably always a bad idea to optimize
// as any store/memset/memcpy is likely using vector instructions so
// shortening it to not vector size is likely to be slower
MemIntrinsic* DepIntrinsic = cast<MemIntrinsic>(DepWrite);
unsigned DepWriteAlign = DepIntrinsic->getAlignment();
if (llvm::isPowerOf2_64(InstWriteOffset) ||
((DepWriteAlign != 0) && InstWriteOffset % DepWriteAlign == 0)) {
DEBUG(dbgs() << "DSE: Remove Dead Store:\n OW END: "
<< *DepWrite << "\n KILLER (offset "
<< InstWriteOffset << ", "
<< DepLoc.Size << ")"
<< *Inst << '\n');
Value* DepWriteLength = DepIntrinsic->getLength();
Value* TrimmedLength = ConstantInt::get(DepWriteLength->getType(),
InstWriteOffset -
DepWriteOffset);
DepIntrinsic->setLength(TrimmedLength);
MadeChange = true;
}
}
}
// If this is a may-aliased store that is clobbering the store value, we

78
test/Transforms/DeadStoreElimination/OverwriteStoreEnd.ll Normal file
View File

@ -0,0 +1,78 @@
; RUN: opt < %s -basicaa -dse -S | 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-S128"
%struct.vec2 = type { <4 x i32>, <4 x i32> }
%struct.vec2plusi = type { <4 x i32>, <4 x i32>, i32 }
@glob1 = global %struct.vec2 zeroinitializer, align 16
@glob2 = global %struct.vec2plusi zeroinitializer, align 16
define void @write24to28(i32* nocapture %p) nounwind uwtable ssp {
; CHECK: @write24to28
entry:
%arrayidx0 = getelementptr inbounds i32* %p, i64 1
%p3 = bitcast i32* %arrayidx0 to i8*
; CHECK: call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 24, i32 4, i1 false)
call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 28, i32 4, i1 false)
%arrayidx1 = getelementptr inbounds i32* %p, i64 7
store i32 1, i32* %arrayidx1, align 4
ret void
}
define void @write28to32(i32* nocapture %p) nounwind uwtable ssp {
; CHECK: @write28to32
entry:
%p3 = bitcast i32* %p to i8*
; CHECK: call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 28, i32 4, i1 false)
call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 32, i32 4, i1 false)
%arrayidx1 = getelementptr inbounds i32* %p, i64 7
store i32 1, i32* %arrayidx1, align 4
ret void
}
define void @dontwrite28to32memset(i32* nocapture %p) nounwind uwtable ssp {
; CHECK: @dontwrite28to32memset
entry:
%p3 = bitcast i32* %p to i8*
; CHECK: call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 32, i32 16, i1 false)
call void @llvm.memset.p0i8.i64(i8* %p3, i8 0, i64 32, i32 16, i1 false)
%arrayidx1 = getelementptr inbounds i32* %p, i64 7
store i32 1, i32* %arrayidx1, align 4
ret void
}
define void @write32to36(%struct.vec2plusi* nocapture %p) nounwind uwtable ssp {
; CHECK: @write32to36
entry:
%0 = bitcast %struct.vec2plusi* %p to i8*
; CHECK: tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2plusi* @glob2 to i8*), i64 32, i32 16, i1 false)
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2plusi* @glob2 to i8*), i64 36, i32 16, i1 false)
%c = getelementptr inbounds %struct.vec2plusi* %p, i64 0, i32 2
store i32 1, i32* %c, align 4
ret void
}
define void @write16to32(%struct.vec2* nocapture %p) nounwind uwtable ssp {
; CHECK: @write16to32
entry:
%0 = bitcast %struct.vec2* %p to i8*
; CHECK: tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2* @glob1 to i8*), i64 16, i32 16, i1 false)
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2* @glob1 to i8*), i64 32, i32 16, i1 false)
%c = getelementptr inbounds %struct.vec2* %p, i64 0, i32 1
store <4 x i32> <i32 1, i32 2, i32 3, i32 4>, <4 x i32>* %c, align 4
ret void
}
define void @dontwrite28to32memcpy(%struct.vec2* nocapture %p) nounwind uwtable ssp {
; CHECK: @dontwrite28to32memcpy
entry:
%0 = bitcast %struct.vec2* %p to i8*
; CHECK: tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2* @glob1 to i8*), i64 32, i32 16, i1 false)
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* %0, i8* bitcast (%struct.vec2* @glob1 to i8*), i64 32, i32 16, i1 false)
%arrayidx1 = getelementptr inbounds %struct.vec2* %p, i64 0, i32 0, i64 7
store i32 1, i32* %arrayidx1, align 4
ret void
}
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture, i8* nocapture, i64, i32, i1) nounwind
declare void @llvm.memset.p0i8.i64(i8* nocapture, i8, i64, i32, i1) nounwind