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

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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
This commit is contained in:
Craig Topper
2017-04-26 16:39:58 +00:00
parent eec01adde3
commit b45eabcf82
25 changed files with 837 additions and 860 deletions
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9
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
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@@ -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 */ &&