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NewGVN: Clean up how we handle the INITIAL class so that everything in
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it is dead or unreachable, as it should be.
This also makes the leader of INITIAL undef, enabling us to handle
irreducibility properly.

Summary:
This lets us verify, more than we do now, that we didn't screw up
value numbering.

Reviewers: davide

Subscribers: Prazek, llvm-commits

Differential Revision: https://reviews.llvm.org/D29842

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@294844 91177308-0d34-0410-b5e6-96231b3b80d8
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@dberlin
dberlin committed Feb 11, 2017
1 parent 5de15e7 commit f6af3ae
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Showing 2 changed files with 76 additions and 20 deletions.
54 changes: 38 additions & 16 deletions lib/Transforms/Scalar/NewGVN.cpp
View file
Original file line number Diff line number Diff line change
Expand Up @@ -213,6 +213,11 @@ class NewGVN : public FunctionPass {
ArrayRecycler<Value *> ArgRecycler;

// Congruence class info.

// This class is called INITIAL in the paper. It is the class everything
// startsout in, and represents any value. Being an optimistic analysis,
// anything in the INITIAL class has the value TOP, which is indeterminate and
// equivalent to everything.
CongruenceClass *InitialClass;
std::vector<CongruenceClass *> CongruenceClasses;
unsigned NextCongruenceNum;
Expand Down Expand Up @@ -691,8 +696,15 @@ const CallExpression *NewGVN::createCallExpression(CallInst *CI,
// return it. Otherwise, return the operand itself.
Value *NewGVN::lookupOperandLeader(Value *V) const {
CongruenceClass *CC = ValueToClass.lookup(V);
if (CC && (CC != InitialClass))
if (CC) {
// Everything in INITIAL is represneted by undef, as it can be any value.
// We do have to make sure we get the type right though, so we can't set the
// RepLeader to undef.
if (CC == InitialClass)
return UndefValue::get(V->getType());
return CC->RepStoredValue ? CC->RepStoredValue : CC->RepLeader;
}

return V;
}

Expand Down Expand Up @@ -957,9 +969,8 @@ const Expression *NewGVN::performSymbolicAggrValueEvaluation(Instruction *I) {
// expression.
assert(II->getNumArgOperands() == 2 &&
"Expect two args for recognised intrinsics.");
return createBinaryExpression(Opcode, EI->getType(),
II->getArgOperand(0),
II->getArgOperand(1));
return createBinaryExpression(
Opcode, EI->getType(), II->getArgOperand(0), II->getArgOperand(1));
}
}
}
Expand Down Expand Up @@ -1200,7 +1211,7 @@ void NewGVN::moveValueToNewCongruenceClass(Instruction *I,
}

// We don't need to sort members if there is only 1, and we don't care about
// sorting the initial class because everything either gets out of it or is
// sorting the INITIAL class because everything either gets out of it or is
// unreachable.
if (OldClass->Members.size() == 1 || OldClass == InitialClass) {
OldClass->RepLeader = *(OldClass->Members.begin());
Expand Down Expand Up @@ -1229,7 +1240,7 @@ void NewGVN::moveValueToNewCongruenceClass(Instruction *I,
void NewGVN::performCongruenceFinding(Instruction *I, const Expression *E) {
ValueToExpression[I] = E;
// This is guaranteed to return something, since it will at least find
// INITIAL.
// TOP.

CongruenceClass *IClass = ValueToClass[I];
assert(IClass && "Should have found a IClass");
Expand Down Expand Up @@ -1418,8 +1429,7 @@ void NewGVN::processOutgoingEdges(TerminatorInst *TI, BasicBlock *B) {
}

// The algorithm initially places the values of the routine in the INITIAL
// congruence
// class. The leader of INITIAL is the undetermined value `TOP`.
// congruence class. The leader of INITIAL is the undetermined value `TOP`.
// When the algorithm has finished, values still in INITIAL are unreachable.
void NewGVN::initializeCongruenceClasses(Function &F) {
// FIXME now i can't remember why this is 2
Expand All @@ -1433,8 +1443,11 @@ void NewGVN::initializeCongruenceClasses(Function &F) {
MemoryAccessToClass[MP] = InitialClass;

for (auto &I : B) {
InitialValues.insert(&I);
ValueToClass[&I] = InitialClass;
// Don't insert void terminators into the class
if (!isa<TerminatorInst>(I) || !I.getType()->isVoidTy()) {
InitialValues.insert(&I);
ValueToClass[&I] = InitialClass;
}
// All memory accesses are equivalent to live on entry to start. They must
// be initialized to something so that initial changes are noticed. For
// the maximal answer, we initialize them all to be the same as
Expand Down Expand Up @@ -1585,7 +1598,8 @@ void NewGVN::valueNumberInstruction(Instruction *I) {
performCongruenceFinding(I, Symbolized);
} else {
// Handle terminators that return values. All of them produce values we
// don't currently understand.
// don't currently understand. We don't place non-value producing
// terminators in a class.
if (!I->getType()->isVoidTy()) {
auto *Symbolized = createUnknownExpression(I);
performCongruenceFinding(I, Symbolized);
Expand Down Expand Up @@ -2198,12 +2212,20 @@ bool NewGVN::eliminateInstructions(Function &F) {
// dead store elimination.
SmallVector<ValueDFS, 8> PossibleDeadStores;

// FIXME: We should eventually be able to replace everything still
// in the initial class with undef, as they should be unreachable.
// Right now, initial still contains some things we skip value
// numbering of (UNREACHABLE's, for example).
if (CC == InitialClass || CC->Dead)
if (CC->Dead)
continue;
// Everything still in the INITIAL class is unreachable or dead.
if (CC == InitialClass) {
#ifndef NDEBUG
for (auto M : CC->Members)
assert((!ReachableBlocks.count(cast<Instruction>(M)->getParent()) ||
InstructionsToErase.count(cast<Instruction>(M))) &&
"Everything in INITIAL should be unreachable or dead at this "
"point");
#endif
continue;
}

assert(CC->RepLeader && "We should have had a leader");

// If this is a leader that is always available, and it's a
Expand Down
42 changes: 38 additions & 4 deletions test/Transforms/NewGVN/basic-cyclic-opt.ll
View file
Original file line number Diff line number Diff line change
Expand Up @@ -228,8 +228,8 @@ bb23: ; preds = %bb4
}

;; This is an irreducible test case that will cause a memoryphi node loop
;; in the two block.
;; it's equivalent to something like
;; in the two blocks.
;; It's equivalent to something like
;; *a = 0
;; if (<....>) goto loopmiddle
;; loopstart:
Expand All @@ -245,8 +245,8 @@ bb23: ; preds = %bb4
;; Both loads should equal 0, but it requires being
;; completely optimistic about MemoryPhis, otherwise
;; we will not be able to see through the cycle.
define i8 @quux(i8* noalias %arg, i8* noalias %arg2) {
; CHECK-LABEL: @quux(
define i8 @irreducible_memoryphi(i8* noalias %arg, i8* noalias %arg2) {
; CHECK-LABEL: @irreducible_memoryphi(
; CHECK-NEXT: bb:
; CHECK-NEXT: store i8 0, i8* [[ARG:%.*]]
; CHECK-NEXT: br i1 undef, label [[BB2:%.*]], label [[BB1:%.*]]
Expand Down Expand Up @@ -274,6 +274,40 @@ bb3: ; preds = %bb2
%tmp3 = add i8 %tmp, %tmp2
ret i8 %tmp3
}
;; This is an irreducible test case that will cause a phi node loop
;; in the two blocks
;;
;; It should return 0, but it requires being
;; completely optimistic about phis, otherwise
;; we will not be able to see through the cycle.
define i32 @irreducible_phi(i32 %arg) {
; CHECK-LABEL: @irreducible_phi(
; CHECK-NEXT: bb:
; CHECK-NEXT: br i1 undef, label [[BB2:%.*]], label [[BB1:%.*]]
; CHECK: bb1:
; CHECK-NEXT: br label [[BB2]]
; CHECK: bb2:
; CHECK-NEXT: br i1 undef, label [[BB1]], label [[BB3:%.*]]
; CHECK: bb3:
; CHECK-NEXT: ret i32 0
;
bb:
%tmp = add i32 0, %arg
br i1 undef, label %bb2, label %bb1

bb1: ; preds = %bb2, %bb
%phi1 = phi i32 [%tmp, %bb], [%phi2, %bb2]
br label %bb2

bb2: ; preds = %bb1, %bb
%phi2 = phi i32 [%tmp, %bb], [%phi1, %bb1]
br i1 undef, label %bb1, label %bb3

bb3: ; preds = %bb2
; This should be zero
%tmp3 = sub i32 %tmp, %phi2
ret i32 %tmp3
}
attributes #0 = { nounwind ssp uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0, !0, !0}
Expand Down

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