1 //===-- BranchRelaxation.cpp ----------------------------------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 #include "llvm/CodeGen/Passes.h"
11 #include "llvm/ADT/SmallVector.h"
12 #include "llvm/ADT/Statistic.h"
13 #include "llvm/CodeGen/LivePhysRegs.h"
14 #include "llvm/CodeGen/MachineFunctionPass.h"
15 #include "llvm/CodeGen/RegisterScavenging.h"
16 #include "llvm/Target/TargetInstrInfo.h"
17 #include "llvm/Target/TargetSubtargetInfo.h"
18 #include "llvm/Support/Debug.h"
19 #include "llvm/Support/Format.h"
20 #include "llvm/Support/raw_ostream.h"
24 #define DEBUG_TYPE "branch-relaxation"
26 STATISTIC(NumSplit, "Number of basic blocks split");
27 STATISTIC(NumConditionalRelaxed, "Number of conditional branches relaxed");
28 STATISTIC(NumUnconditionalRelaxed, "Number of unconditional branches relaxed");
30 #define BRANCH_RELAX_NAME "Branch relaxation pass"
33 class BranchRelaxation : public MachineFunctionPass {
34 /// BasicBlockInfo - Information about the offset and size of a single
36 struct BasicBlockInfo {
37 /// Offset - Distance from the beginning of the function to the beginning
38 /// of this basic block.
40 /// The offset is always aligned as required by the basic block.
43 /// Size - Size of the basic block in bytes. If the block contains
44 /// inline assembly, this is a worst case estimate.
46 /// The size does not include any alignment padding whether from the
47 /// beginning of the block, or from an aligned jump table at the end.
50 BasicBlockInfo() : Offset(0), Size(0) {}
52 /// Compute the offset immediately following this block. \p MBB is the next
54 unsigned postOffset(const MachineBasicBlock &MBB) const {
55 unsigned PO = Offset + Size;
56 unsigned Align = MBB.getAlignment();
60 unsigned AlignAmt = 1 << Align;
61 unsigned ParentAlign = MBB.getParent()->getAlignment();
62 if (Align <= ParentAlign)
63 return PO + OffsetToAlignment(PO, AlignAmt);
65 // The alignment of this MBB is larger than the function's alignment, so we
66 // can't tell whether or not it will insert nops. Assume that it will.
67 return PO + AlignAmt + OffsetToAlignment(PO, AlignAmt);
71 SmallVector<BasicBlockInfo, 16> BlockInfo;
72 std::unique_ptr<RegScavenger> RS;
73 LivePhysRegs LiveRegs;
76 const TargetRegisterInfo *TRI;
77 const TargetInstrInfo *TII;
79 bool relaxBranchInstructions();
82 MachineBasicBlock *createNewBlockAfter(MachineBasicBlock &BB);
84 MachineBasicBlock *splitBlockBeforeInstr(MachineInstr &MI,
85 MachineBasicBlock *DestBB);
86 void adjustBlockOffsets(MachineBasicBlock &MBB);
87 bool isBlockInRange(const MachineInstr &MI, const MachineBasicBlock &BB) const;
89 bool fixupConditionalBranch(MachineInstr &MI);
90 bool fixupUnconditionalBranch(MachineInstr &MI);
91 uint64_t computeBlockSize(const MachineBasicBlock &MBB) const;
92 unsigned getInstrOffset(const MachineInstr &MI) const;
98 BranchRelaxation() : MachineFunctionPass(ID) { }
100 bool runOnMachineFunction(MachineFunction &MF) override;
102 StringRef getPassName() const override {
103 return BRANCH_RELAX_NAME;
109 char BranchRelaxation::ID = 0;
110 char &llvm::BranchRelaxationPassID = BranchRelaxation::ID;
112 INITIALIZE_PASS(BranchRelaxation, DEBUG_TYPE, BRANCH_RELAX_NAME, false, false)
114 /// verify - check BBOffsets, BBSizes, alignment of islands
115 void BranchRelaxation::verify() {
117 unsigned PrevNum = MF->begin()->getNumber();
118 for (MachineBasicBlock &MBB : *MF) {
119 unsigned Align = MBB.getAlignment();
120 unsigned Num = MBB.getNumber();
121 assert(BlockInfo[Num].Offset % (1u << Align) == 0);
122 assert(!Num || BlockInfo[PrevNum].postOffset(MBB) <= BlockInfo[Num].Offset);
123 assert(BlockInfo[Num].Size == computeBlockSize(MBB));
129 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
130 /// print block size and offset information - debugging
131 LLVM_DUMP_METHOD void BranchRelaxation::dumpBBs() {
132 for (auto &MBB : *MF) {
133 const BasicBlockInfo &BBI = BlockInfo[MBB.getNumber()];
134 dbgs() << format("BB#%u\toffset=%08x\t", MBB.getNumber(), BBI.Offset)
135 << format("size=%#x\n", BBI.Size);
140 /// scanFunction - Do the initial scan of the function, building up
141 /// information about each block.
142 void BranchRelaxation::scanFunction() {
144 BlockInfo.resize(MF->getNumBlockIDs());
146 // First thing, compute the size of all basic blocks, and see if the function
147 // has any inline assembly in it. If so, we have to be conservative about
148 // alignment assumptions, as we don't know for sure the size of any
149 // instructions in the inline assembly.
150 for (MachineBasicBlock &MBB : *MF)
151 BlockInfo[MBB.getNumber()].Size = computeBlockSize(MBB);
153 // Compute block offsets and known bits.
154 adjustBlockOffsets(*MF->begin());
157 /// computeBlockSize - Compute the size for MBB.
158 uint64_t BranchRelaxation::computeBlockSize(const MachineBasicBlock &MBB) const {
160 for (const MachineInstr &MI : MBB)
161 Size += TII->getInstSizeInBytes(MI);
165 /// getInstrOffset - Return the current offset of the specified machine
166 /// instruction from the start of the function. This offset changes as stuff is
167 /// moved around inside the function.
168 unsigned BranchRelaxation::getInstrOffset(const MachineInstr &MI) const {
169 const MachineBasicBlock *MBB = MI.getParent();
171 // The offset is composed of two things: the sum of the sizes of all MBB's
172 // before this instruction's block, and the offset from the start of the block
174 unsigned Offset = BlockInfo[MBB->getNumber()].Offset;
176 // Sum instructions before MI in MBB.
177 for (MachineBasicBlock::const_iterator I = MBB->begin(); &*I != &MI; ++I) {
178 assert(I != MBB->end() && "Didn't find MI in its own basic block?");
179 Offset += TII->getInstSizeInBytes(*I);
185 void BranchRelaxation::adjustBlockOffsets(MachineBasicBlock &Start) {
186 unsigned PrevNum = Start.getNumber();
187 for (auto &MBB : make_range(MachineFunction::iterator(Start), MF->end())) {
188 unsigned Num = MBB.getNumber();
189 if (!Num) // block zero is never changed from offset zero.
191 // Get the offset and known bits at the end of the layout predecessor.
192 // Include the alignment of the current block.
193 BlockInfo[Num].Offset = BlockInfo[PrevNum].postOffset(MBB);
199 /// Insert a new empty basic block and insert it after \BB
200 MachineBasicBlock *BranchRelaxation::createNewBlockAfter(MachineBasicBlock &BB) {
201 // Create a new MBB for the code after the OrigBB.
202 MachineBasicBlock *NewBB =
203 MF->CreateMachineBasicBlock(BB.getBasicBlock());
204 MF->insert(++BB.getIterator(), NewBB);
206 // Insert an entry into BlockInfo to align it properly with the block numbers.
207 BlockInfo.insert(BlockInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
212 /// Split the basic block containing MI into two blocks, which are joined by
213 /// an unconditional branch. Update data structures and renumber blocks to
214 /// account for this change and returns the newly created block.
215 MachineBasicBlock *BranchRelaxation::splitBlockBeforeInstr(MachineInstr &MI,
216 MachineBasicBlock *DestBB) {
217 MachineBasicBlock *OrigBB = MI.getParent();
219 // Create a new MBB for the code after the OrigBB.
220 MachineBasicBlock *NewBB =
221 MF->CreateMachineBasicBlock(OrigBB->getBasicBlock());
222 MF->insert(++OrigBB->getIterator(), NewBB);
224 // Splice the instructions starting with MI over to NewBB.
225 NewBB->splice(NewBB->end(), OrigBB, MI.getIterator(), OrigBB->end());
227 // Add an unconditional branch from OrigBB to NewBB.
228 // Note the new unconditional branch is not being recorded.
229 // There doesn't seem to be meaningful DebugInfo available; this doesn't
230 // correspond to anything in the source.
231 TII->insertUnconditionalBranch(*OrigBB, NewBB, DebugLoc());
233 // Insert an entry into BlockInfo to align it properly with the block numbers.
234 BlockInfo.insert(BlockInfo.begin() + NewBB->getNumber(), BasicBlockInfo());
237 NewBB->transferSuccessors(OrigBB);
238 OrigBB->addSuccessor(NewBB);
239 OrigBB->addSuccessor(DestBB);
241 // Cleanup potential unconditional branch to successor block.
242 // Note that updateTerminator may change the size of the blocks.
243 NewBB->updateTerminator();
244 OrigBB->updateTerminator();
246 // Figure out how large the OrigBB is. As the first half of the original
247 // block, it cannot contain a tablejump. The size includes
248 // the new jump we added. (It should be possible to do this without
249 // recounting everything, but it's very confusing, and this is rarely
251 BlockInfo[OrigBB->getNumber()].Size = computeBlockSize(*OrigBB);
253 // Figure out how large the NewMBB is. As the second half of the original
254 // block, it may contain a tablejump.
255 BlockInfo[NewBB->getNumber()].Size = computeBlockSize(*NewBB);
257 // All BBOffsets following these blocks must be modified.
258 adjustBlockOffsets(*OrigBB);
260 // Need to fix live-in lists if we track liveness.
261 if (TRI->trackLivenessAfterRegAlloc(*MF))
262 computeLiveIns(LiveRegs, MF->getRegInfo(), *NewBB);
269 /// isBlockInRange - Returns true if the distance between specific MI and
270 /// specific BB can fit in MI's displacement field.
271 bool BranchRelaxation::isBlockInRange(
272 const MachineInstr &MI, const MachineBasicBlock &DestBB) const {
273 int64_t BrOffset = getInstrOffset(MI);
274 int64_t DestOffset = BlockInfo[DestBB.getNumber()].Offset;
276 if (TII->isBranchOffsetInRange(MI.getOpcode(), DestOffset - BrOffset))
280 dbgs() << "Out of range branch to destination BB#" << DestBB.getNumber()
281 << " from BB#" << MI.getParent()->getNumber()
282 << " to " << DestOffset
283 << " offset " << DestOffset - BrOffset
290 /// fixupConditionalBranch - Fix up a conditional branch whose destination is
291 /// too far away to fit in its displacement field. It is converted to an inverse
292 /// conditional branch + an unconditional branch to the destination.
293 bool BranchRelaxation::fixupConditionalBranch(MachineInstr &MI) {
294 DebugLoc DL = MI.getDebugLoc();
295 MachineBasicBlock *MBB = MI.getParent();
296 MachineBasicBlock *TBB = nullptr, *FBB = nullptr;
297 SmallVector<MachineOperand, 4> Cond;
299 bool Fail = TII->analyzeBranch(*MBB, TBB, FBB, Cond);
300 assert(!Fail && "branches to be relaxed must be analyzable");
303 // Add an unconditional branch to the destination and invert the branch
304 // condition to jump over it:
311 if (FBB && isBlockInRange(MI, *FBB)) {
312 // Last MI in the BB is an unconditional branch. We can simply invert the
313 // condition and swap destinations:
319 DEBUG(dbgs() << " Invert condition and swap "
320 "its destination with " << MBB->back());
322 TII->reverseBranchCondition(Cond);
323 int OldSize = 0, NewSize = 0;
324 TII->removeBranch(*MBB, &OldSize);
325 TII->insertBranch(*MBB, FBB, TBB, Cond, DL, &NewSize);
327 BlockInfo[MBB->getNumber()].Size += (NewSize - OldSize);
330 // We need to split the basic block here to obtain two long-range
331 // unconditional branches.
332 auto &NewBB = *MF->CreateMachineBasicBlock(MBB->getBasicBlock());
333 MF->insert(++MBB->getIterator(), &NewBB);
335 // Insert an entry into BlockInfo to align it properly with the block
337 BlockInfo.insert(BlockInfo.begin() + NewBB.getNumber(), BasicBlockInfo());
339 unsigned &NewBBSize = BlockInfo[NewBB.getNumber()].Size;
341 TII->insertUnconditionalBranch(NewBB, FBB, DL, &NewBrSize);
342 NewBBSize += NewBrSize;
344 // Update the successor lists according to the transformation to follow.
345 // Do it here since if there's no split, no update is needed.
346 MBB->replaceSuccessor(FBB, &NewBB);
347 NewBB.addSuccessor(FBB);
349 // Need to fix live-in lists if we track liveness.
350 if (TRI->trackLivenessAfterRegAlloc(*MF))
351 computeLiveIns(LiveRegs, MF->getRegInfo(), NewBB);
354 // We now have an appropriate fall-through block in place (either naturally or
355 // just created), so we can invert the condition.
356 MachineBasicBlock &NextBB = *std::next(MachineFunction::iterator(MBB));
358 DEBUG(dbgs() << " Insert B to BB#" << TBB->getNumber()
359 << ", invert condition and change dest. to BB#"
360 << NextBB.getNumber() << '\n');
362 unsigned &MBBSize = BlockInfo[MBB->getNumber()].Size;
364 // Insert a new conditional branch and a new unconditional branch.
366 TII->reverseBranchCondition(Cond);
367 TII->removeBranch(*MBB, &RemovedSize);
368 MBBSize -= RemovedSize;
371 TII->insertBranch(*MBB, &NextBB, TBB, Cond, DL, &AddedSize);
372 MBBSize += AddedSize;
374 // Finally, keep the block offsets up to date.
375 adjustBlockOffsets(*MBB);
379 bool BranchRelaxation::fixupUnconditionalBranch(MachineInstr &MI) {
380 MachineBasicBlock *MBB = MI.getParent();
382 unsigned OldBrSize = TII->getInstSizeInBytes(MI);
383 MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI);
385 int64_t DestOffset = BlockInfo[DestBB->getNumber()].Offset;
386 int64_t SrcOffset = getInstrOffset(MI);
388 assert(!TII->isBranchOffsetInRange(MI.getOpcode(), DestOffset - SrcOffset));
390 BlockInfo[MBB->getNumber()].Size -= OldBrSize;
392 MachineBasicBlock *BranchBB = MBB;
394 // If this was an expanded conditional branch, there is already a single
395 // unconditional branch in a block.
397 BranchBB = createNewBlockAfter(*MBB);
400 for (const MachineBasicBlock *Succ : MBB->successors()) {
401 for (const MachineBasicBlock::RegisterMaskPair &LiveIn : Succ->liveins())
402 BranchBB->addLiveIn(LiveIn);
405 BranchBB->sortUniqueLiveIns();
406 BranchBB->addSuccessor(DestBB);
407 MBB->replaceSuccessor(DestBB, BranchBB);
410 DebugLoc DL = MI.getDebugLoc();
411 MI.eraseFromParent();
412 BlockInfo[BranchBB->getNumber()].Size += TII->insertIndirectBranch(
413 *BranchBB, *DestBB, DL, DestOffset - SrcOffset, RS.get());
415 adjustBlockOffsets(*MBB);
419 bool BranchRelaxation::relaxBranchInstructions() {
420 bool Changed = false;
422 // Relaxing branches involves creating new basic blocks, so re-eval
423 // end() for termination.
424 for (MachineFunction::iterator I = MF->begin(); I != MF->end(); ++I) {
425 MachineBasicBlock &MBB = *I;
428 MachineBasicBlock::iterator Last = MBB.getLastNonDebugInstr();
429 if (Last == MBB.end())
432 // Expand the unconditional branch first if necessary. If there is a
433 // conditional branch, this will end up changing the branch destination of
434 // it to be over the newly inserted indirect branch block, which may avoid
435 // the need to try expanding the conditional branch first, saving an extra
437 if (Last->isUnconditionalBranch()) {
438 // Unconditional branch destination might be unanalyzable, assume these
440 if (MachineBasicBlock *DestBB = TII->getBranchDestBlock(*Last)) {
441 if (!isBlockInRange(*Last, *DestBB)) {
442 fixupUnconditionalBranch(*Last);
443 ++NumUnconditionalRelaxed;
449 // Loop over the conditional branches.
450 MachineBasicBlock::iterator Next;
451 for (MachineBasicBlock::iterator J = MBB.getFirstTerminator();
452 J != MBB.end(); J = Next) {
454 MachineInstr &MI = *J;
456 if (MI.isConditionalBranch()) {
457 MachineBasicBlock *DestBB = TII->getBranchDestBlock(MI);
458 if (!isBlockInRange(MI, *DestBB)) {
459 if (Next != MBB.end() && Next->isConditionalBranch()) {
460 // If there are multiple conditional branches, this isn't an
461 // analyzable block. Split later terminators into a new block so
462 // each one will be analyzable.
464 splitBlockBeforeInstr(*Next, DestBB);
466 fixupConditionalBranch(MI);
467 ++NumConditionalRelaxed;
472 // This may have modified all of the terminators, so start over.
473 Next = MBB.getFirstTerminator();
482 bool BranchRelaxation::runOnMachineFunction(MachineFunction &mf) {
485 DEBUG(dbgs() << "***** BranchRelaxation *****\n");
487 const TargetSubtargetInfo &ST = MF->getSubtarget();
488 TII = ST.getInstrInfo();
490 TRI = ST.getRegisterInfo();
491 if (TRI->trackLivenessAfterRegAlloc(*MF))
492 RS.reset(new RegScavenger());
494 // Renumber all of the machine basic blocks in the function, guaranteeing that
495 // the numbers agree with the position of the block in the function.
496 MF->RenumberBlocks();
498 // Do the initial scan of the function, building up information about the
499 // sizes of each block.
502 DEBUG(dbgs() << " Basic blocks before relaxation\n"; dumpBBs(););
504 bool MadeChange = false;
505 while (relaxBranchInstructions())
508 // After a while, this might be made debug-only, but it is not expensive.
511 DEBUG(dbgs() << " Basic blocks after relaxation\n\n"; dumpBBs());