//===- BitTracker.cpp -----------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
// SSA-based bit propagation.
//
// The purpose of this code is, for a given virtual register, to provide
// information about the value of each bit in the register. The values
// of bits are represented by the class BitValue, and take one of four
// cases: 0, 1, "ref" and "bottom". The 0 and 1 are rather clear, the
// "ref" value means that the bit is a copy of another bit (which itself
// cannot be a copy of yet another bit---such chains are not allowed).
// A "ref" value is associated with a BitRef structure, which indicates
// which virtual register, and which bit in that register is the origin
// of the value. For example, given an instruction
// %2 = ASL %1, 1
// assuming that nothing is known about bits of %1, bit 1 of %2
// will be a "ref" to (%1, 0). If there is a subsequent instruction
// %3 = ASL %2, 2
// then bit 3 of %3 will be a "ref" to (%1, 0) as well.
// The "bottom" case means that the bit's value cannot be determined,
// and that this virtual register actually defines it. The "bottom" case
// is discussed in detail in BitTracker.h. In fact, "bottom" is a "ref
// to self", so for the %1 above, the bit 0 of it will be a "ref" to
// (%1, 0), bit 1 will be a "ref" to (%1, 1), etc.
//
// The tracker implements the Wegman-Zadeck algorithm, originally developed
// for SSA-based constant propagation. Each register is represented as
// a sequence of bits, with the convention that bit 0 is the least signi-
// ficant bit. Each bit is propagated individually. The class RegisterCell
// implements the register's representation, and is also the subject of
// the lattice operations in the tracker.
//
// The intended usage of the bit tracker is to create a target-specific
// machine instruction evaluator, pass the evaluator to the BitTracker
// object, and run the tracker. The tracker will then collect the bit
// value information for a given machine function. After that, it can be
// queried for the cells for each virtual register.
// Sample code:
// const TargetSpecificEvaluator TSE(TRI, MRI);
// BitTracker BT(TSE, MF);
// BT.run();
// ...
// unsigned Reg = interestingRegister();
// RegisterCell RC = BT.get(Reg);
// if (RC[3].is(1))
// Reg0bit3 = 1;
//
// The code below is intended to be fully target-independent.
#include "BitTracker.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/CodeGen/MachineBasicBlock.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineOperand.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/TargetRegisterInfo.h"
#include "llvm/IR/Constants.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <cstdint>
#include <iterator>
using namespace llvm;
using BT = BitTracker;
namespace {
// Local trickery to pretty print a register (without the whole "%number"
// business).
struct printv {
printv(unsigned r) : R(r) {}
unsigned R;
};
raw_ostream &operator<< (raw_ostream &OS, const printv &PV) {
if (PV.R)
OS << 'v' << Register::virtReg2Index(PV.R);
else
OS << 's';
return OS;
}
} // end anonymous namespace
namespace llvm {
raw_ostream &operator<<(raw_ostream &OS, const BT::BitValue &BV) {
switch (BV.Type) {
case BT::BitValue::Top:
OS << 'T';
break;
case BT::BitValue::Zero:
OS << '0';
break;
case BT::BitValue::One:
OS << '1';
break;
case BT::BitValue::Ref:
OS << printv(BV.RefI.Reg) << '[' << BV.RefI.Pos << ']';
break;
}
return OS;
}
raw_ostream &operator<<(raw_ostream &OS, const BT::RegisterCell &RC) {
unsigned n = RC.Bits.size();
OS << "{ w:" << n;
// Instead of printing each bit value individually, try to group them
// into logical segments, such as sequences of 0 or 1 bits or references
// to consecutive bits (e.g. "bits 3-5 are same as bits 7-9 of reg xyz").
// "Start" will be the index of the beginning of the most recent segment.
unsigned Start = 0;
bool SeqRef = false; // A sequence of refs to consecutive bits.
bool ConstRef = false; // A sequence of refs to the same bit.
for (unsigned i = 1, n = RC.Bits.size(); i < n; ++i) {
const BT::BitValue &V = RC[i];
const BT::BitValue &SV = RC[Start];
bool IsRef = (V.Type == BT::BitValue::Ref);
// If the current value is the same as Start, skip to the next one.
if (!IsRef && V == SV)
continue;
if (IsRef && SV.Type == BT::BitValue::Ref && V.RefI.Reg == SV.RefI.Reg) {
if (Start+1 == i) {
SeqRef = (V.RefI.Pos == SV.RefI.Pos+1);
ConstRef = (V.RefI.Pos == SV.RefI.Pos);
}
if (SeqRef && V.RefI.Pos == SV.RefI.Pos+(i-Start))
continue;
if (ConstRef && V.RefI.Pos == SV.RefI.Pos)
continue;
}
// The current value is different. Print the previous one and reset
// the Start.
OS << " [" << Start;
unsigned Count = i - Start;
if (Count == 1) {
OS << "]:" << SV;
} else {
OS << '-' << i-1 << "]:";
if (SV.Type == BT::BitValue::Ref && SeqRef)
OS << printv(SV.RefI.Reg) << '[' << SV.RefI.Pos << '-'
<< SV.RefI.Pos+(Count-1) << ']';
else
OS << SV;
}
Start = i;
SeqRef = ConstRef = false;
}
OS << " [" << Start;
unsigned Count = n - Start;
if (n-Start == 1) {
OS << "]:" << RC[Start];
} else {
OS << '-' << n-1 << "]:";
const BT::BitValue &SV = RC[Start];
if (SV.Type == BT::BitValue::Ref && SeqRef)
OS << printv(SV.RefI.Reg) << '[' << SV.RefI.Pos << '-'
<< SV.RefI.Pos+(Count-1) << ']';
else
OS << SV;
}
OS << " }";
return OS;
}
} // end namespace llvm
void BitTracker::print_cells(raw_ostream &OS) const {
for (const std::pair<unsigned, RegisterCell> P : Map)
dbgs() << printReg(P.first, &ME.TRI) << " -> " << P.second << "\n";
}
BitTracker::BitTracker(const MachineEvaluator &E, MachineFunction &F)
: ME(E), MF(F), MRI(F.getRegInfo()), Map(*new CellMapType), Trace(false) {
}
BitTracker::~BitTracker() {
delete ⤅
}
// If we were allowed to update a cell for a part of a register, the meet
// operation would need to be parametrized by the register number and the
// exact part of the register, so that the computer BitRefs correspond to
// the actual bits of the "self" register.
// While this cannot happen in the current implementation, I'm not sure
// if this should be ruled out in the future.
bool BT::RegisterCell::meet(const RegisterCell &RC, unsigned SelfR) {
// An example when "meet" can be invoked with SelfR == 0 is a phi node
// with a physical register as an operand.
assert(SelfR == 0 || Register::isVirtualRegister(SelfR));
bool Changed = false;
for (uint16_t i = 0, n = Bits.size(); i < n; ++i) {
const BitValue &RCV = RC[i];
Changed |= Bits[i].meet(RCV, BitRef(SelfR, i));
}
return Changed;
}
// Insert the entire cell RC into the current cell at position given by M.
BT::RegisterCell &BT::RegisterCell::insert(const BT::RegisterCell &RC,
const BitMask &M) {
uint16_t B = M.first(), E = M.last(), W = width();
// Sanity: M must be a valid mask for *this.
assert(B < W && E < W);
// Sanity: the masked part of *this must have the same number of bits
// as the source.
assert(B > E || E-B+1 == RC.width()); // B <= E => E-B+1 = |RC|.
assert(B <= E || E+(W-B)+1 == RC.width()); // E < B => E+(W-B)+1 = |RC|.
if (B <= E) {
for (uint16_t i = 0; i <= E-B; ++i)
Bits[i+B] = RC[i];
} else {
for (uint16_t i = 0; i < W-B; ++i)
Bits[i+B] = RC[i];
for (uint16_t i = 0; i <= E; ++i)
Bits[i] = RC[i+(W-B)];
}
return *this;
}
BT::RegisterCell BT::RegisterCell::extract(const BitMask &M) const {
uint16_t B = M.first(), E = M.last(), W = width();
assert(B < W && E < W);
if (B <= E) {
RegisterCell RC(E-B+1);
for (uint16_t i = B; i <= E; ++i)
RC.Bits[i-B] = Bits[i];
return RC;
}
RegisterCell RC(E+(W-B)+1);
for (uint16_t i = 0; i < W-B; ++i)
RC.Bits[i] = Bits[i+B];
for (uint16_t i = 0; i <= E; ++i)
RC.Bits[i+(W-B)] = Bits[i];
return RC;
}
BT::RegisterCell &BT::RegisterCell::rol(uint16_t Sh) {
// Rotate left (i.e. towards increasing bit indices).
// Swap the two parts: [0..W-Sh-1] [W-Sh..W-1]
uint16_t W = width();
Sh = Sh % W;
if (Sh == 0)
return *this;
RegisterCell Tmp(W-Sh);
// Tmp = [0..W-Sh-1].
for (uint16_t i = 0; i < W-Sh; ++i)
Tmp[i] = Bits[i];
// Shift [W-Sh..W-1] to [0..Sh-1].
for (uint16_t i = 0; i < Sh; ++i)
Bits[i] = Bits[W-Sh+i];
// Copy Tmp to [Sh..W-1].
for (uint16_t i = 0; i < W-Sh; ++i)
Bits[i+Sh] = Tmp.Bits[i];
return *this;
}
BT::RegisterCell &BT::RegisterCell::fill(uint16_t B, uint16_t E,
const BitValue &V) {
assert(B <= E);
while (B < E)
Bits[B++] = V;
return *this;
}
BT::RegisterCell &BT::RegisterCell::cat(const RegisterCell &RC) {
// Append the cell given as the argument to the "this" cell.
// Bit 0 of RC becomes bit W of the result, where W is this->width().
uint16_t W = width(), WRC = RC.width();
Bits.resize(W+WRC);
for (uint16_t i = 0; i < WRC; ++i)
Bits[i+W] = RC.Bits[i];
return *this;
}
uint16_t BT::RegisterCell::ct(bool B) const {
uint16_t W = width();
uint16_t C = 0;
BitValue V = B;
while (C < W && Bits[C] == V)
C++;
return C;
}
uint16_t BT::RegisterCell::cl(bool B) const {
uint16_t W = width();
uint16_t C = 0;
BitValue V = B;
while (C < W && Bits[W-(C+1)] == V)
C++;
return C;
}
bool BT::RegisterCell::operator== (const RegisterCell &RC) const {
uint16_t W = Bits.size();
if (RC.Bits.size() != W)
return false;
for (uint16_t i = 0; i < W; ++i)
if (Bits[i] != RC[i])
return false;
return true;
}
BT::RegisterCell &BT::RegisterCell::regify(unsigned R) {
for (unsigned i = 0, n = width(); i < n; ++i) {
const BitValue &V = Bits[i];
if (V.Type == BitValue::Ref && V.RefI.Reg == 0)
Bits[i].RefI = BitRef(R, i);
}
return *this;
}
uint16_t BT::MachineEvaluator::getRegBitWidth(const RegisterRef &RR) const {
// The general problem is with finding a register class that corresponds
// to a given reference reg:sub. There can be several such classes, and
// since we only care about the register size, it does not matter which
// such class we would find.
// The easiest way to accomplish what we want is to
// 1. find a physical register PhysR from the same class as RR.Reg,
// 2. find a physical register PhysS that corresponds to PhysR:RR.Sub,
// 3. find a register class that contains PhysS.
if (Register::isVirtualRegister(RR.Reg)) {
const auto &VC = composeWithSubRegIndex(*MRI.getRegClass(RR.Reg), RR.Sub);
return TRI.getRegSizeInBits(VC);
}
assert(Register::isPhysicalRegister(RR.Reg));
Register PhysR =
(RR.Sub == 0) ? Register(RR.Reg) : TRI.getSubReg(RR.Reg, RR.Sub);
return getPhysRegBitWidth(PhysR);
}
BT::RegisterCell BT::MachineEvaluator::getCell(const RegisterRef &RR,
const CellMapType &M) const {
uint16_t BW = getRegBitWidth(RR);
// Physical registers are assumed to be present in the map with an unknown
// value. Don't actually insert anything in the map, just return the cell.
if (Register::isPhysicalRegister(RR.Reg))
return RegisterCell::self(0, BW);
assert(Register::isVirtualRegister(RR.Reg));
// For virtual registers that belong to a class that is not tracked,
// generate an "unknown" value as well.
const TargetRegisterClass *C = MRI.getRegClass(RR.Reg);
if (!track(C))
return RegisterCell::self(0, BW);
CellMapType::const_iterator F = M.find(RR.Reg);
if (F != M.end()) {
if (!RR.Sub)
return F->second;
BitMask M = mask(RR.Reg, RR.Sub);
return F->second.extract(M);
}
// If not found, create a "top" entry, but do not insert it in the map.
return RegisterCell::top(BW);
}
void BT::MachineEvaluator::putCell(const RegisterRef &RR, RegisterCell RC,
CellMapType &M) const {
// While updating the cell map can be done in a meaningful way for
// a part of a register, it makes little sense to implement it as the
// SSA representation would never contain such "partial definitions".
if (!Register::isVirtualRegister(RR.Reg))
return;
assert(RR.Sub == 0 && "Unexpected sub-register in definition");
// Eliminate all ref-to-reg-0 bit values: replace them with "self".
M[RR.Reg] = RC.regify(RR.Reg);
}
// Check if the cell represents a compile-time integer value.
bool BT::MachineEvaluator::isInt(const RegisterCell &A) const {
uint16_t W = A.width();
for (uint16_t i = 0; i < W; ++i)
if (!A[i].is(0) && !A[i].is(1))
return false;
return true;
}
// Convert a cell to the integer value. The result must fit in uint64_t.
uint64_t BT::MachineEvaluator::toInt(const RegisterCell &A) const {
assert(isInt(A));
uint64_t Val = 0;
uint16_t W = A.width();
for (uint16_t i = 0; i < W; ++i) {
Val <<= 1;
Val |= A[i].is(1);
}
return Val;
}
// Evaluator helper functions. These implement some common operation on
// register cells that can be used to implement target-specific instructions
// in a target-specific evaluator.
BT::RegisterCell BT::MachineEvaluator::eIMM(int64_t V, uint16_t W) const {
RegisterCell Res(W);
// For bits beyond the 63rd, this will generate the sign bit of V.
for (uint16_t i = 0; i < W; ++i) {
Res[i] = BitValue(V & 1);
V >>= 1;
}
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eIMM(const ConstantInt *CI) const {
const APInt &A = CI->getValue();
uint16_t BW = A.getBitWidth();
assert((unsigned)BW == A.getBitWidth() && "BitWidth overflow");
RegisterCell Res(BW);
for (uint16_t i = 0; i < BW; ++i)
Res[i] = A[i];
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eADD(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width();
assert(W == A2.width());
RegisterCell Res(W);
bool Carry = false;
uint16_t I;
for (I = 0; I < W; ++I) {
const BitValue &V1 = A1[I];
const BitValue &V2 = A2[I];
if (!V1.num() || !V2.num())
break;
unsigned S = bool(V1) + bool(V2) + Carry;
Res[I] = BitValue(S & 1);
Carry = (S > 1);
}
for (; I < W; ++I) {
const BitValue &V1 = A1[I];
const BitValue &V2 = A2[I];
// If the next bit is same as Carry, the result will be 0 plus the
// other bit. The Carry bit will remain unchanged.
if (V1.is(Carry))
Res[I] = BitValue::ref(V2);
else if (V2.is(Carry))
Res[I] = BitValue::ref(V1);
else
break;
}
for (; I < W; ++I)
Res[I] = BitValue::self();
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eSUB(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width();
assert(W == A2.width());
RegisterCell Res(W);
bool Borrow = false;
uint16_t I;
for (I = 0; I < W; ++I) {
const BitValue &V1 = A1[I];
const BitValue &V2 = A2[I];
if (!V1.num() || !V2.num())
break;
unsigned S = bool(V1) - bool(V2) - Borrow;
Res[I] = BitValue(S & 1);
Borrow = (S > 1);
}
for (; I < W; ++I) {
const BitValue &V1 = A1[I];
const BitValue &V2 = A2[I];
if (V1.is(Borrow)) {
Res[I] = BitValue::ref(V2);
break;
}
if (V2.is(Borrow))
Res[I] = BitValue::ref(V1);
else
break;
}
for (; I < W; ++I)
Res[I] = BitValue::self();
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eMLS(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width() + A2.width();
uint16_t Z = A1.ct(false) + A2.ct(false);
RegisterCell Res(W);
Res.fill(0, Z, BitValue::Zero);
Res.fill(Z, W, BitValue::self());
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eMLU(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width() + A2.width();
uint16_t Z = A1.ct(false) + A2.ct(false);
RegisterCell Res(W);
Res.fill(0, Z, BitValue::Zero);
Res.fill(Z, W, BitValue::self());
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eASL(const RegisterCell &A1,
uint16_t Sh) const {
assert(Sh <= A1.width());
RegisterCell Res = RegisterCell::ref(A1);
Res.rol(Sh);
Res.fill(0, Sh, BitValue::Zero);
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eLSR(const RegisterCell &A1,
uint16_t Sh) const {
uint16_t W = A1.width();
assert(Sh <= W);
RegisterCell Res = RegisterCell::ref(A1);
Res.rol(W-Sh);
Res.fill(W-Sh, W, BitValue::Zero);
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eASR(const RegisterCell &A1,
uint16_t Sh) const {
uint16_t W = A1.width();
assert(Sh <= W);
RegisterCell Res = RegisterCell::ref(A1);
BitValue Sign = Res[W-1];
Res.rol(W-Sh);
Res.fill(W-Sh, W, Sign);
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eAND(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width();
assert(W == A2.width());
RegisterCell Res(W);
for (uint16_t i = 0; i < W; ++i) {
const BitValue &V1 = A1[i];
const BitValue &V2 = A2[i];
if (V1.is(1))
Res[i] = BitValue::ref(V2);
else if (V2.is(1))
Res[i] = BitValue::ref(V1);
else if (V1.is(0) || V2.is(0))
Res[i] = BitValue::Zero;
else if (V1 == V2)
Res[i] = V1;
else
Res[i] = BitValue::self();
}
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eORL(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width();
assert(W == A2.width());
RegisterCell Res(W);
for (uint16_t i = 0; i < W; ++i) {
const BitValue &V1 = A1[i];
const BitValue &V2 = A2[i];
if (V1.is(1) || V2.is(1))
Res[i] = BitValue::One;
else if (V1.is(0))
Res[i] = BitValue::ref(V2);
else if (V2.is(0))
Res[i] = BitValue::ref(V1);
else if (V1 == V2)
Res[i] = V1;
else
Res[i] = BitValue::self();
}
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eXOR(const RegisterCell &A1,
const RegisterCell &A2) const {
uint16_t W = A1.width();
assert(W == A2.width());
RegisterCell Res(W);
for (uint16_t i = 0; i < W; ++i) {
const BitValue &V1 = A1[i];
const BitValue &V2 = A2[i];
if (V1.is(0))
Res[i] = BitValue::ref(V2);
else if (V2.is(0))
Res[i] = BitValue::ref(V1);
else if (V1 == V2)
Res[i] = BitValue::Zero;
else
Res[i] = BitValue::self();
}
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eNOT(const RegisterCell &A1) const {
uint16_t W = A1.width();
RegisterCell Res(W);
for (uint16_t i = 0; i < W; ++i) {
const BitValue &V = A1[i];
if (V.is(0))
Res[i] = BitValue::One;
else if (V.is(1))
Res[i] = BitValue::Zero;
else
Res[i] = BitValue::self();
}
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eSET(const RegisterCell &A1,
uint16_t BitN) const {
assert(BitN < A1.width());
RegisterCell Res = RegisterCell::ref(A1);
Res[BitN] = BitValue::One;
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eCLR(const RegisterCell &A1,
uint16_t BitN) const {
assert(BitN < A1.width());
RegisterCell Res = RegisterCell::ref(A1);
Res[BitN] = BitValue::Zero;
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eCLB(const RegisterCell &A1, bool B,
uint16_t W) const {
uint16_t C = A1.cl(B), AW = A1.width();
// If the last leading non-B bit is not a constant, then we don't know
// the real count.
if ((C < AW && A1[AW-1-C].num()) || C == AW)
return eIMM(C, W);
return RegisterCell::self(0, W);
}
BT::RegisterCell BT::MachineEvaluator::eCTB(const RegisterCell &A1, bool B,
uint16_t W) const {
uint16_t C = A1.ct(B), AW = A1.width();
// If the last trailing non-B bit is not a constant, then we don't know
// the real count.
if ((C < AW && A1[C].num()) || C == AW)
return eIMM(C, W);
return RegisterCell::self(0, W);
}
BT::RegisterCell BT::MachineEvaluator::eSXT(const RegisterCell &A1,
uint16_t FromN) const {
uint16_t W = A1.width();
assert(FromN <= W);
RegisterCell Res = RegisterCell::ref(A1);
BitValue Sign = Res[FromN-1];
// Sign-extend "inreg".
Res.fill(FromN, W, Sign);
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eZXT(const RegisterCell &A1,
uint16_t FromN) const {
uint16_t W = A1.width();
assert(FromN <= W);
RegisterCell Res = RegisterCell::ref(A1);
Res.fill(FromN, W, BitValue::Zero);
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eXTR(const RegisterCell &A1,
uint16_t B, uint16_t E) const {
uint16_t W = A1.width();
assert(B < W && E <= W);
if (B == E)
return RegisterCell(0);
uint16_t Last = (E > 0) ? E-1 : W-1;
RegisterCell Res = RegisterCell::ref(A1).extract(BT::BitMask(B, Last));
// Return shorter cell.
return Res;
}
BT::RegisterCell BT::MachineEvaluator::eINS(const RegisterCell &A1,
const RegisterCell &A2, uint16_t AtN) const {
uint16_t W1 = A1.width(), W2 = A2.width();
(void)W1;
assert(AtN < W1 && AtN+W2 <= W1);
// Copy bits from A1, insert A2 at position AtN.
RegisterCell Res = RegisterCell::ref(A1);
if (W2 > 0)
Res.insert(RegisterCell::ref(A2), BT::BitMask(AtN, AtN+W2-1));
return Res;
}
BT::BitMask BT::MachineEvaluator::mask(unsigned Reg, unsigned Sub) const {
assert(Sub == 0 && "Generic BitTracker::mask called for Sub != 0");
uint16_t W = getRegBitWidth(Reg);
assert(W > 0 && "Cannot generate mask for empty register");
return BitMask(0, W-1);
}
uint16_t BT::MachineEvaluator::getPhysRegBitWidth(unsigned Reg) const {
assert(Register::isPhysicalRegister(Reg));
const TargetRegisterClass &PC = *TRI.getMinimalPhysRegClass(Reg);
return TRI.getRegSizeInBits(PC);
}
bool BT::MachineEvaluator::evaluate(const MachineInstr &MI,
const CellMapType &Inputs,
CellMapType &Outputs) const {
unsigned Opc = MI.getOpcode();
switch (Opc) {
case TargetOpcode::REG_SEQUENCE: {
RegisterRef RD = MI.getOperand(0);
assert(RD.Sub == 0);
RegisterRef RS = MI.getOperand(1);
unsigned SS = MI.getOperand(2).getImm();
RegisterRef RT = MI.getOperand(3);
unsigned ST = MI.getOperand(4).getImm();
assert(SS != ST);
uint16_t W = getRegBitWidth(RD);
RegisterCell Res(W);
Res.insert(RegisterCell::ref(getCell(RS, Inputs)), mask(RD.Reg, SS));
Res.insert(RegisterCell::ref(getCell(RT, Inputs)), mask(RD.Reg, ST));
putCell(RD, Res, Outputs);
break;
}
case TargetOpcode::COPY: {
// COPY can transfer a smaller register into a wider one.
// If that is the case, fill the remaining high bits with 0.
RegisterRef RD = MI.getOperand(0);
RegisterRef RS = MI.getOperand(1);
assert(RD.Sub == 0);
uint16_t WD = getRegBitWidth(RD);
uint16_t WS = getRegBitWidth(RS);
assert(WD >= WS);
RegisterCell Src = getCell(RS, Inputs);
RegisterCell Res(WD);
Res.insert(Src, BitMask(0, WS-1));
Res.fill(WS, WD, BitValue::Zero);
putCell(RD, Res, Outputs);
break;
}
default:
return false;
}
return true;
}
bool BT::UseQueueType::Cmp::operator()(const MachineInstr *InstA,
const MachineInstr *InstB) const {
// This is a comparison function for a priority queue: give higher priority
// to earlier instructions.
// This operator is used as "less", so returning "true" gives InstB higher
// priority (because then InstA < InstB).
if (InstA == InstB)
return false;
const MachineBasicBlock *BA = InstA->getParent();
const MachineBasicBlock *BB = InstB->getParent();
if (BA != BB) {
// If the blocks are different, ideally the dominating block would
// have a higher priority, but it may be too expensive to check.
return BA->getNumber() > BB->getNumber();
}
auto getDist = [this] (const MachineInstr *MI) {
auto F = Dist.find(MI);
if (F != Dist.end())
return F->second;
MachineBasicBlock::const_iterator I = MI->getParent()->begin();
MachineBasicBlock::const_iterator E = MI->getIterator();
unsigned D = std::distance(I, E);
Dist.insert(std::make_pair(MI, D));
return D;
};
return getDist(InstA) > getDist(InstB);
}
// Main W-Z implementation.
void BT::visitPHI(const MachineInstr &PI) {
int ThisN = PI.getParent()->getNumber();
if (Trace)
dbgs() << "Visit FI(" << printMBBReference(*PI.getParent()) << "): " << PI;
const MachineOperand &MD = PI.getOperand(0);
assert(MD.getSubReg() == 0 && "Unexpected sub-register in definition");
RegisterRef DefRR(MD);
uint16_t DefBW = ME.getRegBitWidth(DefRR);
RegisterCell DefC = ME.getCell(DefRR, Map);
if (DefC == RegisterCell::self(DefRR.Reg, DefBW)) // XXX slow
return;
bool Changed = false;
for (unsigned i = 1, n = PI.getNumOperands(); i < n; i += 2) {
const MachineBasicBlock *PB = PI.getOperand(i + 1).getMBB();
int PredN = PB->getNumber();
if (Trace)
dbgs() << " edge " << printMBBReference(*PB) << "->"
<< printMBBReference(*PI.getParent());
if (!EdgeExec.count(CFGEdge(PredN, ThisN))) {
if (Trace)
dbgs() << " not executable\n";
continue;
}
RegisterRef RU = PI.getOperand(i);
RegisterCell ResC = ME.getCell(RU, Map);
if (Trace)
dbgs() << " input reg: " << printReg(RU.Reg, &ME.TRI, RU.Sub)
<< " cell: " << ResC << "\n";
Changed |= DefC.meet(ResC, DefRR.Reg);
}
if (Changed) {
if (Trace)
dbgs() << "Output: " << printReg(DefRR.Reg, &ME.TRI, DefRR.Sub)
<< " cell: " << DefC << "\n";
ME.putCell(DefRR, DefC, Map);
visitUsesOf(DefRR.Reg);
}
}
void BT::visitNonBranch(const MachineInstr &MI) {
if (Trace)
dbgs() << "Visit MI(" << printMBBReference(*MI.getParent()) << "): " << MI;
if (MI.isDebugInstr())
return;
assert(!MI.isBranch() && "Unexpected branch instruction");
CellMapType ResMap;
bool Eval = ME.evaluate(MI, Map, ResMap);
if (Trace && Eval) {
for (unsigned i = 0, n = MI.getNumOperands(); i < n; ++i) {
const MachineOperand &MO = MI.getOperand(i);
if (!MO.isReg() || !MO.isUse())
continue;
RegisterRef RU(MO);
dbgs() << " input reg: " << printReg(RU.Reg, &ME.TRI, RU.Sub)
<< " cell: " << ME.getCell(RU, Map) << "\n";
}
dbgs() << "Outputs:\n";
for (const std::pair<const unsigned, RegisterCell> &P : ResMap) {
RegisterRef RD(P.first);
dbgs() << " " << printReg(P.first, &ME.TRI) << " cell: "
<< ME.getCell(RD, ResMap) << "\n";
}
}
// Iterate over all definitions of the instruction, and update the
// cells accordingly.
for (const MachineOperand &MO : MI.operands()) {
// Visit register defs only.
if (!MO.isReg() || !MO.isDef())
continue;
RegisterRef RD(MO);
assert(RD.Sub == 0 && "Unexpected sub-register in definition");
if (!Register::isVirtualRegister(RD.Reg))
continue;
bool Changed = false;
if (!Eval || ResMap.count(RD.Reg) == 0) {
// Set to "ref" (aka "bottom").
uint16_t DefBW = ME.getRegBitWidth(RD);
RegisterCell RefC = RegisterCell::self(RD.Reg, DefBW);
if (RefC != ME.getCell(RD, Map)) {
ME.putCell(RD, RefC, Map);
Changed = true;
}
} else {
RegisterCell DefC = ME.getCell(RD, Map);
RegisterCell ResC = ME.getCell(RD, ResMap);
// This is a non-phi instruction, so the values of the inputs come
// from the same registers each time this instruction is evaluated.
// During the propagation, the values of the inputs can become lowered
// in the sense of the lattice operation, which may cause different
// results to be calculated in subsequent evaluations. This should
// not cause the bottoming of the result in the map, since the new
// result is already reflecting the lowered inputs.
for (uint16_t i = 0, w = DefC.width(); i < w; ++i) {
BitValue &V = DefC[i];
// Bits that are already "bottom" should not be updated.
if (V.Type == BitValue::Ref && V.RefI.Reg == RD.Reg)
continue;
// Same for those that are identical in DefC and ResC.
if (V == ResC[i])
continue;
V = ResC[i];
Changed = true;
}
if (Changed)
ME.putCell(RD, DefC, Map);
}
if (Changed)
visitUsesOf(RD.Reg);
}
}
void BT::visitBranchesFrom(const MachineInstr &BI) {
const MachineBasicBlock &B = *BI.getParent();
MachineBasicBlock::const_iterator It = BI, End = B.end();
BranchTargetList Targets, BTs;
bool FallsThrough = true, DefaultToAll = false;
int ThisN = B.getNumber();
do {
BTs.clear();
const MachineInstr &MI = *It;
if (Trace)
dbgs() << "Visit BR(" << printMBBReference(B) << "): " << MI;
assert(MI.isBranch() && "Expecting branch instruction");
InstrExec.insert(&MI);
bool Eval = ME.evaluate(MI, Map, BTs, FallsThrough);
if (!Eval) {
// If the evaluation failed, we will add all targets. Keep going in
// the loop to mark all executable branches as such.
DefaultToAll = true;
FallsThrough = true;
if (Trace)
dbgs() << " failed to evaluate: will add all CFG successors\n";
} else if (!DefaultToAll) {
// If evaluated successfully add the targets to the cumulative list.
if (Trace) {
dbgs() << " adding targets:";
for (unsigned i = 0, n = BTs.size(); i < n; ++i)
dbgs() << " " << printMBBReference(*BTs[i]);
if (FallsThrough)
dbgs() << "\n falls through\n";
else
dbgs() << "\n does not fall through\n";
}
Targets.insert(BTs.begin(), BTs.end());
}
++It;
} while (FallsThrough && It != End);
if (B.mayHaveInlineAsmBr())
DefaultToAll = true;
if (!DefaultToAll) {
// Need to add all CFG successors that lead to EH landing pads.
// There won't be explicit branches to these blocks, but they must
// be processed.
for (const MachineBasicBlock *SB : B.successors()) {
if (SB->isEHPad())
Targets.insert(SB);
}
if (FallsThrough) {
MachineFunction::const_iterator BIt = B.getIterator();
MachineFunction::const_iterator Next = std::next(BIt);
if (Next != MF.end())
Targets.insert(&*Next);
}
} else {
for (const MachineBasicBlock *SB : B.successors())
Targets.insert(SB);
}
for (const MachineBasicBlock *TB : Targets)
FlowQ.push(CFGEdge(ThisN, TB->getNumber()));
}
void BT::visitUsesOf(unsigned Reg) {
if (Trace)
dbgs() << "queuing uses of modified reg " << printReg(Reg, &ME.TRI)
<< " cell: " << ME.getCell(Reg, Map) << '\n';
for (MachineInstr &UseI : MRI.use_nodbg_instructions(Reg))
UseQ.push(&UseI);
}
BT::RegisterCell BT::get(RegisterRef RR) const {
return ME.getCell(RR, Map);
}
void BT::put(RegisterRef RR, const RegisterCell &RC) {
ME.putCell(RR, RC, Map);
}
// Replace all references to bits from OldRR with the corresponding bits
// in NewRR.
void BT::subst(RegisterRef OldRR, RegisterRef NewRR) {
assert(Map.count(OldRR.Reg) > 0 && "OldRR not present in map");
BitMask OM = ME.mask(OldRR.Reg, OldRR.Sub);
BitMask NM = ME.mask(NewRR.Reg, NewRR.Sub);
uint16_t OMB = OM.first(), OME = OM.last();
uint16_t NMB = NM.first(), NME = NM.last();
(void)NME;
assert((OME-OMB == NME-NMB) &&
"Substituting registers of different lengths");
for (std::pair<const unsigned, RegisterCell> &P : Map) {
RegisterCell &RC = P.second;
for (uint16_t i = 0, w = RC.width(); i < w; ++i) {
BitValue &V = RC[i];
if (V.Type != BitValue::Ref || V.RefI.Reg != OldRR.Reg)
continue;
if (V.RefI.Pos < OMB || V.RefI.Pos > OME)
continue;
V.RefI.Reg = NewRR.Reg;
V.RefI.Pos += NMB-OMB;
}
}
}
// Check if the block has been "executed" during propagation. (If not, the
// block is dead, but it may still appear to be reachable.)
bool BT::reached(const MachineBasicBlock *B) const {
int BN = B->getNumber();
assert(BN >= 0);
return ReachedBB.count(BN);
}
// Visit an individual instruction. This could be a newly added instruction,
// or one that has been modified by an optimization.
void BT::visit(const MachineInstr &MI) {
assert(!MI.isBranch() && "Only non-branches are allowed");
InstrExec.insert(&MI);
visitNonBranch(MI);
// Make sure to flush all the pending use updates.
runUseQueue();
// The call to visitNonBranch could propagate the changes until a branch
// is actually visited. This could result in adding CFG edges to the flow
// queue. Since the queue won't be processed, clear it.
while (!FlowQ.empty())
FlowQ.pop();
}
void BT::reset() {
EdgeExec.clear();
InstrExec.clear();
Map.clear();
ReachedBB.clear();
ReachedBB.reserve(MF.size());
}
void BT::runEdgeQueue(BitVector &BlockScanned) {
while (!FlowQ.empty()) {
CFGEdge Edge = FlowQ.front();
FlowQ.pop();
if (EdgeExec.count(Edge))
return;
EdgeExec.insert(Edge);
ReachedBB.insert(Edge.second);
const MachineBasicBlock &B = *MF.getBlockNumbered(Edge.second);
MachineBasicBlock::const_iterator It = B.begin(), End = B.end();
// Visit PHI nodes first.
while (It != End && It->isPHI()) {
const MachineInstr &PI = *It++;
InstrExec.insert(&PI);
visitPHI(PI);
}
// If this block has already been visited through a flow graph edge,
// then the instructions have already been processed. Any updates to
// the cells would now only happen through visitUsesOf...
if (BlockScanned[Edge.second])
return;
BlockScanned[Edge.second] = true;
// Visit non-branch instructions.
while (It != End && !It->isBranch()) {
const MachineInstr &MI = *It++;
InstrExec.insert(&MI);
visitNonBranch(MI);
}
// If block end has been reached, add the fall-through edge to the queue.
if (It == End) {
MachineFunction::const_iterator BIt = B.getIterator();
MachineFunction::const_iterator Next = std::next(BIt);
if (Next != MF.end() && B.isSuccessor(&*Next)) {
int ThisN = B.getNumber();
int NextN = Next->getNumber();
FlowQ.push(CFGEdge(ThisN, NextN));
}
} else {
// Handle the remaining sequence of branches. This function will update
// the work queue.
visitBranchesFrom(*It);
}
} // while (!FlowQ->empty())
}
void BT::runUseQueue() {
while (!UseQ.empty()) {
MachineInstr &UseI = *UseQ.front();
UseQ.pop();
if (!InstrExec.count(&UseI))
continue;
if (UseI.isPHI())
visitPHI(UseI);
else if (!UseI.isBranch())
visitNonBranch(UseI);
else
visitBranchesFrom(UseI);
}
}
void BT::run() {
reset();
assert(FlowQ.empty());
using MachineFlowGraphTraits = GraphTraits<const MachineFunction*>;
const MachineBasicBlock *Entry = MachineFlowGraphTraits::getEntryNode(&MF);
unsigned MaxBN = 0;
for (const MachineBasicBlock &B : MF) {
assert(B.getNumber() >= 0 && "Disconnected block");
unsigned BN = B.getNumber();
if (BN > MaxBN)
MaxBN = BN;
}
// Keep track of visited blocks.
BitVector BlockScanned(MaxBN+1);
int EntryN = Entry->getNumber();
// Generate a fake edge to get something to start with.
FlowQ.push(CFGEdge(-1, EntryN));
while (!FlowQ.empty() || !UseQ.empty()) {
runEdgeQueue(BlockScanned);
runUseQueue();
}
UseQ.reset();
if (Trace)
print_cells(dbgs() << "Cells after propagation:\n");
}