[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/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 1c2c763cea0..f86e97b6cc7 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -60,6 +60,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/KnownBits.h"
 #include "llvm/Support/MathExtras.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
@@ -4367,8 +4368,8 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
                                      const DataLayout &DL) {
   Value *Cond = SI->getCondition();
   unsigned Bits = Cond->getType()->getIntegerBitWidth();
-  APInt KnownZero(Bits, 0), KnownOne(Bits, 0);
-  computeKnownBits(Cond, KnownZero, KnownOne, DL, 0, AC, SI);
+  KnownBits Known(Bits);
+  computeKnownBits(Cond, Known, DL, 0, AC, SI);
 
   // We can also eliminate cases by determining that their values are outside of
   // the limited range of the condition based on how many significant (non-sign)
@@ -4380,7 +4381,7 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
   SmallVector<ConstantInt *, 8> DeadCases;
   for (auto &Case : SI->cases()) {
     APInt CaseVal = Case.getCaseValue()->getValue();
-    if (KnownZero.intersects(CaseVal) || !KnownOne.isSubsetOf(CaseVal) ||
+    if (Known.Zero.intersects(CaseVal) || !Known.One.isSubsetOf(CaseVal) ||
         (CaseVal.getMinSignedBits() > MaxSignificantBitsInCond)) {
       DeadCases.push_back(Case.getCaseValue());
       DEBUG(dbgs() << "SimplifyCFG: switch case " << CaseVal << " is dead.\n");
@@ -4394,7 +4395,7 @@ static bool EliminateDeadSwitchCases(SwitchInst *SI, AssumptionCache *AC,
   bool HasDefault =
       !isa<UnreachableInst>(SI->getDefaultDest()->getFirstNonPHIOrDbg());
   const unsigned NumUnknownBits =
-      Bits - (KnownZero | KnownOne).countPopulation();
+      Bits - (Known.Zero | Known.One).countPopulation();
   assert(NumUnknownBits <= Bits);
   if (HasDefault && DeadCases.empty() &&
       NumUnknownBits < 64 /* avoid overflow */ &&