NewGVN: Clean up how we handle the INITIAL class so that everything in

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

diff --git a/lib/Transforms/Scalar/NewGVN.cpp b/lib/Transforms/Scalar/NewGVN.cpp
index e67cd1f..594f677 100644 (file)
--- a/lib/Transforms/Scalar/NewGVN.cpp
+++ b/lib/Transforms/Scalar/NewGVN.cpp
@@ -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;
@@ -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;
 }
 
@@ -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));
       }
     }
   }
@@ -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());
@@ -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");
@@ -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
@@ -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
@@ -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);
@@ -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

diff --git a/test/Transforms/NewGVN/basic-cyclic-opt.ll b/test/Transforms/NewGVN/basic-cyclic-opt.ll
index 4901a30..7830d7e 100644 (file)
--- a/test/Transforms/NewGVN/basic-cyclic-opt.ll
+++ b/test/Transforms/NewGVN/basic-cyclic-opt.ll
@@ -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:
@@ -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:%.*]]
@@ -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}