[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

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 5d6d899da4b..76829c5e457 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -17,6 +17,7 @@
 #include "llvm/Analysis/ValueTracking.h"
 #include "llvm/IR/MDBuilder.h"
 #include "llvm/IR/PatternMatch.h"
+#include "llvm/Support/KnownBits.h"
 using namespace llvm;
 using namespace PatternMatch;
 
@@ -1476,11 +1477,11 @@ Instruction *InstCombiner::visitSelectInst(SelectInst &SI) {
   // The motivation for this call into value tracking is to take advantage of
   // the assumption cache, so make sure that is populated.
   if (!CondVal->getType()->isVectorTy() && !AC.assumptions().empty()) {
-    APInt KnownOne(1, 0), KnownZero(1, 0);
-    computeKnownBits(CondVal, KnownZero, KnownOne, 0, &SI);
-    if (KnownOne == 1)
+    KnownBits Known(1);
+    computeKnownBits(CondVal, Known, 0, &SI);
+    if (Known.One == 1)
       return replaceInstUsesWith(SI, TrueVal);
-    if (KnownZero == 1)
+    if (Known.Zero == 1)
       return replaceInstUsesWith(SI, FalseVal);
   }