Return "[SCEV] Prove implicaitons via AddRec start"

The initial version of the patch was reverted because it missed the check that
the predicate being proved is actually guarded by this check on 1st iteration.
If it was not executed on 1st iteration (but possibly executes after that), then
it is incorrect to use reasoning about IV start to prove it.

Added the test where the miscompile was seen. Unfortunately, my attempts
to reduce it with bugpoint did not succeed; it can further be reduced when
we understand how to do it without losing the initial bug's notion.

Returning assuming the miscompiles are now gone.

Differential Revision: https://reviews.llvm.org/D88208

Author: xortator

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1677,23 +1677,30 @@ class ScalarEvolution {
   getPredecessorWithUniqueSuccessorForBB(const BasicBlock *BB) const;
 
   /// Test whether the condition described by Pred, LHS, and RHS is true
-  /// whenever the given FoundCondValue value evaluates to true.
+  /// whenever the given FoundCondValue value evaluates to true in given
+  /// Context. If Context is nullptr, then the found predicate is true
+  /// everywhere.
   bool isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS, const SCEV *RHS,
-                     const Value *FoundCondValue, bool Inverse);
+                     const Value *FoundCondValue, bool Inverse,
+                     const Instruction *Context = nullptr);
 
   /// Test whether the condition described by Pred, LHS, and RHS is true
   /// whenever the condition described by FoundPred, FoundLHS, FoundRHS is
-  /// true.
+  /// true in given Context. If Context is nullptr, then the found predicate is
+  /// true everywhere.
   bool isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS, const SCEV *RHS,
                      ICmpInst::Predicate FoundPred, const SCEV *FoundLHS,
-                     const SCEV *FoundRHS);
+                     const SCEV *FoundRHS,
+                     const Instruction *Context = nullptr);
 
   /// Test whether the condition described by Pred, LHS, and RHS is true
   /// whenever the condition described by Pred, FoundLHS, and FoundRHS is
-  /// true.
+  /// true in given Context. If Context is nullptr, then the found predicate is
+  /// true everywhere.
   bool isImpliedCondOperands(ICmpInst::Predicate Pred, const SCEV *LHS,
                              const SCEV *RHS, const SCEV *FoundLHS,
-                             const SCEV *FoundRHS);
+                             const SCEV *FoundRHS,
+                             const Instruction *Context = nullptr);
 
   /// Test whether the condition described by Pred, LHS, and RHS is true
   /// whenever the condition described by Pred, FoundLHS, and FoundRHS is
@@ -1740,6 +1747,18 @@ class ScalarEvolution {
                                           const SCEV *FoundLHS,
                                           const SCEV *FoundRHS);
 
+  /// Test whether the condition described by Pred, LHS, and RHS is true
+  /// whenever the condition described by Pred, FoundLHS, and FoundRHS is
+  /// true.
+  ///
+  /// This routine tries to weaken the known condition basing on fact that
+  /// FoundLHS is an AddRec.
+  bool isImpliedCondOperandsViaAddRecStart(ICmpInst::Predicate Pred,
+                                           const SCEV *LHS, const SCEV *RHS,
+                                           const SCEV *FoundLHS,
+                                           const SCEV *FoundRHS,
+                                           const Instruction *Context);
+
   /// Test whether the condition described by Pred, LHS, and RHS is true
   /// whenever the condition described by Pred, FoundLHS, and FoundRHS is
   /// true.

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -9549,15 +9549,16 @@ bool ScalarEvolution::isBasicBlockEntryGuardedByCond(const BasicBlock *BB,
 
   // Try to prove (Pred, LHS, RHS) using isImpliedCond.
   auto ProveViaCond = [&](const Value *Condition, bool Inverse) {
-    if (isImpliedCond(Pred, LHS, RHS, Condition, Inverse))
+    const Instruction *Context = &BB->front();
+    if (isImpliedCond(Pred, LHS, RHS, Condition, Inverse, Context))
       return true;
     if (ProvingStrictComparison) {
       if (!ProvedNonStrictComparison)
-        ProvedNonStrictComparison =
-            isImpliedCond(NonStrictPredicate, LHS, RHS, Condition, Inverse);
+        ProvedNonStrictComparison = isImpliedCond(NonStrictPredicate, LHS, RHS,
+                                                  Condition, Inverse, Context);
       if (!ProvedNonEquality)
-        ProvedNonEquality =
-            isImpliedCond(ICmpInst::ICMP_NE, LHS, RHS, Condition, Inverse);
+        ProvedNonEquality = isImpliedCond(ICmpInst::ICMP_NE, LHS, RHS,
+                                          Condition, Inverse, Context);
       if (ProvedNonStrictComparison && ProvedNonEquality)
         return true;
     }
@@ -9623,7 +9624,8 @@ bool ScalarEvolution::isLoopEntryGuardedByCond(const Loop *L,
 
 bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
                                     const SCEV *RHS,
-                                    const Value *FoundCondValue, bool Inverse) {
+                                    const Value *FoundCondValue, bool Inverse,
+                                    const Instruction *Context) {
   if (!PendingLoopPredicates.insert(FoundCondValue).second)
     return false;
 
@@ -9634,12 +9636,16 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
   if (const BinaryOperator *BO = dyn_cast<BinaryOperator>(FoundCondValue)) {
     if (BO->getOpcode() == Instruction::And) {
       if (!Inverse)
-        return isImpliedCond(Pred, LHS, RHS, BO->getOperand(0), Inverse) ||
-               isImpliedCond(Pred, LHS, RHS, BO->getOperand(1), Inverse);
+        return isImpliedCond(Pred, LHS, RHS, BO->getOperand(0), Inverse,
+                             Context) ||
+               isImpliedCond(Pred, LHS, RHS, BO->getOperand(1), Inverse,
+                             Context);
     } else if (BO->getOpcode() == Instruction::Or) {
       if (Inverse)
-        return isImpliedCond(Pred, LHS, RHS, BO->getOperand(0), Inverse) ||
-               isImpliedCond(Pred, LHS, RHS, BO->getOperand(1), Inverse);
+        return isImpliedCond(Pred, LHS, RHS, BO->getOperand(0), Inverse,
+                             Context) ||
+               isImpliedCond(Pred, LHS, RHS, BO->getOperand(1), Inverse,
+                             Context);
     }
   }
 
@@ -9657,14 +9663,14 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
   const SCEV *FoundLHS = getSCEV(ICI->getOperand(0));
   const SCEV *FoundRHS = getSCEV(ICI->getOperand(1));
 
-  return isImpliedCond(Pred, LHS, RHS, FoundPred, FoundLHS, FoundRHS);
+  return isImpliedCond(Pred, LHS, RHS, FoundPred, FoundLHS, FoundRHS, Context);
 }
 
 bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
                                     const SCEV *RHS,
                                     ICmpInst::Predicate FoundPred,
-                                    const SCEV *FoundLHS,
-                                    const SCEV *FoundRHS) {
+                                    const SCEV *FoundLHS, const SCEV *FoundRHS,
+                                    const Instruction *Context) {
   // Balance the types.
   if (getTypeSizeInBits(LHS->getType()) <
       getTypeSizeInBits(FoundLHS->getType())) {
@@ -9708,24 +9714,24 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
 
   // Check whether the found predicate is the same as the desired predicate.
   if (FoundPred == Pred)
-    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS);
+    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, Context);
 
   // Check whether swapping the found predicate makes it the same as the
   // desired predicate.
   if (ICmpInst::getSwappedPredicate(FoundPred) == Pred) {
     if (isa<SCEVConstant>(RHS))
-      return isImpliedCondOperands(Pred, LHS, RHS, FoundRHS, FoundLHS);
+      return isImpliedCondOperands(Pred, LHS, RHS, FoundRHS, FoundLHS, Context);
     else
-      return isImpliedCondOperands(ICmpInst::getSwappedPredicate(Pred),
-                                   RHS, LHS, FoundLHS, FoundRHS);
+      return isImpliedCondOperands(ICmpInst::getSwappedPredicate(Pred), RHS,
+                                   LHS, FoundLHS, FoundRHS, Context);
   }
 
   // Unsigned comparison is the same as signed comparison when both the operands
   // are non-negative.
   if (CmpInst::isUnsigned(FoundPred) &&
       CmpInst::getSignedPredicate(FoundPred) == Pred &&
       isKnownNonNegative(FoundLHS) && isKnownNonNegative(FoundRHS))
-    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS);
+    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, Context);
 
   // Check if we can make progress by sharpening ranges.
   if (FoundPred == ICmpInst::ICMP_NE &&
@@ -9762,8 +9768,8 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
         case ICmpInst::ICMP_UGE:
           // We know V `Pred` SharperMin.  If this implies LHS `Pred`
           // RHS, we're done.
-          if (isImpliedCondOperands(Pred, LHS, RHS, V,
-                                    getConstant(SharperMin)))
+          if (isImpliedCondOperands(Pred, LHS, RHS, V, getConstant(SharperMin),
+                                    Context))
             return true;
           LLVM_FALLTHROUGH;
 
@@ -9778,22 +9784,23 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
           //
           // If V `Pred` Min implies LHS `Pred` RHS, we're done.
 
-          if (isImpliedCondOperands(Pred, LHS, RHS, V, getConstant(Min)))
+          if (isImpliedCondOperands(Pred, LHS, RHS, V, getConstant(Min),
+                                    Context))
             return true;
           break;
 
         // `LHS < RHS` and `LHS <= RHS` are handled in the same way as `RHS > LHS` and `RHS >= LHS` respectively.
         case ICmpInst::ICMP_SLE:
         case ICmpInst::ICMP_ULE:
           if (isImpliedCondOperands(CmpInst::getSwappedPredicate(Pred), RHS,
-                                    LHS, V, getConstant(SharperMin)))
+                                    LHS, V, getConstant(SharperMin), Context))
             return true;
           LLVM_FALLTHROUGH;
 
         case ICmpInst::ICMP_SLT:
         case ICmpInst::ICMP_ULT:
           if (isImpliedCondOperands(CmpInst::getSwappedPredicate(Pred), RHS,
-                                    LHS, V, getConstant(Min)))
+                                    LHS, V, getConstant(Min), Context))
             return true;
           break;
 
@@ -9807,11 +9814,12 @@ bool ScalarEvolution::isImpliedCond(ICmpInst::Predicate Pred, const SCEV *LHS,
   // Check whether the actual condition is beyond sufficient.
   if (FoundPred == ICmpInst::ICMP_EQ)
     if (ICmpInst::isTrueWhenEqual(Pred))
-      if (isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS))
+      if (isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, FoundRHS, Context))
         return true;
   if (Pred == ICmpInst::ICMP_NE)
     if (!ICmpInst::isTrueWhenEqual(FoundPred))
-      if (isImpliedCondOperands(FoundPred, LHS, RHS, FoundLHS, FoundRHS))
+      if (isImpliedCondOperands(FoundPred, LHS, RHS, FoundLHS, FoundRHS,
+                                Context))
         return true;
 
   // Otherwise assume the worst.
@@ -9890,6 +9898,51 @@ Optional<APInt> ScalarEvolution::computeConstantDifference(const SCEV *More,
   return None;
 }
 
+bool ScalarEvolution::isImpliedCondOperandsViaAddRecStart(
+    ICmpInst::Predicate Pred, const SCEV *LHS, const SCEV *RHS,
+    const SCEV *FoundLHS, const SCEV *FoundRHS, const Instruction *Context) {
+  // Try to recognize the following pattern:
+  //
+  //   FoundRHS = ...
+  // ...
+  // loop:
+  //   FoundLHS = {Start,+,W}
+  // context_bb: // Basic block from the same loop
+  //   known(Pred, FoundLHS, FoundRHS)
+  //
+  // If some predicate is known in the context of a loop, it is also known on
+  // each iteration of this loop, including the first iteration. Therefore, in
+  // this case, `FoundLHS Pred FoundRHS` implies `Start Pred FoundRHS`. Try to
+  // prove the original pred using this fact.
+  if (!Context)
+    return false;
+  const BasicBlock *ContextBB = Context->getParent();
+  // Make sure AR varies in the context block.
+  if (auto *AR = dyn_cast<SCEVAddRecExpr>(FoundLHS)) {
+    const Loop *L = AR->getLoop();
+    // Make sure that context belongs to the loop and executes on 1st iteration
+    // (if it ever executes at all).
+    if (!L->contains(ContextBB) || !DT.dominates(ContextBB, L->getLoopLatch()))
+      return false;
+    if (!isAvailableAtLoopEntry(FoundRHS, AR->getLoop()))
+      return false;
+    return isImpliedCondOperands(Pred, LHS, RHS, AR->getStart(), FoundRHS);
+  }
+
+  if (auto *AR = dyn_cast<SCEVAddRecExpr>(FoundRHS)) {
+    const Loop *L = AR->getLoop();
+    // Make sure that context belongs to the loop and executes on 1st iteration
+    // (if it ever executes at all).
+    if (!L->contains(ContextBB) || !DT.dominates(ContextBB, L->getLoopLatch()))
+      return false;
+    if (!isAvailableAtLoopEntry(FoundLHS, AR->getLoop()))
+      return false;
+    return isImpliedCondOperands(Pred, LHS, RHS, FoundLHS, AR->getStart());
+  }
+
+  return false;
+}
+
 bool ScalarEvolution::isImpliedCondOperandsViaNoOverflow(
     ICmpInst::Predicate Pred, const SCEV *LHS, const SCEV *RHS,
     const SCEV *FoundLHS, const SCEV *FoundRHS) {
@@ -10080,13 +10133,18 @@ bool ScalarEvolution::isImpliedViaMerge(ICmpInst::Predicate Pred,
 bool ScalarEvolution::isImpliedCondOperands(ICmpInst::Predicate Pred,
                                             const SCEV *LHS, const SCEV *RHS,
                                             const SCEV *FoundLHS,
-                                            const SCEV *FoundRHS) {
+                                            const SCEV *FoundRHS,
+                                            const Instruction *Context) {
   if (isImpliedCondOperandsViaRanges(Pred, LHS, RHS, FoundLHS, FoundRHS))
     return true;
 
   if (isImpliedCondOperandsViaNoOverflow(Pred, LHS, RHS, FoundLHS, FoundRHS))
     return true;
 
+  if (isImpliedCondOperandsViaAddRecStart(Pred, LHS, RHS, FoundLHS, FoundRHS,
+                                          Context))
+    return true;
+
   return isImpliedCondOperandsHelper(Pred, LHS, RHS,
                                      FoundLHS, FoundRHS) ||
          // ~x < ~y --> x > y