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//===-- SIModeRegister.cpp - Mode Register --------------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
/// \file
/// This pass inserts changes to the Mode register settings as required.
/// Note that currently it only deals with the Double Precision Floating Point
/// rounding mode setting, but is intended to be generic enough to be easily
/// expanded.
#include "AMDGPU.h"
#include "GCNSubtarget.h"
#include "MCTargetDesc/AMDGPUMCTargetDesc.h"
#include "llvm/ADT/Statistic.h"
#include <queue>
#define DEBUG_TYPE "si-mode-register"
STATISTIC(NumSetregInserted, "Number of setreg of mode register inserted.");
using namespace llvm;
struct Status {
// Mask is a bitmask where a '1' indicates the corresponding Mode bit has a
// known value
unsigned Mask;
unsigned Mode;
Status() : Mask(0), Mode(0){};
Status(unsigned NewMask, unsigned NewMode) : Mask(NewMask), Mode(NewMode) {
Mode &= Mask;
// merge two status values such that only values that don't conflict are
// preserved
Status merge(const Status &S) const {
return Status((Mask | S.Mask), ((Mode & ~S.Mask) | (S.Mode & S.Mask)));
// merge an unknown value by using the unknown value's mask to remove bits
// from the result
Status mergeUnknown(unsigned newMask) {
return Status(Mask & ~newMask, Mode & ~newMask);
// intersect two Status values to produce a mode and mask that is a subset
// of both values
Status intersect(const Status &S) const {
unsigned NewMask = (Mask & S.Mask) & (Mode ^ ~S.Mode);
unsigned NewMode = (Mode & NewMask);
return Status(NewMask, NewMode);
// produce the delta required to change the Mode to the required Mode
Status delta(const Status &S) const {
return Status((S.Mask & (Mode ^ S.Mode)) | (~Mask & S.Mask), S.Mode);
bool operator==(const Status &S) const {
return (Mask == S.Mask) && (Mode == S.Mode);
bool operator!=(const Status &S) const { return !(*this == S); }
bool isCompatible(Status &S) {
return ((Mask & S.Mask) == S.Mask) && ((Mode & S.Mask) == S.Mode);
bool isCombinable(Status &S) { return !(Mask & S.Mask) || isCompatible(S); }
class BlockData {
// The Status that represents the mode register settings required by the
// FirstInsertionPoint (if any) in this block. Calculated in Phase 1.
Status Require;
// The Status that represents the net changes to the Mode register made by
// this block, Calculated in Phase 1.
Status Change;
// The Status that represents the mode register settings on exit from this
// block. Calculated in Phase 2.
Status Exit;
// The Status that represents the intersection of exit Mode register settings
// from all predecessor blocks. Calculated in Phase 2, and used by Phase 3.
Status Pred;
// In Phase 1 we record the first instruction that has a mode requirement,
// which is used in Phase 3 if we need to insert a mode change.
MachineInstr *FirstInsertionPoint;
// A flag to indicate whether an Exit value has been set (we can't tell by
// examining the Exit value itself as all values may be valid results).
bool ExitSet;
BlockData() : FirstInsertionPoint(nullptr), ExitSet(false){};
namespace {
class SIModeRegister : public MachineFunctionPass {
static char ID;
std::vector<std::unique_ptr<BlockData>> BlockInfo;
std::queue<MachineBasicBlock *> Phase2List;
// The default mode register setting currently only caters for the floating
// point double precision rounding mode.
// We currently assume the default rounding mode is Round to Nearest
// NOTE: this should come from a per function rounding mode setting once such
// a setting exists.
unsigned DefaultMode = FP_ROUND_ROUND_TO_NEAREST;
Status DefaultStatus =
Status(FP_ROUND_MODE_DP(0x3), FP_ROUND_MODE_DP(DefaultMode));
bool Changed = false;
SIModeRegister() : MachineFunctionPass(ID) {}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
void processBlockPhase1(MachineBasicBlock &MBB, const SIInstrInfo *TII);
void processBlockPhase2(MachineBasicBlock &MBB, const SIInstrInfo *TII);
void processBlockPhase3(MachineBasicBlock &MBB, const SIInstrInfo *TII);
Status getInstructionMode(MachineInstr &MI, const SIInstrInfo *TII);
void insertSetreg(MachineBasicBlock &MBB, MachineInstr *I,
const SIInstrInfo *TII, Status InstrMode);
} // End anonymous namespace.
"Insert required mode register values", false, false)
char SIModeRegister::ID = 0;
char &llvm::SIModeRegisterID = SIModeRegister::ID;
FunctionPass *llvm::createSIModeRegisterPass() { return new SIModeRegister(); }
// Determine the Mode register setting required for this instruction.
// Instructions which don't use the Mode register return a null Status.
// Note this currently only deals with instructions that use the floating point
// double precision setting.
Status SIModeRegister::getInstructionMode(MachineInstr &MI,
const SIInstrInfo *TII) {
if (TII->usesFPDPRounding(MI)) {
switch (MI.getOpcode()) {
// f16 interpolation instructions need double precision round to zero
return Status(FP_ROUND_MODE_DP(3),
return DefaultStatus;
return Status();
// Insert a setreg instruction to update the Mode register.
// It is possible (though unlikely) for an instruction to require a change to
// the value of disjoint parts of the Mode register when we don't know the
// value of the intervening bits. In that case we need to use more than one
// setreg instruction.
void SIModeRegister::insertSetreg(MachineBasicBlock &MBB, MachineInstr *MI,
const SIInstrInfo *TII, Status InstrMode) {
while (InstrMode.Mask) {
unsigned Offset = countTrailingZeros<unsigned>(InstrMode.Mask);
unsigned Width = countTrailingOnes<unsigned>(InstrMode.Mask >> Offset);
unsigned Value = (InstrMode.Mode >> Offset) & ((1 << Width) - 1);
BuildMI(MBB, MI, 0, TII->get(AMDGPU::S_SETREG_IMM32_B32))
.addImm(((Width - 1) << AMDGPU::Hwreg::WIDTH_M1_SHIFT_) |
(Offset << AMDGPU::Hwreg::OFFSET_SHIFT_) |
(AMDGPU::Hwreg::ID_MODE << AMDGPU::Hwreg::ID_SHIFT_));
Changed = true;
InstrMode.Mask &= ~(((1 << Width) - 1) << Offset);
// In Phase 1 we iterate through the instructions of the block and for each
// instruction we get its mode usage. If the instruction uses the Mode register
// we:
// - update the Change status, which tracks the changes to the Mode register
// made by this block
// - if this instruction's requirements are compatible with the current setting
// of the Mode register we merge the modes
// - if it isn't compatible and an InsertionPoint isn't set, then we set the
// InsertionPoint to the current instruction, and we remember the current
// mode
// - if it isn't compatible and InsertionPoint is set we insert a seteg before
// that instruction (unless this instruction forms part of the block's
// entry requirements in which case the insertion is deferred until Phase 3
// when predecessor exit values are known), and move the insertion point to
// this instruction
// - if this is a setreg instruction we treat it as an incompatible instruction.
// This is sub-optimal but avoids some nasty corner cases, and is expected to
// occur very rarely.
// - on exit we have set the Require, Change, and initial Exit modes.
void SIModeRegister::processBlockPhase1(MachineBasicBlock &MBB,
const SIInstrInfo *TII) {
auto NewInfo = std::make_unique<BlockData>();
MachineInstr *InsertionPoint = nullptr;
// RequirePending is used to indicate whether we are collecting the initial
// requirements for the block, and need to defer the first InsertionPoint to
// Phase 3. It is set to false once we have set FirstInsertionPoint, or when
// we discover an explicit setreg that means this block doesn't have any
// initial requirements.
bool RequirePending = true;
Status IPChange;
for (MachineInstr &MI : MBB) {
Status InstrMode = getInstructionMode(MI, TII);
if (MI.getOpcode() == AMDGPU::S_SETREG_B32 ||
MI.getOpcode() == AMDGPU::S_SETREG_B32_mode ||
MI.getOpcode() == AMDGPU::S_SETREG_IMM32_B32 ||
MI.getOpcode() == AMDGPU::S_SETREG_IMM32_B32_mode) {
// We preserve any explicit mode register setreg instruction we encounter,
// as we assume it has been inserted by a higher authority (this is
// likely to be a very rare occurrence).
unsigned Dst = TII->getNamedOperand(MI, AMDGPU::OpName::simm16)->getImm();
if (((Dst & AMDGPU::Hwreg::ID_MASK_) >> AMDGPU::Hwreg::ID_SHIFT_) !=
unsigned Width = ((Dst & AMDGPU::Hwreg::WIDTH_M1_MASK_) >>
unsigned Offset =
unsigned Mask = ((1 << Width) - 1) << Offset;
// If an InsertionPoint is set we will insert a setreg there.
if (InsertionPoint) {
insertSetreg(MBB, InsertionPoint, TII,>Change));
InsertionPoint = nullptr;
// If this is an immediate then we know the value being set, but if it is
// not an immediate then we treat the modified bits of the mode register
// as unknown.
if (MI.getOpcode() == AMDGPU::S_SETREG_IMM32_B32 ||
MI.getOpcode() == AMDGPU::S_SETREG_IMM32_B32_mode) {
unsigned Val = TII->getNamedOperand(MI, AMDGPU::OpName::imm)->getImm();
unsigned Mode = (Val << Offset) & Mask;
Status Setreg = Status(Mask, Mode);
// If we haven't already set the initial requirements for the block we
// don't need to as the requirements start from this explicit setreg.
RequirePending = false;
NewInfo->Change = NewInfo->Change.merge(Setreg);
} else {
NewInfo->Change = NewInfo->Change.mergeUnknown(Mask);
} else if (!NewInfo->Change.isCompatible(InstrMode)) {
// This instruction uses the Mode register and its requirements aren't
// compatible with the current mode.
if (InsertionPoint) {
// If the required mode change cannot be included in the current
// InsertionPoint changes, we need a setreg and start a new
// InsertionPoint.
if (!>Change).isCombinable(InstrMode)) {
if (RequirePending) {
// This is the first insertionPoint in the block so we will defer
// the insertion of the setreg to Phase 3 where we know whether or
// not it is actually needed.
NewInfo->FirstInsertionPoint = InsertionPoint;
NewInfo->Require = NewInfo->Change;
RequirePending = false;
} else {
insertSetreg(MBB, InsertionPoint, TII,>Change));
IPChange = NewInfo->Change;
// Set the new InsertionPoint
InsertionPoint = &MI;
NewInfo->Change = NewInfo->Change.merge(InstrMode);
} else {
// No InsertionPoint is currently set - this is either the first in
// the block or we have previously seen an explicit setreg.
InsertionPoint = &MI;
IPChange = NewInfo->Change;
NewInfo->Change = NewInfo->Change.merge(InstrMode);
if (RequirePending) {
// If we haven't yet set the initial requirements for the block we set them
// now.
NewInfo->FirstInsertionPoint = InsertionPoint;
NewInfo->Require = NewInfo->Change;
} else if (InsertionPoint) {
// We need to insert a setreg at the InsertionPoint
insertSetreg(MBB, InsertionPoint, TII,>Change));
NewInfo->Exit = NewInfo->Change;
BlockInfo[MBB.getNumber()] = std::move(NewInfo);
// In Phase 2 we revisit each block and calculate the common Mode register
// value provided by all predecessor blocks. If the Exit value for the block
// is changed, then we add the successor blocks to the worklist so that the
// exit value is propagated.
void SIModeRegister::processBlockPhase2(MachineBasicBlock &MBB,
const SIInstrInfo *TII) {
bool RevisitRequired = false;
bool ExitSet = false;
unsigned ThisBlock = MBB.getNumber();
if (MBB.pred_empty()) {
// There are no predecessors, so use the default starting status.
BlockInfo[ThisBlock]->Pred = DefaultStatus;
ExitSet = true;
} else {
// Build a status that is common to all the predecessors by intersecting
// all the predecessor exit status values.
// Mask bits (which represent the Mode bits with a known value) can only be
// added by explicit SETREG instructions or the initial default value -
// the intersection process may remove Mask bits.
// If we find a predecessor that has not yet had an exit value determined
// (this can happen for example if a block is its own predecessor) we defer
// use of that value as the Mask will be all zero, and we will revisit this
// block again later (unless the only predecessor without an exit value is
// this block).
MachineBasicBlock::pred_iterator P = MBB.pred_begin(), E = MBB.pred_end();
MachineBasicBlock &PB = *(*P);
unsigned PredBlock = PB.getNumber();
if ((ThisBlock == PredBlock) && (std::next(P) == E)) {
BlockInfo[ThisBlock]->Pred = DefaultStatus;
ExitSet = true;
} else if (BlockInfo[PredBlock]->ExitSet) {
BlockInfo[ThisBlock]->Pred = BlockInfo[PredBlock]->Exit;
ExitSet = true;
} else if (PredBlock != ThisBlock)
RevisitRequired = true;
for (P = std::next(P); P != E; P = std::next(P)) {
MachineBasicBlock *Pred = *P;
unsigned PredBlock = Pred->getNumber();
if (BlockInfo[PredBlock]->ExitSet) {
if (BlockInfo[ThisBlock]->ExitSet) {
BlockInfo[ThisBlock]->Pred =
} else {
BlockInfo[ThisBlock]->Pred = BlockInfo[PredBlock]->Exit;
ExitSet = true;
} else if (PredBlock != ThisBlock)
RevisitRequired = true;
Status TmpStatus =
if (BlockInfo[ThisBlock]->Exit != TmpStatus) {
BlockInfo[ThisBlock]->Exit = TmpStatus;
// Add the successors to the work list so we can propagate the changed exit
// status.
for (MachineBasicBlock *Succ : MBB.successors())
BlockInfo[ThisBlock]->ExitSet = ExitSet;
if (RevisitRequired)
// In Phase 3 we revisit each block and if it has an insertion point defined we
// check whether the predecessor mode meets the block's entry requirements. If
// not we insert an appropriate setreg instruction to modify the Mode register.
void SIModeRegister::processBlockPhase3(MachineBasicBlock &MBB,
const SIInstrInfo *TII) {
unsigned ThisBlock = MBB.getNumber();
if (!BlockInfo[ThisBlock]->Pred.isCompatible(BlockInfo[ThisBlock]->Require)) {
Status Delta =
if (BlockInfo[ThisBlock]->FirstInsertionPoint)
insertSetreg(MBB, BlockInfo[ThisBlock]->FirstInsertionPoint, TII, Delta);
insertSetreg(MBB, &MBB.instr_front(), TII, Delta);
bool SIModeRegister::runOnMachineFunction(MachineFunction &MF) {
const GCNSubtarget &ST = MF.getSubtarget<GCNSubtarget>();
const SIInstrInfo *TII = ST.getInstrInfo();
// Processing is performed in a number of phases
// Phase 1 - determine the initial mode required by each block, and add setreg
// instructions for intra block requirements.
for (MachineBasicBlock &BB : MF)
processBlockPhase1(BB, TII);
// Phase 2 - determine the exit mode from each block. We add all blocks to the
// list here, but will also add any that need to be revisited during Phase 2
// processing.
for (MachineBasicBlock &BB : MF)
while (!Phase2List.empty()) {
processBlockPhase2(*Phase2List.front(), TII);
// Phase 3 - add an initial setreg to each block where the required entry mode
// is not satisfied by the exit mode of all its predecessors.
for (MachineBasicBlock &BB : MF)
processBlockPhase3(BB, TII);
return Changed;