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

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


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@94875 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bob Wilson 2010-01-30 04:42:39 +00:00
parent a6bff982c1
commit 49db68fba0
6 changed files with 70 additions and 16 deletions
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2
include/llvm/Transforms/Utils/Local.h
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@ -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
lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
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@ -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
lib/Transforms/Scalar/GVN.cpp
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@ -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
lib/Transforms/Scalar/TailRecursionElimination.cpp
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@ -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
lib/Transforms/Utils/Local.cpp
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@ -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
test/Transforms/GVN/load-pre-align.ll Normal file
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@ -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
}