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rewrite of constant islands 

llvm-svn: 34523
This commit is contained in:
Dale Johannesen 2007-02-23 05:02:36 +00:00
parent fa6532a3b9
commit e18b13b213
2 changed files with 274 additions and 85 deletions
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335
llvm/lib/Target/ARM/ARMConstantIslandPass.cpp
View File

@ -35,10 +35,10 @@ STATISTIC(NumCBrFixed, "Number of cond branches fixed");
STATISTIC(NumUBrFixed, "Number of uncond branches fixed");
namespace {
/// ARMConstantIslands - Due to limited pc-relative displacements, ARM
/// ARMConstantIslands - Due to limited PC-relative displacements, ARM
/// requires constant pool entries to be scattered among the instructions
/// inside a function. To do this, it completely ignores the normal LLVM
/// constant pool, instead, it places constants where-ever it feels like with
/// constant pool; instead, it places constants wherever it feels like with
/// special instructions.
///
/// The terminology used in this pass includes:
@ -59,6 +59,11 @@ namespace {
/// to a return, unreachable, or unconditional branch).
std::vector<MachineBasicBlock*> WaterList;
// WaterListOffsets - the offset of the beginning of each WaterList block.
// This is computed as needed in HandleConstantPoolUser; not necessarily
// valid at arbitrary times.
std::vector<unsigned> WaterListOffsets;
/// CPUser - One user of a constant pool, keeping the machine instruction
/// pointer, the constant pool being referenced, and the max displacement
/// allowed from the instruction to the CP.
@ -86,7 +91,10 @@ namespace {
};
/// CPEntries - Keep track of all of the constant pool entry machine
/// instructions. For each constpool index, it keeps a vector of entries.
/// instructions. For each original constpool index (i.e. those that
/// existed upon entry to this pass), it keeps a vector of entries.
/// Original elements are cloned as we go along; the clones are
/// put in the vector of the original element, but have distinct CPIs.
std::vector<std::vector<CPEntry> > CPEntries;
/// ImmBranch - One per immediate branch, keeping the machine instruction
@ -131,8 +139,16 @@ namespace {
const std::vector<MachineInstr*> &CPEMIs);
MachineBasicBlock *SplitBlockBeforeInstr(MachineInstr *MI);
void UpdateForInsertedWaterBlock(MachineBasicBlock *NewBB);
bool DecrementOldEntry(unsigned CPI, MachineInstr* CPEMI, unsigned Size);
void ComputeWaterListOffsets(MachineFunction &Fn);
int LookForExistingCPEntry(CPUser& U, unsigned UserOffset);
bool HandleConstantPoolUser(MachineFunction &Fn, CPUser &U);
bool CPEIsInRange(MachineInstr *MI, MachineInstr *CPEMI, unsigned Disp);
bool CPEIsInRange(MachineInstr *MI, unsigned UserOffset,
MachineInstr *CPEMI, unsigned Disp,
bool DoDump);
bool WaterIsInRange(unsigned UserOffset,
std::vector<MachineBasicBlock*>::iterator IP,
unsigned Disp);
bool BBIsInRange(MachineInstr *MI, MachineBasicBlock *BB, unsigned Disp);
bool FixUpImmediateBr(MachineFunction &Fn, ImmBranch &Br);
bool FixUpConditionalBr(MachineFunction &Fn, ImmBranch &Br);
@ -240,7 +256,7 @@ void ARMConstantIslands::DoInitialPlacement(MachineFunction &Fn,
}
}
/// BBHasFallthrough - Return true of the specified basic block can fallthrough
/// BBHasFallthrough - Return true if the specified basic block can fallthrough
/// into the block immediately after it.
static bool BBHasFallthrough(MachineBasicBlock *MBB) {
// Get the next machine basic block in the function.
@ -394,8 +410,9 @@ void ARMConstantIslands::InitialFunctionScan(MachineFunction &Fn,
}
}
// In thumb mode, if this block is a constpool island, pessmisticly assume
// it needs to be padded by two byte so it's aligned on 4 byte boundary.
// In thumb mode, if this block is a constpool island, pessimistically
// assume it needs to be padded by two byte so it's aligned on 4 byte
// boundary.
if (AFI->isThumbFunction() &&
!MBB.empty() &&
MBB.begin()->getOpcode() == ARM::CONSTPOOL_ENTRY)
@ -503,8 +520,27 @@ MachineBasicBlock *ARMConstantIslands::SplitBlockBeforeInstr(MachineInstr *MI) {
OrigBB->addSuccessor(NewBB);
// Update internal data structures to account for the newly inserted MBB.
UpdateForInsertedWaterBlock(NewBB);
// This is almost the same as UpdateForInsertedWaterBlock, except that
// the Water goes after OrigBB, not NewBB.
NewBB->getParent()->RenumberBlocks(NewBB);
// Insert a size into BBSizes to align it properly with the (newly
// renumbered) block numbers.
BBSizes.insert(BBSizes.begin()+NewBB->getNumber(), 0);
// Next, update WaterList. Specifically, we need to add OrigMBB as having
// available water after it (but not if it's already there, which happens
// when splitting before a conditional branch that is followed by an
// unconditional branch - in that case we want to insert NewBB).
std::vector<MachineBasicBlock*>::iterator IP =
std::lower_bound(WaterList.begin(), WaterList.end(), OrigBB,
CompareMBBNumbers);
MachineBasicBlock* WaterBB = *IP;
if (WaterBB == OrigBB)
WaterList.insert(next(IP), NewBB);
else
WaterList.insert(IP, OrigBB);
// Figure out how large the first NewMBB is.
unsigned NewBBSize = 0;
for (MachineBasicBlock::iterator I = NewBB->begin(), E = NewBB->end();
@ -521,20 +557,53 @@ MachineBasicBlock *ARMConstantIslands::SplitBlockBeforeInstr(MachineInstr *MI) {
return NewBB;
}
/// CPEIsInRange - Returns true is the distance between specific MI and
/// WaterIsInRange - Returns true if a CPE placed after the specified
/// Water (a basic block) will be in range for the specific MI.
bool ARMConstantIslands::WaterIsInRange(unsigned UserOffset,
std::vector<MachineBasicBlock*>::iterator IP,
unsigned MaxDisp)
{
MachineBasicBlock *Water = *IP;
unsigned Index = IP - WaterList.begin();
unsigned CPEOffset = WaterListOffsets[Index] +
BBSizes[Water->getNumber()];
// If the Water is a constpool island, it has already been aligned.
// If not, align it.
if (AFI->isThumbFunction() &&
(Water->empty() ||
Water->begin()->getOpcode() != ARM::CONSTPOOL_ENTRY))
CPEOffset += 2;
if (UserOffset <= CPEOffset) {
// User before the CPE.
if (CPEOffset-UserOffset <= MaxDisp)
return true;
} else if (!AFI->isThumbFunction()) {
// Thumb LDR cannot encode negative offset.
if (UserOffset-CPEOffset <= MaxDisp)
return true;
}
return false;
}
/// CPEIsInRange - Returns true if the distance between specific MI and
/// specific ConstPool entry instruction can fit in MI's displacement field.
bool ARMConstantIslands::CPEIsInRange(MachineInstr *MI, MachineInstr *CPEMI,
unsigned MaxDisp) {
unsigned PCAdj = AFI->isThumbFunction() ? 4 : 8;
unsigned UserOffset = GetOffsetOf(MI) + PCAdj;
// In thumb mode, pessmisticly assumes the .align 2 before the first CPE
bool ARMConstantIslands::CPEIsInRange(MachineInstr *MI, unsigned UserOffset,
MachineInstr *CPEMI,
unsigned MaxDisp, bool DoDump) {
// In thumb mode, pessimistically assumes the .align 2 before the first CPE
// in the island adds two byte padding.
unsigned AlignAdj = AFI->isThumbFunction() ? 2 : 0;
unsigned CPEOffset = GetOffsetOf(CPEMI) + AlignAdj;
DOUT << "User of CPE#" << CPEMI->getOperand(0).getImm()
<< " max delta=" << MaxDisp
<< " at offset " << int(CPEOffset-UserOffset) << "\t" << *MI;
if (DoDump) {
DOUT << "User of CPE#" << CPEMI->getOperand(0).getImm()
<< " max delta=" << MaxDisp
<< " insn address=" << UserOffset
<< " CPE address=" << CPEOffset
<< " offset=" << int(CPEOffset-UserOffset) << "\t" << *MI;
}
if (UserOffset <= CPEOffset) {
// User before the CPE.
@ -562,52 +631,13 @@ static bool BBIsJumpedOver(MachineBasicBlock *MBB) {
return false;
}
/// HandleConstantPoolUser - Analyze the specified user, checking to see if it
/// is out-of-range. If so, pick it up the constant pool value and move it some
/// place in-range.
bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
// Check to see if the CPE is already in-range.
if (CPEIsInRange(UserMI, CPEMI, U.MaxDisp))
return false;
// Solution guaranteed to work: split the user's MBB right after the user and
// insert a clone the CPE into the newly created water.
MachineBasicBlock *UserMBB = UserMI->getParent();
MachineBasicBlock *NewMBB;
// TODO: Search for the best place to split the code. In practice, using
// loop nesting information to insert these guys outside of loops would be
// sufficient.
bool isThumb = AFI->isThumbFunction();
if (&UserMBB->back() == UserMI) {
assert(BBHasFallthrough(UserMBB) && "Expected a fallthrough BB!");
NewMBB = next(MachineFunction::iterator(UserMBB));
// Add an unconditional branch from UserMBB to fallthrough block.
// Note the new unconditional branch is not being recorded.
BuildMI(UserMBB, TII->get(isThumb ? ARM::tB : ARM::B)).addMBB(NewMBB);
BBSizes[UserMBB->getNumber()] += isThumb ? 2 : 4;
} else {
MachineInstr *NextMI = next(MachineBasicBlock::iterator(UserMI));
NewMBB = SplitBlockBeforeInstr(NextMI);
}
// Okay, we know we can put an island before UserMBB now, do it!
MachineBasicBlock *NewIsland = new MachineBasicBlock();
Fn.getBasicBlockList().insert(NewMBB, NewIsland);
// Update internal data structures to account for the newly inserted MBB.
UpdateForInsertedWaterBlock(NewIsland);
// Now that we have an island to add the CPE to, clone the original CPE and
// add it to the island.
unsigned ID = NextUID++;
unsigned CPI = CPEMI->getOperand(1).getConstantPoolIndex();
unsigned Size = CPEMI->getOperand(2).getImm();
/// DecrementOldEntry - find the constant pool entry with index CPI
/// and instruction CPEMI, and decrement its refcount. If the refcount
/// becomes 0 remove the entry and instruction. Returns true if we removed
/// the entry, false if we didn't.
bool ARMConstantIslands::DecrementOldEntry(unsigned CPI, MachineInstr *CPEMI,
unsigned Size) {
// Find the old entry. Eliminate it if it is no longer used.
CPEntry *OldCPE = findConstPoolEntry(CPI, CPEMI);
assert(OldCPE && "Unexpected!");
@ -615,7 +645,8 @@ bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
MachineBasicBlock *OldCPEBB = OldCPE->CPEMI->getParent();
if (OldCPEBB->empty()) {
// In thumb mode, the size of island is padded by two to compensate for
// the alignment requirement.
// the alignment requirement. Thus it will now be 2 when the block is
// empty, so fix this.
BBSizes[OldCPEBB->getNumber()] = 0;
// An island has only one predecessor BB and one successor BB. Check if
// this BB's predecessor jumps directly to this BB's successor. This
@ -627,12 +658,171 @@ bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
OldCPE->CPEMI->eraseFromParent();
OldCPE->CPEMI = NULL;
NumCPEs--;
return true;
}
return false;
}
/// ComputeWaterListOffsets - just what you think.
/// This vector is built to avoid re-adding BBSizes for each WaterBB under test
/// (which would cause the algorithm to be n^2).
void ARMConstantIslands::ComputeWaterListOffsets(MachineFunction &Fn) {
unsigned WaterListIndex = 0;
unsigned Offset = 0;
unsigned BB = 0;
WaterListOffsets.clear();
for (MachineFunction::iterator MBBI = Fn.begin(), E = Fn.end();
MBBI != E; ++BB, ++MBBI) {
MachineBasicBlock *MBB = MBBI;
if (MBB == WaterList[WaterListIndex]) {
WaterListOffsets.push_back(Offset);
WaterListIndex++;
}
Offset += BBSizes[BB];
}
}
/// LookForCPEntryInRange - see if the currently referenced CPE is in range;
/// if not, see if an in-range clone of the CPE is in range, and if so,
/// change the data structures so the user references the clone. Returns:
/// 0 = no existing entry found
/// 1 = entry found, and there were no code insertions or deletions
/// 2 = entry found, and there were code insertions or deletions
int ARMConstantIslands::LookForExistingCPEntry(CPUser& U, unsigned UserOffset)
{
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
// Check to see if the CPE is already in-range.
if (CPEIsInRange(UserMI, UserOffset, CPEMI, U.MaxDisp, true)) {
DOUT << "In range\n";
return 1;
}
// Build a new CPE for this user.
// No. Look for previously created clones of the CPE that are in range.
unsigned CPI = CPEMI->getOperand(1).getConstantPoolIndex();
std::vector<CPEntry> &CPEs = CPEntries[CPI];
for (unsigned i = 0, e = CPEs.size(); i != e; ++i) {
// We already tried this one
if (CPEs[i].CPEMI == CPEMI)
continue;
// Removing CPEs can leave empty entries, skip
if (CPEs[i].CPEMI == NULL)
continue;
if (CPEIsInRange(UserMI, UserOffset, CPEs[i].CPEMI, U.MaxDisp, false)) {
DOUT << "Replacing CPE#" << CPI << " with CPE#" << CPEs[i].CPI << "\n";
// Point the CPUser node to the replacement
U.CPEMI = CPEs[i].CPEMI;
// Change the CPI in the instruction operand to refer to the clone.
for (unsigned j = 0, e = UserMI->getNumOperands(); j != e; ++j)
if (UserMI->getOperand(j).isConstantPoolIndex()) {
UserMI->getOperand(j).setConstantPoolIndex(CPEs[i].CPI);
break;
}
// Adjust the refcount of the clone...
CPEs[i].RefCount++;
// ...and the original. If we didn't remove the old entry, none of the
// addresses changed, so we don't need another pass.
unsigned Size = CPEMI->getOperand(2).getImm();
return DecrementOldEntry(CPI, CPEMI, Size) ? 2 : 1;
}
}
return 0;
}
/// HandleConstantPoolUser - Analyze the specified user, checking to see if it
/// is out-of-range. If so, pick it up the constant pool value and move it some
/// place in-range. Return true if we changed any addresses (thus must run
/// another pass of branch lengthening), false otherwise.
bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
MachineInstr *UserMI = U.MI;
MachineInstr *CPEMI = U.CPEMI;
unsigned CPI = CPEMI->getOperand(1).getConstantPoolIndex();
unsigned Size = CPEMI->getOperand(2).getImm();
bool isThumb = AFI->isThumbFunction();
MachineBasicBlock *NewMBB;
// Compute this only once, it's expensive
unsigned UserOffset = GetOffsetOf(UserMI) + (isThumb ? 4 : 8);
// See if the current entry is within range, or there is a clone of it
// in range.
int result = LookForExistingCPEntry(U, UserOffset);
if (result==1) return false;
else if (result==2) return true;
// No existing clone of this CPE is within range.
// We will be generating a new clone. Get a UID for it.
unsigned ID = NextUID++;
// Look for water where we can place this CPE. We look for the farthest one
// away that will work. Forward references only for now (although later
// we might find some that are backwards).
bool WaterFound = false;
if (!WaterList.empty()) {
// Compute offsets for the blocks in the current WaterList.
// It is a big compile-time speed win to do this only once
// rather than for each WaterList entry.
ComputeWaterListOffsets(Fn);
assert(WaterList.size() == WaterListOffsets.size());
for (std::vector<MachineBasicBlock*>::iterator IP = prior(WaterList.end()),
B = WaterList.begin();; --IP) {
MachineBasicBlock* WaterBB = *IP;
if (WaterIsInRange(UserOffset, IP, U.MaxDisp)) {
WaterFound = true;
DOUT << "found water in range\n";
// CPE goes before following block (NewMBB).
NewMBB = next(MachineFunction::iterator(WaterBB));
// Remove the original WaterList entry; we want subsequent
// insertions in this vicinity to go after the one we're
// about to insert. This considerably reduces the number
// of times we have to move the same CPE more than once.
WaterList.erase(IP);
break;
}
if (IP == B)
break;
}
}
if (!WaterFound) {
// No water found.
// Solution of last resort: split the user's MBB right after the user
// and insert a clone of the CPE into the newly created water.
DOUT << "No water found\n";
MachineBasicBlock *UserMBB = UserMI->getParent();
// TODO: Search for the best place to split the code. In practice, using
// loop nesting information to insert these guys outside of loops would be
// sufficient.
if (&UserMBB->back() == UserMI) {
assert(BBHasFallthrough(UserMBB) && "Expected a fallthrough BB!");
NewMBB = next(MachineFunction::iterator(UserMBB));
// Add an unconditional branch from UserMBB to fallthrough block.
// Note the new unconditional branch is not being recorded.
BuildMI(UserMBB, TII->get(isThumb ? ARM::tB : ARM::B)).addMBB(NewMBB);
BBSizes[UserMBB->getNumber()] += isThumb ? 2 : 4;
} else {
MachineInstr *NextMI = next(MachineBasicBlock::iterator(UserMI));
NewMBB = SplitBlockBeforeInstr(NextMI);
}
}
// Okay, we know we can put an island before NewMBB now, do it!
MachineBasicBlock *NewIsland = new MachineBasicBlock();
Fn.getBasicBlockList().insert(NewMBB, NewIsland);
// Update internal data structures to account for the newly inserted MBB.
UpdateForInsertedWaterBlock(NewIsland);
// Decrement the old entry, and remove it if refcount becomes 0.
DecrementOldEntry(CPI, CPEMI, Size);
// Now that we have an island to add the CPE to, clone the original CPE and
// add it to the island.
U.CPEMI = BuildMI(NewIsland, TII->get(ARM::CONSTPOOL_ENTRY))
.addImm(ID).addConstantPoolIndex(CPI).addImm(Size);
CPEntries[CPI].push_back(CPEntry(U.CPEMI, CPI, 1));
CPEntries[CPI].push_back(CPEntry(U.CPEMI, ID, 1));
NumCPEs++;
// Compensate for .align 2 in thumb mode.
@ -652,7 +842,7 @@ bool ARMConstantIslands::HandleConstantPoolUser(MachineFunction &Fn, CPUser &U){
return true;
}
/// BBIsInRange - Returns true is the distance between specific MI and
/// BBIsInRange - Returns true if the distance between specific MI and
/// specific BB can fit in MI's displacement field.
bool ARMConstantIslands::BBIsInRange(MachineInstr *MI,MachineBasicBlock *DestBB,
unsigned MaxDisp) {
@ -690,10 +880,10 @@ bool ARMConstantIslands::FixUpImmediateBr(MachineFunction &Fn, ImmBranch &Br) {
return FixUpConditionalBr(Fn, Br);
}
/// FixUpUnconditionalBr - Fix up an unconditional branches whose destination is
/// too far away to fit in its displacement field. If LR register ha been
/// FixUpUnconditionalBr - Fix up an unconditional branch whose destination is
/// too far away to fit in its displacement field. If the LR register has been
/// spilled in the epilogue, then we can use BL to implement a far jump.
/// Otherwise, add a intermediate branch instruction to to a branch.
/// Otherwise, add an intermediate branch instruction to to a branch.
bool
ARMConstantIslands::FixUpUnconditionalBr(MachineFunction &Fn, ImmBranch &Br) {
MachineInstr *MI = Br.MI;
@ -712,13 +902,13 @@ ARMConstantIslands::FixUpUnconditionalBr(MachineFunction &Fn, ImmBranch &Br) {
return true;
}
/// getUnconditionalBrDisp - Returns the maximum displacement that can fit in the
/// specific unconditional branch instruction.
/// getUnconditionalBrDisp - Returns the maximum displacement that can fit in
/// the specific unconditional branch instruction.
static inline unsigned getUnconditionalBrDisp(int Opc) {
return (Opc == ARM::tB) ? (1<<10)*2 : (1<<23)*4;
}
/// FixUpConditionalBr - Fix up a conditional branches whose destination is too
/// FixUpConditionalBr - Fix up a conditional branch whose destination is too
/// far away to fit in its displacement field. It is converted to an inverse
/// conditional branch + an unconditional branch to the destination.
bool
@ -791,7 +981,6 @@ ARMConstantIslands::FixUpConditionalBr(MachineFunction &Fn, ImmBranch &Br) {
return true;
}
/// UndoLRSpillRestore - Remove Thumb push / pop instructions that only spills
/// LR / restores LR to pc.
bool ARMConstantIslands::UndoLRSpillRestore() {

24
llvm/lib/Target/ARM/README.txt
View File

@ -17,20 +17,20 @@ Reimplement 'select' in terms of 'SEL'.
//===---------------------------------------------------------------------===//
The constant island pass is extremely naive. If a constant pool entry is
out of range, it *always* splits a block and inserts a copy of the cp
entry inline. It should:
The constant island pass has been much improved; all the todo items in the
previous version of this document have been addressed. However, there are still
things that can be done:
1. Check to see if there is already a copy of this constant nearby. If so,
reuse it.
2. Instead of always splitting blocks to insert the constant, insert it in
nearby 'water'.
3. Constant island references should be ref counted. If a constant reference
is out-of-range, and the last reference to a constant is relocated, the
dead constant should be removed.
1. When there isn't an existing water, the current MBB is split right after the
use. It would be profitable to look farther forward, especially on Thumb,
where negative offsets won't work.
This pass has all the framework needed to implement this, but it hasn't
been done.
2. WaterBlockListOffsets might be maintained throughout, rather than computed
when it is needed. This would probably lead to faster compile times.
Similarly, the offsets of blocks might be maintained throughout.
3. There may be some advantage to trying to be smarter about the initial
placement, rather than putting everything at the end.
//===---------------------------------------------------------------------===//