[SCEV] Change the interface of computeConstantDifference; NFC

This is in preparation of
s/getConstantDifference/computeConstantDifference/ in a later change.

llvm-svn: 276503

llvm/include/llvm/Analysis/ScalarEvolution.h

@@ -1099,10 +1099,14 @@ namespace llvm {
     bool splitBinaryAdd(const SCEV *Expr, const SCEV *&L, const SCEV *&R,
                         SCEV::NoWrapFlags &Flags);
 
-    /// Return true if More == (Less + C), where C is a constant.  This is
-    /// intended to be used as a cheaper substitute for full SCEV subtraction.
-    bool computeConstantDifference(const SCEV *Less, const SCEV *More,
-                                   APInt &C);
+    /// Compute \p LHS - \p RHS and returns the result as an APInt if it is a
+    /// constant, and None if it isn't.
+    ///
+    /// This is intended to be a cheaper version of getMinusSCEV.  We can be
+    /// frugal here since we just bail out of actually constructing and
+    /// canonicalizing an expression in the cases where the result isn't going
+    /// to be a constant.
+    Optional<APInt> computeConstantDifference(const SCEV *LHS, const SCEV *RHS);
 
     /// Drop memoized information computed for S.
     void forgetMemoizedResults(const SCEV *S);

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -8275,9 +8275,8 @@ bool ScalarEvolution::splitBinaryAdd(const SCEV *Expr,
   return true;
 }
 
-bool ScalarEvolution::computeConstantDifference(const SCEV *Less,
-                                                const SCEV *More,
-                                                APInt &C) {
+Optional<APInt> ScalarEvolution::computeConstantDifference(const SCEV *More,
+                                                           const SCEV *Less) {
   // We avoid subtracting expressions here because this function is usually
   // fairly deep in the call stack (i.e. is called many times).
 
@@ -8286,15 +8285,15 @@ bool ScalarEvolution::computeConstantDifference(const SCEV *Less,
     const auto *MAR = cast<SCEVAddRecExpr>(More);
 
     if (LAR->getLoop() != MAR->getLoop())
-      return false;
+      return None;
 
     // We look at affine expressions only; not for correctness but to keep
     // getStepRecurrence cheap.
     if (!LAR->isAffine() || !MAR->isAffine())
-      return false;
+      return None;
 
     if (LAR->getStepRecurrence(*this) != MAR->getStepRecurrence(*this))
-      return false;
+      return None;
 
     Less = LAR->getStart();
     More = MAR->getStart();
@@ -8305,27 +8304,22 @@ bool ScalarEvolution::computeConstantDifference(const SCEV *Less,
   if (isa<SCEVConstant>(Less) && isa<SCEVConstant>(More)) {
     const auto &M = cast<SCEVConstant>(More)->getAPInt();
     const auto &L = cast<SCEVConstant>(Less)->getAPInt();
-    C = M - L;
-    return true;
+    return M - L;
   }
 
   const SCEV *L, *R;
   SCEV::NoWrapFlags Flags;
   if (splitBinaryAdd(Less, L, R, Flags))
     if (const auto *LC = dyn_cast<SCEVConstant>(L))
-      if (R == More) {
-        C = -(LC->getAPInt());
-        return true;
-      }
+      if (R == More)
+        return -(LC->getAPInt());
 
   if (splitBinaryAdd(More, L, R, Flags))
     if (const auto *LC = dyn_cast<SCEVConstant>(L))
-      if (R == Less) {
-        C = LC->getAPInt();
-        return true;
-      }
+      if (R == Less)
+        return LC->getAPInt();
 
-  return false;
+  return None;
 }
 
 bool ScalarEvolution::isImpliedCondOperandsViaNoOverflow(
@@ -8382,22 +8376,21 @@ bool ScalarEvolution::isImpliedCondOperandsViaNoOverflow(
   // neither necessary nor sufficient to prove "(FoundLHS + C) s< (FoundRHS +
   // C)".
 
-  APInt LDiff, RDiff;
-  if (!computeConstantDifference(FoundLHS, LHS, LDiff) ||
-      !computeConstantDifference(FoundRHS, RHS, RDiff) ||
-      LDiff != RDiff)
+  Optional<APInt> LDiff = computeConstantDifference(LHS, FoundLHS);
+  Optional<APInt> RDiff = computeConstantDifference(RHS, FoundRHS);
+  if (!LDiff || !RDiff || *LDiff != *RDiff)
     return false;
 
-  if (LDiff == 0)
+  if (LDiff->isMinValue())
     return true;
 
   APInt FoundRHSLimit;
 
   if (Pred == CmpInst::ICMP_ULT) {
-    FoundRHSLimit = -RDiff;
+    FoundRHSLimit = -(*RDiff);
   } else {
     assert(Pred == CmpInst::ICMP_SLT && "Checked above!");
-    FoundRHSLimit = APInt::getSignedMinValue(getTypeSizeInBits(RHS->getType())) - RDiff;
+    FoundRHSLimit = APInt::getSignedMinValue(getTypeSizeInBits(RHS->getType())) - *RDiff;
   }
 
   // Try to prove (1) or (2), as needed.