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Make isSafeToLoadUnconditionally a bit smarter, implementing PR362 and

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Regression/Transforms/InstCombine/CPP_min_max.llx


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@16409 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-09-19 19:18:10 +00:00
parent c2fa9894ee
commit 8a37520f80
1 changed files with 28 additions and 6 deletions
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34
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -2993,9 +2993,30 @@ static Instruction *InstCombineLoadCast(InstCombiner &IC, LoadInst &LI) {
}
/// isSafeToLoadUnconditionally - Return true if we know that executing a load
/// from this value cannot trap.
static bool isSafeToLoadUnconditionally(Value *V) {
return isa<AllocaInst>(V) || isa<GlobalVariable>(V);
/// from this value cannot trap. If it is not obviously safe to load from the
/// specified pointer, we do a quick local scan of the basic block containing
/// ScanFrom, to determine if the address is already accessed.
static bool isSafeToLoadUnconditionally(Value *V, Instruction *ScanFrom) {
// If it is an alloca or global variable, it is always safe to load from.
if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
// Otherwise, be a little bit agressive by scanning the local block where we
// want to check to see if the pointer is already being loaded or stored
// from/to. If so, the previous load or store would hav already trapped, so
// there is no harm doing an extra load (also, CSE will later eliminate the
// load entirely).
BasicBlock::iterator BBI = ScanFrom, E = ScanFrom->getParent()->begin();
do {
--BBI;
if (LoadInst *LI = dyn_cast<LoadInst>(BBI)) {
if (LI->getOperand(0) == V) return true;
} else if (StoreInst *SI = dyn_cast<StoreInst>(BBI))
if (SI->getOperand(1) == V) return true;
} while (BBI != E);
return false;
}
Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
@ -3040,8 +3061,8 @@ 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)) &&
isSafeToLoadUnconditionally(SI->getOperand(2))) {
if (isSafeToLoadUnconditionally(SI->getOperand(1), SI) &&
isSafeToLoadUnconditionally(SI->getOperand(2), SI)) {
Value *V1 = InsertNewInstBefore(new LoadInst(SI->getOperand(1),
SI->getOperand(1)->getName()+".val"), *SI);
Value *V2 = InsertNewInstBefore(new LoadInst(SI->getOperand(2),
@ -3053,7 +3074,8 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
// load (phi (&V1, &V2, &V3)) --> phi(load &V1, load &V2, load &V3)
bool Safe = true;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
if (!isSafeToLoadUnconditionally(PN->getIncomingValue(i))) {
if (!isSafeToLoadUnconditionally(PN->getIncomingValue(i),
PN->getIncomingBlock(i)->getTerminator())) {
Safe = false;
break;
}