[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, 6 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
index 2640c1f447a..2c1c30463a2 100644
--- a/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyLibCalls.cpp
@@ -30,6 +30,7 @@
 #include "llvm/IR/Module.h"
 #include "llvm/IR/PatternMatch.h"
 #include "llvm/Support/CommandLine.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Transforms/Utils/BuildLibCalls.h"
 #include "llvm/Transforms/Utils/Local.h"
 
@@ -455,11 +456,9 @@ Value *LibCallSimplifier::optimizeStrLen(CallInst *CI, IRBuilder<> &B) {
      
       Value *Offset = GEP->getOperand(2);
       unsigned BitWidth = Offset->getType()->getIntegerBitWidth();
-      APInt KnownZero(BitWidth, 0);
-      APInt KnownOne(BitWidth, 0);
-      computeKnownBits(Offset, KnownZero, KnownOne, DL, 0, nullptr, CI, 
-                       nullptr);
-      KnownZero.flipAllBits();
+      KnownBits Known(BitWidth);
+      computeKnownBits(Offset, Known, DL, 0, nullptr, CI, nullptr);
+      Known.Zero.flipAllBits();
       size_t ArrSize = 
              cast<ArrayType>(GEP->getSourceElementType())->getNumElements();
 
@@ -473,7 +472,7 @@ Value *LibCallSimplifier::optimizeStrLen(CallInst *CI, IRBuilder<> &B) {
       // optimize if we can prove that the program has undefined behavior when 
       // Offset is outside that range. That is the case when GEP->getOperand(0) 
       // is a pointer to an object whose memory extent is NullTermIdx+1.
-      if ((KnownZero.isNonNegative() && KnownZero.ule(NullTermIdx)) || 
+      if ((Known.Zero.isNonNegative() && Known.Zero.ule(NullTermIdx)) || 
           (GEP->isInBounds() && isa<GlobalVariable>(GEP->getOperand(0)) &&
            NullTermIdx == ArrSize - 1))
         return B.CreateSub(ConstantInt::get(CI->getType(), NullTermIdx),