[ConstantRange][SimplifyCFG] Add a helper method to allow SimplifyCFG to determine if a ConstantRange has more than 8 elements without requiring an allocation if the ConstantRange is 64-bits wide.

Previously SimplifyCFG used getSetSize which returns an APInt that is 1 bit wider than the ConstantRange's bit width. In the reasonably common case that the ConstantRange is 64-bits wide, this requires returning a 65-bit APInt. APInt's can only store 64-bits without a memory allocation so this is inefficient.

The new method takes the 8 as an input and tells if the range contains more than that many elements without requiring any wider math.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302385 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/ConstantRange.h

@@ -169,6 +169,9 @@ public:
   /// Compare set size of this range with the range CR.
   bool isSizeStrictlySmallerThan(const ConstantRange &CR) const;
 
+  // Compare set size of this range with Value.
+  bool isSizeLargerThan(uint64_t MaxSize) const;
+
   /// Return the largest unsigned value contained in the ConstantRange.
   APInt getUnsignedMax() const;
 

lib/IR/ConstantRange.cpp

@@ -260,6 +260,17 @@ ConstantRange::isSizeStrictlySmallerThan(const ConstantRange &Other) const {
   return (Upper - Lower).ult(Other.Upper - Other.Lower);
 }
 
+bool
+ConstantRange::isSizeLargerThan(uint64_t MaxSize) const {
+  assert(MaxSize && "MaxSize can't be 0.");
+  // If this a full set, we need special handling to avoid needing an extra bit
+  // to represent the size.
+  if (isFullSet())
+    return APInt::getMaxValue(getBitWidth()).ugt(MaxSize - 1);
+
+  return (Upper - Lower).ugt(MaxSize);
+}
+
 APInt ConstantRange::getUnsignedMax() const {
   if (isFullSet() || isWrappedSet())
     return APInt::getMaxValue(getBitWidth());

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -596,7 +596,7 @@ private:
       Span = Span.inverse();
 
     // If there are a ton of values, we don't want to make a ginormous switch.
-    if (Span.getSetSize().ugt(8) || Span.isEmptySet()) {
+    if (Span.isSizeLargerThan(8) || Span.isEmptySet()) {
       return false;
     }