[LVI] Look through negations when evaluating conditions

This teaches LVI (and thus CVP) to extract range information
from branches whose condition is negated using (`xor %c, true`).
On the implementation side, we switch the cache to additionally
track whether we're looking for the inverted value or not and
otherwise using the existing support for computing inverted
conditions.

I think the biggest question here is why this negation shows up
here at all. After all, it should always be possible for some
other pass to fold such a negation into a branch, comparison or
some other logical operation. Indeed, instcombine does just that.
However, these negations can be otherwise fairly persistent, e.g.
instsimplify is not able to exchange branch conditions from
negations. In addition, jumpthreading, which sits at the same
point in default pass pipeline also handles this pattern, which
adds further evidence that we might expect these negations to
not have been canonicalized away yet at this point in the pass
pipeline.

In the particular case I was looking at there was a bit of a
circular dependency where flags computed by cvp were needed
by instcombine, and incstombine's folding of the negation was
needed for cvp. Adding a second instombine pass would have
worked of course, but instcombine can be somewhat expensive,
so it appeared desirable to not require it to have run
before cvp (as is the case in the default pass pipeline).

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D140933

(cherry picked from commit 1436a92)

Author: Keno

llvm/lib/Analysis/LazyValueInfo.cpp

@@ -1161,11 +1161,17 @@ static ValueLatticeElement getValueFromOverflowCondition(
   return ValueLatticeElement::getRange(NWR);
 }
 
+// Tracks a Value * condition and whether we're interested in it or its inverse
+typedef PointerIntPair<Value *, 1, bool> CondValue;
+
 static Optional<ValueLatticeElement>
-getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
-                          bool isRevisit,
-                          SmallDenseMap<Value *, ValueLatticeElement> &Visited,
-                          SmallVectorImpl<Value *> &Worklist) {
+getValueFromConditionImpl(
+    Value *Val, CondValue CondVal, bool isRevisit,
+    SmallDenseMap<CondValue, ValueLatticeElement> &Visited,
+    SmallVectorImpl<CondValue> &Worklist) {
+
+  Value *Cond = CondVal.getPointer();
+  bool isTrueDest = CondVal.getInt();
   if (!isRevisit) {
     if (ICmpInst *ICI = dyn_cast<ICmpInst>(Cond))
       return getValueFromICmpCondition(Val, ICI, isTrueDest);
@@ -1176,6 +1182,17 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
           return getValueFromOverflowCondition(Val, WO, isTrueDest);
   }
 
+  Value *N;
+  if (match(Cond, m_Not(m_Value(N)))) {
+    CondValue NKey(N, !isTrueDest);
+    auto NV = Visited.find(NKey);
+    if (NV == Visited.end()) {
+      Worklist.push_back(NKey);
+      return None;
+    }
+    return NV->second;
+  }
+
   Value *L, *R;
   bool IsAnd;
   if (match(Cond, m_LogicalAnd(m_Value(L), m_Value(R))))
@@ -1185,13 +1202,13 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
   else
     return ValueLatticeElement::getOverdefined();
 
-  auto LV = Visited.find(L);
-  auto RV = Visited.find(R);
+  auto LV = Visited.find(CondValue(L, isTrueDest));
+  auto RV = Visited.find(CondValue(R, isTrueDest));
 
   // if (L && R) -> intersect L and R
-  // if (!(L || R)) -> intersect L and R
+  // if (!(L || R)) -> intersect !L and !R
   // if (L || R) -> union L and R
-  // if (!(L && R)) -> union L and R
+  // if (!(L && R)) -> union !L and !R
   if ((isTrueDest ^ IsAnd) && (LV != Visited.end())) {
     ValueLatticeElement V = LV->second;
     if (V.isOverdefined())
@@ -1205,9 +1222,9 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
   if (LV == Visited.end() || RV == Visited.end()) {
     assert(!isRevisit);
     if (LV == Visited.end())
-      Worklist.push_back(L);
+      Worklist.push_back(CondValue(L, isTrueDest));
     if (RV == Visited.end())
-      Worklist.push_back(R);
+      Worklist.push_back(CondValue(R, isTrueDest));
     return None;
   }
 
@@ -1217,12 +1234,13 @@ getValueFromConditionImpl(Value *Val, Value *Cond, bool isTrueDest,
 ValueLatticeElement getValueFromCondition(Value *Val, Value *Cond,
                                           bool isTrueDest) {
   assert(Cond && "precondition");
-  SmallDenseMap<Value*, ValueLatticeElement> Visited;
-  SmallVector<Value *> Worklist;
+  SmallDenseMap<CondValue, ValueLatticeElement> Visited;
+  SmallVector<CondValue> Worklist;
 
-  Worklist.push_back(Cond);
+  CondValue CondKey(Cond, isTrueDest);
+  Worklist.push_back(CondKey);
   do {
-    Value *CurrentCond = Worklist.back();
+    CondValue CurrentCond = Worklist.back();
     // Insert an Overdefined placeholder into the set to prevent
     // infinite recursion if there exists IRs that use not
     // dominated by its def as in this example:
@@ -1232,14 +1250,14 @@ ValueLatticeElement getValueFromCondition(Value *Val, Value *Cond,
         Visited.try_emplace(CurrentCond, ValueLatticeElement::getOverdefined());
     bool isRevisit = !Iter.second;
     Optional<ValueLatticeElement> Result = getValueFromConditionImpl(
-        Val, CurrentCond, isTrueDest, isRevisit, Visited, Worklist);
+        Val, CurrentCond, isRevisit, Visited, Worklist);
     if (Result) {
       Visited[CurrentCond] = *Result;
       Worklist.pop_back();
     }
   } while (!Worklist.empty());
 
-  auto Result = Visited.find(Cond);
+  auto Result = Visited.find(CondKey);
   assert(Result != Visited.end());
   return Result->second;
 }

llvm/test/Transforms/CorrelatedValuePropagation/basic.ll

@@ -1853,6 +1853,63 @@ define void @xor(i8 %a, i1* %p) {
   ret void
 }
 
+define i1 @xor_neg_cond(i32 %a) {
+; CHECK-LABEL: @xor_neg_cond(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp eq i32 [[A:%.*]], 10
+; CHECK-NEXT:    [[XOR:%.*]] = xor i1 [[CMP1]], true
+; CHECK-NEXT:    br i1 [[XOR]], label [[EXIT:%.*]], label [[GUARD:%.*]]
+; CHECK:       guard:
+; CHECK-NEXT:    ret i1 true
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 false
+;
+  %cmp1 = icmp eq i32 %a, 10
+  %xor = xor i1 %cmp1, true
+  br i1 %xor, label %exit, label %guard
+
+guard:
+  %cmp2 = icmp eq i32 %a, 10
+  ret i1 %cmp2
+
+exit:
+  ret i1 false
+}
+
+define i1 @xor_approx(i32 %a) {
+; CHECK-LABEL: @xor_approx(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp ugt i32 [[A:%.*]], 2
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp ult i32 [[A]], 5
+; CHECK-NEXT:    [[CMP3:%.*]] = icmp ugt i32 [[A]], 7
+; CHECK-NEXT:    [[CMP4:%.*]] = icmp ult i32 [[A]], 9
+; CHECK-NEXT:    [[AND1:%.*]] = and i1 [[CMP1]], [[CMP2]]
+; CHECK-NEXT:    [[AND2:%.*]] = and i1 [[CMP3]], [[CMP4]]
+; CHECK-NEXT:    [[OR:%.*]] = or i1 [[AND1]], [[AND2]]
+; CHECK-NEXT:    [[XOR:%.*]] = xor i1 [[OR]], true
+; CHECK-NEXT:    br i1 [[XOR]], label [[EXIT:%.*]], label [[GUARD:%.*]]
+; CHECK:       guard:
+; CHECK-NEXT:    [[CMP5:%.*]] = icmp eq i32 [[A]], 6
+; CHECK-NEXT:    ret i1 [[CMP5]]
+; CHECK:       exit:
+; CHECK-NEXT:    ret i1 false
+;
+  %cmp1 = icmp ugt i32 %a, 2
+  %cmp2 = icmp ult i32 %a, 5
+  %cmp3 = icmp ugt i32 %a, 7
+  %cmp4 = icmp ult i32 %a, 9
+  %and1 = and i1 %cmp1, %cmp2
+  %and2 = and i1 %cmp3, %cmp4
+  %or = or i1 %and1, %and2
+  %xor = xor i1 %or, true
+  br i1 %xor, label %exit, label %guard
+
+guard:
+  %cmp5 = icmp eq i32 %a, 6
+  ret i1 %cmp5
+
+exit:
+  ret i1 false
+}
+
 declare i32 @llvm.uadd.sat.i32(i32, i32)
 declare i32 @llvm.usub.sat.i32(i32, i32)
 declare i32 @llvm.sadd.sat.i32(i32, i32)