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teach GVN's load PRE to insert computations of the address in predecessors

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where it is not available.  It's unclear how to get this inserted 
computation into GVN's scalar availability sets, Owen, help? :)


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@89997 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-11-27 08:25:10 +00:00
parent d280d85791
commit 616613d7a4
4 changed files with 135 additions and 24 deletions
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7
include/llvm/Analysis/MemoryDependenceAnalysis.h
View File

@ -250,6 +250,13 @@ namespace llvm {
Value *PHITranslatePointer(Value *V,
BasicBlock *CurBB, BasicBlock *PredBB,
const TargetData *TD) const;
/// InsertPHITranslatedPointer - Insert a computation of the PHI translated
/// version of 'V' for the edge PredBB->CurBB into the end of the PredBB
/// block.
Value *InsertPHITranslatedPointer(Value *V,
BasicBlock *CurBB, BasicBlock *PredBB,
const TargetData *TD) const;
/// removeInstruction - Remove an instruction from the dependence analysis,
/// updating the dependence of instructions that previously depended on it.

72
lib/Analysis/MemoryDependenceAnalysis.cpp
View File

@ -752,19 +752,35 @@ PHITranslatePointer(Value *InVal, BasicBlock *CurBB, BasicBlock *Pred,
// Handle getelementptr with at least one PHI operand.
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Inst)) {
SmallVector<Value*, 8> GEPOps;
Value *APHIOp = 0;
BasicBlock *CurBB = GEP->getParent();
for (unsigned i = 0, e = GEP->getNumOperands(); i != e; ++i) {
GEPOps.push_back(GEP->getOperand(i)->DoPHITranslation(CurBB, Pred));
if (!isa<Constant>(GEPOps.back()))
APHIOp = GEPOps.back();
Value *GEPOp = GEP->getOperand(i);
// No PHI translation is needed of operands whose values are live in to
// the predecessor block.
if (!isa<Instruction>(GEPOp) ||
cast<Instruction>(GEPOp)->getParent() != CurBB) {
GEPOps.push_back(GEPOp);
continue;
}
// If the operand is a phi node, do phi translation.
if (PHINode *PN = dyn_cast<PHINode>(GEPOp)) {
GEPOps.push_back(PN->getIncomingValueForBlock(Pred));
continue;
}
// Otherwise, we can't PHI translate this random value defined in this
// block.
return 0;
}
// Simplify the GEP to handle 'gep x, 0' -> x etc.
if (Value *V = SimplifyGEPInst(&GEPOps[0], GEPOps.size(), TD))
return V;
// Scan to see if we have this GEP available.
Value *APHIOp = GEPOps[0];
for (Value::use_iterator UI = APHIOp->use_begin(), E = APHIOp->use_end();
UI != E; ++UI) {
if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(*UI))
@ -787,6 +803,52 @@ PHITranslatePointer(Value *InVal, BasicBlock *CurBB, BasicBlock *Pred,
return 0;
}
/// InsertPHITranslatedPointer - Insert a computation of the PHI translated
/// version of 'V' for the edge PredBB->CurBB into the end of the PredBB
/// block.
///
/// This is only called when PHITranslatePointer returns a value that doesn't
/// dominate the block, so we don't need to handle the trivial cases here.
Value *MemoryDependenceAnalysis::
InsertPHITranslatedPointer(Value *InVal, BasicBlock *CurBB,
BasicBlock *PredBB, const TargetData *TD) const {
// If the input value isn't an instruction in CurBB, it doesn't need phi
// translation.
Instruction *Inst = cast<Instruction>(InVal);
assert(Inst->getParent() == CurBB && "Doesn't need phi trans");
// Handle bitcast of PHI.
if (BitCastInst *BC = dyn_cast<BitCastInst>(Inst)) {
PHINode *BCPN = cast<PHINode>(BC->getOperand(0));
Value *PHIIn = BCPN->getIncomingValueForBlock(PredBB);
// Otherwise insert a bitcast at the end of PredBB.
return new BitCastInst(PHIIn, InVal->getType(),
InVal->getName()+".phi.trans.insert",
PredBB->getTerminator());
}
// Handle getelementptr with at least one PHI operand.
if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Inst)) {
SmallVector<Value*, 8> GEPOps;
Value *APHIOp = 0;
BasicBlock *CurBB = GEP->getParent();
for (unsigned i = 0, e = GEP->getNumOperands(); i != e; ++i) {
GEPOps.push_back(GEP->getOperand(i)->DoPHITranslation(CurBB, PredBB));
if (!isa<Constant>(GEPOps.back()))
APHIOp = GEPOps.back();
}
GetElementPtrInst *Result =
GetElementPtrInst::Create(GEPOps[0], GEPOps.begin()+1, GEPOps.end(),
InVal->getName()+".phi.trans.insert",
PredBB->getTerminator());
Result->setIsInBounds(GEP->isInBounds());
return Result;
}
return 0;
}
/// getNonLocalPointerDepFromBB - Perform a dependency query based on
/// pointer/pointeesize starting at the end of StartBB. Add any clobber/def

46
lib/Transforms/Scalar/GVN.cpp
View File

@ -1425,32 +1425,40 @@ bool GVN::processNonLocalLoad(LoadInst *LI,
assert(UnavailablePred != 0 &&
"Fully available value should be eliminated above!");
// If the loaded pointer is PHI node defined in this block, do PHI translation
// to get its value in the predecessor.
Value *LoadPtr = MD->PHITranslatePointer(LI->getOperand(0),
LoadBB, UnavailablePred, TD);
if (LoadPtr == 0) {
DEBUG(errs() << "COULDN'T PRE LOAD BECAUSE PTR CAN'T BE PHI TRANSLATED: "
<< *LI->getOperand(0) << '\n' << *LI << "\n");
return false;
}
// Make sure the value is live in the predecessor. If it was defined by a
// non-PHI instruction in this block, we don't know how to recompute it above.
if (Instruction *LPInst = dyn_cast<Instruction>(LoadPtr))
if (!DT->dominates(LPInst->getParent(), UnavailablePred)) {
DEBUG(errs() << "COULDN'T PRE LOAD BECAUSE PTR DOES NOT DOMINATE PRED: "
<< *LPInst << '\n' << *LI << "\n");
return false;
}
// We don't currently handle critical edges :(
if (UnavailablePred->getTerminator()->getNumSuccessors() != 1) {
DEBUG(errs() << "COULD NOT PRE LOAD BECAUSE OF CRITICAL EDGE '"
<< UnavailablePred->getName() << "': " << *LI << '\n');
return false;
}
// If the loaded pointer is PHI node defined in this block, do PHI translation
// to get its value in the predecessor.
Value *LoadPtr = MD->PHITranslatePointer(LI->getOperand(0),
LoadBB, UnavailablePred, TD);
// Make sure the value is live in the predecessor. MemDep found a computation
// of LPInst with the right value, but that does not dominate UnavailablePred,
// then we can't use it.
if (Instruction *LPInst = dyn_cast_or_null<Instruction>(LoadPtr))
if (!DT->dominates(LPInst->getParent(), UnavailablePred))
LoadPtr = 0;
// If we don't have a computation of this phi translated value, try to insert
// one.
if (LoadPtr == 0) {
LoadPtr = MD->InsertPHITranslatedPointer(LI->getOperand(0),
LoadBB, UnavailablePred, TD);
if (LoadPtr == 0) {
DEBUG(errs() << "COULDN'T INSERT PHI TRANSLATED VALUE OF: "
<< *LI->getOperand(0) << "\n");
return false;
}
// FIXME: This inserts a computation, but we don't tell scalar GVN
// optimization stuff about it. How do we do this?
DEBUG(errs() << "INSERTED PHI TRANSLATED VALUE: " << *LoadPtr << "\n");
}
// Make sure it is valid to move this load here. We have to watch out for:
// @1 = getelementptr (i8* p, ...
// test p and branch if == 0

34
test/Transforms/GVN/pre-load.ll
View File

@ -22,6 +22,7 @@ block4:
; CHECK-NEXT: ret i32
}
; This is a simple phi translation case.
define i32 @test2(i32* %p, i32* %q, i1 %C) {
; CHECK: @test2
block1:
@ -46,6 +47,7 @@ block4:
; CHECK: ret i32
}
; This is a PRE case that requires phi translation through a GEP.
define i32 @test3(i32* %p, i32* %q, i32** %Hack, i1 %C) {
; CHECK: @test3
block1:
@ -73,3 +75,35 @@ block4:
; CHECK-NOT: load
; CHECK: ret i32
}
;; Here the loaded address is available, but the computation is in 'block3'
;; which does not dominate 'block2'.
define i32 @test4(i32* %p, i32* %q, i32** %Hack, i1 %C) {
; CHECK: @test4
block1:
br i1 %C, label %block2, label %block3
block2:
br label %block4
; CHECK: block2:
; CHECK: load i32*
; CHECK: br label %block4
block3:
%B = getelementptr i32* %q, i32 1
store i32* %B, i32** %Hack
%A = getelementptr i32* %p, i32 1
store i32 0, i32* %A
br label %block4
block4:
%P2 = phi i32* [%p, %block3], [%q, %block2]
%P3 = getelementptr i32* %P2, i32 1
%PRE = load i32* %P3
ret i32 %PRE
; CHECK: block4:
; CHECK-NEXT: phi i32 [
; CHECK-NOT: load
; CHECK: ret i32
}