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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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31
llvm/lib/Analysis/DemandedBits.cpp
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@@ -37,6 +37,7 @@
#include "llvm/IR/Operator.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
@@ -72,8 +73,7 @@ static bool isAlwaysLive(Instruction *I) {
void DemandedBits::determineLiveOperandBits(
const Instruction *UserI, const Instruction *I, unsigned OperandNo,
const APInt &AOut, APInt &AB, APInt &KnownZero, APInt &KnownOne,
APInt &KnownZero2, APInt &KnownOne2) {
const APInt &AOut, APInt &AB, KnownBits &Known, KnownBits &Known2) {
unsigned BitWidth = AB.getBitWidth();
// We're called once per operand, but for some instructions, we need to
@@ -85,15 +85,13 @@ void DemandedBits::determineLiveOperandBits(
auto ComputeKnownBits =
[&](unsigned BitWidth, const Value *V1, const Value *V2) {
const DataLayout &DL = I->getModule()->getDataLayout();
KnownZero = APInt(BitWidth, 0);
KnownOne = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V1), KnownZero, KnownOne, DL, 0,
Known = KnownBits(BitWidth);
computeKnownBits(const_cast<Value *>(V1), Known, DL, 0,
&AC, UserI, &DT);
if (V2) {
KnownZero2 = APInt(BitWidth, 0);
KnownOne2 = APInt(BitWidth, 0);
computeKnownBits(const_cast<Value *>(V2), KnownZero2, KnownOne2, DL,
Known2 = KnownBits(BitWidth);
computeKnownBits(const_cast<Value *>(V2), Known2, DL,
0, &AC, UserI, &DT);
}
};
@@ -120,7 +118,7 @@ void DemandedBits::determineLiveOperandBits(
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getHighBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countLeadingZeros()+1));
std::min(BitWidth, Known.One.countLeadingZeros()+1));
}
break;
case Intrinsic::cttz:
@@ -130,7 +128,7 @@ void DemandedBits::determineLiveOperandBits(
// known to be one.
ComputeKnownBits(BitWidth, I, nullptr);
AB = APInt::getLowBitsSet(BitWidth,
std::min(BitWidth, KnownOne.countTrailingZeros()+1));
std::min(BitWidth, Known.One.countTrailingZeros()+1));
}
break;
}
@@ -200,11 +198,11 @@ void DemandedBits::determineLiveOperandBits(
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownZero2;
AB &= ~Known2.Zero;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownZero & ~KnownZero2);
AB &= ~(Known.Zero & ~Known2.Zero);
}
break;
case Instruction::Or:
@@ -216,11 +214,11 @@ void DemandedBits::determineLiveOperandBits(
// dead).
if (OperandNo == 0) {
ComputeKnownBits(BitWidth, I, UserI->getOperand(1));
AB &= ~KnownOne2;
AB &= ~Known2.One;
} else {
if (!isa<Instruction>(UserI->getOperand(0)))
ComputeKnownBits(BitWidth, UserI->getOperand(0), I);
AB &= ~(KnownOne & ~KnownOne2);
AB &= ~(Known.One & ~Known2.One);
}
break;
case Instruction::Xor:
@@ -318,7 +316,7 @@ void DemandedBits::performAnalysis() {
if (!UserI->getType()->isIntegerTy())
Visited.insert(UserI);
APInt KnownZero, KnownOne, KnownZero2, KnownOne2;
KnownBits Known, Known2;
// Compute the set of alive bits for each operand. These are anded into the
// existing set, if any, and if that changes the set of alive bits, the
// operand is added to the work-list.
@@ -335,8 +333,7 @@ void DemandedBits::performAnalysis() {
// Bits of each operand that are used to compute alive bits of the
// output are alive, all others are dead.
determineLiveOperandBits(UserI, I, OI.getOperandNo(), AOut, AB,
KnownZero, KnownOne,
KnownZero2, KnownOne2);
Known, Known2);
}
// If we've added to the set of alive bits (or the operand has not