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Check alignment of loads when deciding whether it is safe to execute them 

unconditionally.  Besides checking the offset, also check that the underlying
object is aligned as much as the load itself.

llvm-svn: 94875
This commit is contained in:
Bob Wilson 2010-01-30 04:42:39 +00:00
parent a18f0fbca2
commit 56600a15ad
6 changed files with 70 additions and 16 deletions
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2
llvm/include/llvm/Transforms/Utils/Local.h
View File

@ -39,7 +39,7 @@ template<typename T> class SmallVectorImpl;
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
const TargetData *TD = 0);
unsigned Align, const TargetData *TD = 0);
//===----------------------------------------------------------------------===//
// Local constant propagation.

13
llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
View File

@ -200,14 +200,15 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
//
if (SelectInst *SI = dyn_cast<SelectInst>(Op)) {
// load (select (Cond, &V1, &V2)) --> select(Cond, load &V1, load &V2).
if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, TD) &&
isSafeToLoadUnconditionally(SI->getOperand(2), SI, TD)) {
unsigned Align = LI.getAlignment();
if (isSafeToLoadUnconditionally(SI->getOperand(1), SI, Align, TD) &&
isSafeToLoadUnconditionally(SI->getOperand(2), SI, Align, TD)) {
LoadInst *V1 = Builder->CreateLoad(SI->getOperand(1),
SI->getOperand(1)->getName()+".val");
SI->getOperand(1)->getName()+".val");
LoadInst *V2 = Builder->CreateLoad(SI->getOperand(2),
SI->getOperand(2)->getName()+".val");
V1->setAlignment(LI.getAlignment());
V2->setAlignment(LI.getAlignment());
SI->getOperand(2)->getName()+".val");
V1->setAlignment(Align);
V2->setAlignment(Align);
return SelectInst::Create(SI->getCondition(), V1, V2);
}

3
llvm/lib/Transforms/Scalar/GVN.cpp
View File

@ -1651,7 +1651,8 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
if (!allSingleSucc &&
// FIXME: REEVALUTE THIS.
!isSafeToLoadUnconditionally(LoadPtr,
UnavailablePred->getTerminator(), TD)) {
UnavailablePred->getTerminator(),
LI->getAlignment(), TD)) {
assert(NewInsts.empty() && "Should not have inserted instructions");
return false;
}

3
llvm/lib/Transforms/Scalar/TailRecursionElimination.cpp
View File

@ -211,7 +211,8 @@ bool TailCallElim::CanMoveAboveCall(Instruction *I, CallInst *CI) {
// FIXME: Writes to memory only matter if they may alias the pointer
// being loaded from.
if (CI->mayWriteToMemory() ||
!isSafeToLoadUnconditionally(L->getPointerOperand(), L))
!isSafeToLoadUnconditionally(L->getPointerOperand(), L,
L->getAlignment()))
return false;
}
}

21
llvm/lib/Transforms/Utils/Local.cpp
View File

@ -75,31 +75,38 @@ static Value *getUnderlyingObjectWithOffset(Value *V, const TargetData *TD,
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
bool llvm::isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom,
const TargetData *TD) {
unsigned Align, const TargetData *TD) {
uint64_t ByteOffset = 0;
Value *Base = V;
if (TD)
Base = getUnderlyingObjectWithOffset(V, TD, ByteOffset);
const Type *BaseType = 0;
if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base))
// If it is an alloca it is always safe to load from.
unsigned BaseAlign = 0;
if (const AllocaInst *AI = dyn_cast<AllocaInst>(Base)) {
// An alloca is safe to load from as load as it is suitably aligned.
BaseType = AI->getAllocatedType();
else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) {
BaseAlign = AI->getAlignment();
} else if (const GlobalValue *GV = dyn_cast<GlobalValue>(Base)) {
// Global variables are safe to load from but their size cannot be
// guaranteed if they are overridden.
if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden())
if (!isa<GlobalAlias>(GV) && !GV->mayBeOverridden()) {
BaseType = GV->getType()->getElementType();
BaseAlign = GV->getAlignment();
}
}
if (TD && BaseType && BaseAlign == 0)
BaseAlign = TD->getPrefTypeAlignment(BaseType);
if (BaseType) {
if (BaseType && Align <= BaseAlign) {
if (!TD)
return true; // Loading directly from an alloca or global is OK.
if (BaseType->isSized()) {
// Check if the load is within the bounds of the underlying object.
const PointerType *AddrTy = cast<PointerType>(V->getType());
uint64_t LoadSize = TD->getTypeStoreSize(AddrTy->getElementType());
if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType))
if (ByteOffset + LoadSize <= TD->getTypeAllocSize(BaseType) &&
(Align == 0 || (ByteOffset % Align) == 0))
return true;
}
}

44
llvm/test/Transforms/GVN/load-pre-align.ll Normal file
View File

@ -0,0 +1,44 @@
; RUN: opt < %s -gvn -S | FileCheck %s
target datalayout = "e-p:32:32:32-i1:8:32-i8:8:32-i16:16:32-i32:32:32-i64:32:32-f32:32:32-f64:32:32-v64:64:64-v128:128:128-a0:0:32-n32"
@p = external global i32
define arm_apcscc i32 @test(i32 %n) nounwind {
; CHECK: @test
entry:
br label %for.cond
; loads aligned greater than the memory should not be moved past conditionals
; CHECK-NOT: load
; CHECK: br i1
for.cond:
%i.0 = phi i32 [ 0, %entry ], [ %indvar.next, %for.inc ]
%cmp = icmp slt i32 %i.0, %n
br i1 %cmp, label %for.body, label %for.cond.for.end_crit_edge
for.cond.for.end_crit_edge:
; ...but PRE can still move the load out of for.end to here.
; CHECK: for.cond.for.end_crit_edge:
; CHECK-NEXT: load
br label %for.end
for.body:
%tmp3 = load i32* @p, align 8
%dec = add i32 %tmp3, -1
store i32 %dec, i32* @p
%cmp6 = icmp slt i32 %dec, 0
br i1 %cmp6, label %for.body.for.end_crit_edge, label %for.inc
for.body.for.end_crit_edge:
br label %for.end
for.inc:
%indvar.next = add i32 %i.0, 1
br label %for.cond
for.end:
%tmp9 = load i32* @p, align 8
ret i32 %tmp9
}