[ValueTracking] Introduce a KnownBits struct to wrap the two APInts for computeKnownBits

This patch introduces a new KnownBits struct that wraps the two APInt used by computeKnownBits. This allows us to treat them as more of a unit.

Initially I've just altered the signatures of computeKnownBits and InstCombine's simplifyDemandedBits to pass a KnownBits reference instead of two separate APInt references. I'll do similar to the SelectionDAG version of computeKnownBits/simplifyDemandedBits as a separate patch.

I've added a constructor that allows initializing both APInts to the same bit width with a starting value of 0. This reduces the repeated pattern of initializing both APInts. Once place default constructed the APInts so I added a default constructor for those cases.

Going forward I would like to add more methods that will work on the pairs. For example trunc, zext, and sext occur on both APInts together in several places. We should probably add a clear method that can be used to clear both pieces. Maybe a method to check for conflicting information. A method to return (Zero|One) so we don't write it out everywhere. Maybe a method for (Zero|One).isAllOnesValue() to determine if all bits are known. I'm sure there are many other methods we can come up with.

Differential Revision: https://reviews.llvm.org/D32376

llvm-svn: 301432

llvm/lib/Transforms/Vectorize/LoadStoreVectorizer.cpp

@@ -30,6 +30,7 @@
 #include "llvm/IR/Value.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Vectorize.h"
@@ -343,10 +344,9 @@ bool Vectorizer::isConsecutiveAccess(Value *A, Value *B) {
   // If any bits are known to be zero other than the sign bit in OpA, we can
   // add 1 to it while guaranteeing no overflow of any sort.
   if (!Safe) {
-    APInt KnownZero(BitWidth, 0);
-    APInt KnownOne(BitWidth, 0);
-    computeKnownBits(OpA, KnownZero, KnownOne, DL, 0, nullptr, OpA, &DT);
-    if (KnownZero.countTrailingZeros() < (BitWidth - 1))
+    KnownBits Known(BitWidth);
+    computeKnownBits(OpA, Known, DL, 0, nullptr, OpA, &DT);
+    if (Known.Zero.countTrailingZeros() < (BitWidth - 1))
       Safe = true;
   }