[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, 4 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 63db98afecd..439f67f1e15 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -36,6 +36,7 @@
 #include "llvm/IR/Intrinsics.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/ManagedStatic.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/Mutex.h"
@@ -7539,10 +7540,10 @@ unsigned SelectionDAG::InferPtrAlignment(SDValue Ptr) const {
   int64_t GVOffset = 0;
   if (TLI->isGAPlusOffset(Ptr.getNode(), GV, GVOffset)) {
     unsigned PtrWidth = getDataLayout().getPointerTypeSizeInBits(GV->getType());
-    APInt KnownZero(PtrWidth, 0), KnownOne(PtrWidth, 0);
-    llvm::computeKnownBits(const_cast<GlobalValue *>(GV), KnownZero, KnownOne,
+    KnownBits Known(PtrWidth);
+    llvm::computeKnownBits(const_cast<GlobalValue *>(GV), Known,
                            getDataLayout());
-    unsigned AlignBits = KnownZero.countTrailingOnes();
+    unsigned AlignBits = Known.Zero.countTrailingOnes();
     unsigned Align = AlignBits ? 1 << std::min(31U, AlignBits) : 0;
     if (Align)
       return MinAlign(Align, GVOffset);