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RISCVMakeCompressible.cpp
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RISCVMakeCompressible.cpp
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//===-- RISCVMakeCompressible.cpp - Make more instructions compressible ---===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// This pass searches for instructions that are prevented from being compressed
// by one of the following:
//
// 1. The use of a single uncompressed register.
// 2. A base register + offset where the offset is too large to be compressed
// and the base register may or may not be compressed.
//
//
// For case 1, if a compressed register is available, then the uncompressed
// register is copied to the compressed register and its uses are replaced.
//
// For example, storing zero uses the uncompressible zero register:
// sw zero, 0(a0) # if zero
// sw zero, 8(a0) # if zero
// sw zero, 4(a0) # if zero
// sw zero, 24(a0) # if zero
//
// If a compressed register (e.g. a1) is available, the above can be transformed
// to the following to improve code size:
// li a1, 0
// c.sw a1, 0(a0)
// c.sw a1, 8(a0)
// c.sw a1, 4(a0)
// c.sw a1, 24(a0)
//
//
// For case 2, if a compressed register is available, then the original base
// is copied and adjusted such that:
//
// new_base_register = base_register + adjustment
// base_register + large_offset = new_base_register + small_offset
//
// For example, the following offsets are too large for c.sw:
// lui a2, 983065
// sw a1, -236(a2)
// sw a1, -240(a2)
// sw a1, -244(a2)
// sw a1, -248(a2)
// sw a1, -252(a2)
// sw a0, -256(a2)
//
// If a compressed register is available (e.g. a3), a new base could be created
// such that the addresses can accessed with a compressible offset, thus
// improving code size:
// lui a2, 983065
// addi a3, a2, -256
// c.sw a1, 20(a3)
// c.sw a1, 16(a3)
// c.sw a1, 12(a3)
// c.sw a1, 8(a3)
// c.sw a1, 4(a3)
// c.sw a0, 0(a3)
//
//
// This optimization is only applied if there are enough uses of the copied
// register for code size to be reduced.
//
//===----------------------------------------------------------------------===//
#include "RISCV.h"
#include "RISCVSubtarget.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/RegisterScavenging.h"
#include "llvm/MC/TargetRegistry.h"
#include "llvm/Support/Debug.h"
using namespace llvm;
#define DEBUG_TYPE "riscv-make-compressible"
#define RISCV_COMPRESS_INSTRS_NAME "RISCV Make Compressible"
namespace {
struct RISCVMakeCompressibleOpt : public MachineFunctionPass {
static char ID;
bool runOnMachineFunction(MachineFunction &Fn) override;
RISCVMakeCompressibleOpt() : MachineFunctionPass(ID) {
initializeRISCVMakeCompressibleOptPass(*PassRegistry::getPassRegistry());
}
StringRef getPassName() const override { return RISCV_COMPRESS_INSTRS_NAME; }
};
} // namespace
char RISCVMakeCompressibleOpt::ID = 0;
INITIALIZE_PASS(RISCVMakeCompressibleOpt, "riscv-make-compressible",
RISCV_COMPRESS_INSTRS_NAME, false, false)
// Return log2(widthInBytes) of load/store done by Opcode.
static unsigned log2LdstWidth(unsigned Opcode) {
switch (Opcode) {
default:
llvm_unreachable("Unexpected opcode");
case RISCV::LW:
case RISCV::SW:
case RISCV::FLW:
case RISCV::FSW:
return 2;
case RISCV::LD:
case RISCV::SD:
case RISCV::FLD:
case RISCV::FSD:
return 3;
}
}
// Return a mask for the offset bits of a non-stack-pointer based compressed
// load/store.
static uint8_t compressedLDSTOffsetMask(unsigned Opcode) {
return 0x1f << log2LdstWidth(Opcode);
}
// Return true if Offset fits within a compressed stack-pointer based
// load/store.
static bool compressibleSPOffset(int64_t Offset, unsigned Opcode) {
return log2LdstWidth(Opcode) == 2 ? isShiftedUInt<6, 2>(Offset)
: isShiftedUInt<6, 3>(Offset);
}
// Given an offset for a load/store, return the adjustment required to the base
// register such that the address can be accessed with a compressible offset.
// This will return 0 if the offset is already compressible.
static int64_t getBaseAdjustForCompression(int64_t Offset, unsigned Opcode) {
// Return the excess bits that do not fit in a compressible offset.
return Offset & ~compressedLDSTOffsetMask(Opcode);
}
// Return true if Reg is in a compressed register class.
static bool isCompressedReg(Register Reg) {
return RISCV::GPRCRegClass.contains(Reg) ||
RISCV::FPR32CRegClass.contains(Reg) ||
RISCV::FPR64CRegClass.contains(Reg);
}
// Return true if MI is a load for which there exists a compressed version.
static bool isCompressibleLoad(const MachineInstr &MI) {
const RISCVSubtarget &STI = MI.getMF()->getSubtarget<RISCVSubtarget>();
const unsigned Opcode = MI.getOpcode();
return Opcode == RISCV::LW || (!STI.is64Bit() && Opcode == RISCV::FLW) ||
Opcode == RISCV::LD || Opcode == RISCV::FLD;
}
// Return true if MI is a store for which there exists a compressed version.
static bool isCompressibleStore(const MachineInstr &MI) {
const RISCVSubtarget &STI = MI.getMF()->getSubtarget<RISCVSubtarget>();
const unsigned Opcode = MI.getOpcode();
return Opcode == RISCV::SW || (!STI.is64Bit() && Opcode == RISCV::FSW) ||
Opcode == RISCV::SD || Opcode == RISCV::FSD;
}
// Find a single register and/or large offset which, if compressible, would
// allow the given instruction to be compressed.
//
// Possible return values:
//
// {Reg, 0} - Uncompressed Reg needs replacing with a compressed
// register.
// {Reg, N} - Reg needs replacing with a compressed register and
// N needs adding to the new register. (Reg may be
// compressed or uncompressed).
// {RISCV::NoRegister, 0} - No suitable optimization found for this
// instruction.
static RegImmPair getRegImmPairPreventingCompression(const MachineInstr &MI) {
const unsigned Opcode = MI.getOpcode();
if (isCompressibleLoad(MI) || isCompressibleStore(MI)) {
const MachineOperand &MOImm = MI.getOperand(2);
if (!MOImm.isImm())
return RegImmPair(RISCV::NoRegister, 0);
int64_t Offset = MOImm.getImm();
int64_t NewBaseAdjust = getBaseAdjustForCompression(Offset, Opcode);
Register Base = MI.getOperand(1).getReg();
// Memory accesses via the stack pointer do not have a requirement for
// either of the registers to be compressible and can take a larger offset.
if (RISCV::SPRegClass.contains(Base)) {
if (!compressibleSPOffset(Offset, Opcode) && NewBaseAdjust)
return RegImmPair(Base, NewBaseAdjust);
} else {
Register SrcDest = MI.getOperand(0).getReg();
bool SrcDestCompressed = isCompressedReg(SrcDest);
bool BaseCompressed = isCompressedReg(Base);
// If only Base and/or offset prevent compression, then return Base and
// any adjustment required to make the offset compressible.
if ((!BaseCompressed || NewBaseAdjust) && SrcDestCompressed)
return RegImmPair(Base, NewBaseAdjust);
// For loads, we can only change the base register since dest is defined
// rather than used.
//
// For stores, we can change SrcDest (and Base if SrcDest == Base) but
// cannot resolve an uncompressible offset in this case.
if (isCompressibleStore(MI)) {
if (!SrcDestCompressed && (BaseCompressed || SrcDest == Base) &&
!NewBaseAdjust)
return RegImmPair(SrcDest, NewBaseAdjust);
}
}
}
return RegImmPair(RISCV::NoRegister, 0);
}
// Check all uses after FirstMI of the given register, keeping a vector of
// instructions that would be compressible if the given register (and offset if
// applicable) were compressible.
//
// If there are enough uses for this optimization to improve code size and a
// compressed register is available, return that compressed register.
static Register analyzeCompressibleUses(MachineInstr &FirstMI,
RegImmPair RegImm,
SmallVectorImpl<MachineInstr *> &MIs) {
MachineBasicBlock &MBB = *FirstMI.getParent();
const TargetRegisterInfo *TRI =
MBB.getParent()->getSubtarget().getRegisterInfo();
RegScavenger RS;
RS.enterBasicBlock(MBB);
for (MachineBasicBlock::instr_iterator I = FirstMI.getIterator(),
E = MBB.instr_end();
I != E; ++I) {
MachineInstr &MI = *I;
// Determine if this is an instruction which would benefit from using the
// new register.
RegImmPair CandidateRegImm = getRegImmPairPreventingCompression(MI);
if (CandidateRegImm.Reg == RegImm.Reg &&
CandidateRegImm.Imm == RegImm.Imm) {
// Advance tracking since the value in the new register must be live for
// this instruction too.
RS.forward(I);
MIs.push_back(&MI);
}
// If RegImm.Reg is modified by this instruction, then we cannot optimize
// past this instruction. If the register is already compressed, then it may
// possible to optimize a large offset in the current instruction - this
// will have been detected by the preceeding call to
// getRegImmPairPreventingCompression.
if (MI.modifiesRegister(RegImm.Reg, TRI))
break;
}
// Adjusting the base costs one new uncompressed addi and therefore three uses
// are required for a code size reduction. If no base adjustment is required,
// then copying the register costs one new c.mv (or c.li Rd, 0 for "copying"
// the zero register) and therefore two uses are required for a code size
// reduction.
if (MIs.size() < 2 || (RegImm.Imm != 0 && MIs.size() < 3))
return RISCV::NoRegister;
// Find a compressible register which will be available from the first
// instruction we care about to the last.
const TargetRegisterClass *RCToScavenge;
// Work out the compressed register class from which to scavenge.
if (RISCV::GPRRegClass.contains(RegImm.Reg))
RCToScavenge = &RISCV::GPRCRegClass;
else if (RISCV::FPR32RegClass.contains(RegImm.Reg))
RCToScavenge = &RISCV::FPR32CRegClass;
else if (RISCV::FPR64RegClass.contains(RegImm.Reg))
RCToScavenge = &RISCV::FPR64CRegClass;
else
return RISCV::NoRegister;
return RS.scavengeRegisterBackwards(*RCToScavenge, FirstMI.getIterator(),
/*RestoreAfter=*/false, /*SPAdj=*/0,
/*AllowSpill=*/false);
}
// Update uses of the old register in the given instruction to the new register.
static void updateOperands(MachineInstr &MI, RegImmPair OldRegImm,
Register NewReg) {
unsigned Opcode = MI.getOpcode();
// If this pass is extended to support more instructions, the check for
// definedness may need to be strengthened.
assert((isCompressibleLoad(MI) || isCompressibleStore(MI)) &&
"Unsupported instruction for this optimization.");
int SkipN = 0;
// Skip the first (value) operand to a store instruction (except if the store
// offset is zero) in order to avoid an incorrect transformation.
// e.g. sd a0, 808(a0) to addi a2, a0, 768; sd a2, 40(a2)
if (isCompressibleStore(MI) && OldRegImm.Imm != 0)
SkipN = 1;
// Update registers
for (MachineOperand &MO : drop_begin(MI.operands(), SkipN))
if (MO.isReg() && MO.getReg() == OldRegImm.Reg) {
// Do not update operands that define the old register.
//
// The new register was scavenged for the range of instructions that are
// being updated, therefore it should not be defined within this range
// except possibly in the final instruction.
if (MO.isDef()) {
assert(isCompressibleLoad(MI));
continue;
}
// Update reg
MO.setReg(NewReg);
}
// Update offset
MachineOperand &MOImm = MI.getOperand(2);
int64_t NewOffset = MOImm.getImm() & compressedLDSTOffsetMask(Opcode);
MOImm.setImm(NewOffset);
}
bool RISCVMakeCompressibleOpt::runOnMachineFunction(MachineFunction &Fn) {
// This is a size optimization.
if (skipFunction(Fn.getFunction()) || !Fn.getFunction().hasMinSize())
return false;
const RISCVSubtarget &STI = Fn.getSubtarget<RISCVSubtarget>();
const RISCVInstrInfo &TII = *STI.getInstrInfo();
// This optimization only makes sense if compressed instructions are emitted.
if (!STI.hasStdExtC())
return false;
for (MachineBasicBlock &MBB : Fn) {
LLVM_DEBUG(dbgs() << "MBB: " << MBB.getName() << "\n");
for (MachineInstr &MI : MBB) {
// Determine if this instruction would otherwise be compressed if not for
// an uncompressible register or offset.
RegImmPair RegImm = getRegImmPairPreventingCompression(MI);
if (!RegImm.Reg && RegImm.Imm == 0)
continue;
// Determine if there is a set of instructions for which replacing this
// register with a compressed register (and compressible offset if
// applicable) is possible and will allow compression.
SmallVector<MachineInstr *, 8> MIs;
Register NewReg = analyzeCompressibleUses(MI, RegImm, MIs);
if (!NewReg)
continue;
// Create the appropriate copy and/or offset.
if (RISCV::GPRRegClass.contains(RegImm.Reg)) {
assert(isInt<12>(RegImm.Imm));
BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(RISCV::ADDI), NewReg)
.addReg(RegImm.Reg)
.addImm(RegImm.Imm);
} else {
// If we are looking at replacing an FPR register we don't expect to
// have any offset. The only compressible FP instructions with an offset
// are loads and stores, for which the offset applies to the GPR operand
// not the FPR operand.
assert(RegImm.Imm == 0);
unsigned Opcode = RISCV::FPR32RegClass.contains(RegImm.Reg)
? RISCV::FSGNJ_S
: RISCV::FSGNJ_D;
BuildMI(MBB, MI, MI.getDebugLoc(), TII.get(Opcode), NewReg)
.addReg(RegImm.Reg)
.addReg(RegImm.Reg);
}
// Update the set of instructions to use the compressed register and
// compressible offset instead. These instructions should now be
// compressible.
// TODO: Update all uses if RegImm.Imm == 0? Not just those that are
// expected to become compressible.
for (MachineInstr *UpdateMI : MIs)
updateOperands(*UpdateMI, RegImm, NewReg);
}
}
return true;
}
/// Returns an instance of the Make Compressible Optimization pass.
FunctionPass *llvm::createRISCVMakeCompressibleOptPass() {
return new RISCVMakeCompressibleOpt();
}