#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
#include "WebAssemblySubtarget.h"
#include "llvm/ADT/SCCIterator.h"
+#include "llvm/ADT/SetVector.h"
#include "llvm/CodeGen/MachineDominators.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineInstrBuilder.h"
});
}
+/// Return the "bottom" block of a loop. This differs from
+/// MachineLoop::getBottomBlock in that it works even if the loop is
+/// discontiguous.
+static MachineBasicBlock *LoopBottom(const MachineLoop *Loop) {
+ MachineBasicBlock *Bottom = Loop->getHeader();
+ for (MachineBasicBlock *MBB : Loop->blocks())
+ if (MBB->getNumber() > Bottom->getNumber())
+ Bottom = MBB;
+ return Bottom;
+}
+
/// Sort the blocks in RPO, taking special care to make sure that loops are
/// contiguous even in the case of split backedges.
+///
+/// TODO: Determine whether RPO is actually worthwhile, or whether we should
+/// move to just a stable-topological-sort-based approach that would preserve
+/// more of the original order.
static void SortBlocks(MachineFunction &MF, const MachineLoopInfo &MLI) {
// Note that we do our own RPO rather than using
// "llvm/ADT/PostOrderIterator.h" because we want control over the order that
MF.RenumberBlocks();
#ifndef NDEBUG
- for (auto &MBB : MF)
- if (MachineLoop *Loop = MLI.getLoopFor(&MBB)) {
- // Assert that all containing loops are contiguous.
- for (MachineLoop *L = Loop; L; L = L->getParentLoop()) {
- if (&MBB == L->getHeader()) {
- assert(&MBB == L->getTopBlock());
- } else {
- assert(&MBB != L->getTopBlock());
- assert(L->contains(
- MLI.getLoopFor(&*prev(MachineFunction::iterator(&MBB)))) &&
- "Loop isn't contiguous");
- }
- }
+ SmallSetVector<MachineLoop *, 8> OnStack;
+
+ // Insert a sentinel representing the degenerate loop that starts at the
+ // function entry block and includes the entire function as a "loop" that
+ // executes once.
+ OnStack.insert(nullptr);
+
+ for (auto &MBB : MF) {
+ assert(MBB.getNumber() >= 0 && "Renumbered blocks should be non-negative.");
+
+ MachineLoop *Loop = MLI.getLoopFor(&MBB);
+ if (Loop && &MBB == Loop->getHeader()) {
+ // Loop header. The loop predecessor should be sorted above, and the other
+ // predecessors should be backedges below.
+ for (auto Pred : MBB.predecessors())
+ assert(
+ (Pred->getNumber() < MBB.getNumber() || Loop->contains(Pred)) &&
+ "Loop header predecessors must be loop predecessors or backedges");
+ assert(OnStack.insert(Loop) && "Loops should be declared at most once.");
} else {
- // Assert that non-loops have no backedge predecessors.
+ // Not a loop header. All predecessors should be sorted above.
for (auto Pred : MBB.predecessors())
assert(Pred->getNumber() < MBB.getNumber() &&
- "CFG still has multiple-entry loops");
+ "Non-loop-header predecessors should be topologically sorted");
+ assert(OnStack.count(MLI.getLoopFor(&MBB)) &&
+ "Blocks must be nested in their loops");
}
+ while (OnStack.size() > 1 && &MBB == LoopBottom(OnStack.back()))
+ OnStack.pop_back();
+ }
+ assert(OnStack.pop_back_val() == nullptr &&
+ "The function entry block shouldn't actually be a loop header");
+ assert(OnStack.empty() &&
+ "Control flow stack pushes and pops should be balanced.");
#endif
}
-static unsigned GetLoopDepth(const MachineLoop *Loop) {
- return Loop ? Loop->getLoopDepth() : 0;
-}
-
/// Insert a BLOCK marker for branches to MBB (if needed).
-static void PlaceBlockMarkers(MachineBasicBlock &MBB,
- const WebAssemblyInstrInfo &TII,
- MachineDominatorTree &MDT,
- const MachineLoopInfo &MLI) {
- // Place the BLOCK for forward non-fallthrough branches. Put it at the nearest
- // common dominator of all preceding predecesors so that we minimize the time
- // that it's on the stack, which reduces overall stack height.
+static void PlaceBlockMarker(MachineBasicBlock &MBB, MachineFunction &MF,
+ SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
+ const WebAssemblyInstrInfo &TII,
+ const MachineLoopInfo &MLI,
+ MachineDominatorTree &MDT) {
+ // First compute the nearest common dominator of all forward non-fallthrough
+ // predecessors so that we minimize the time that the BLOCK is on the stack,
+ // which reduces overall stack height.
MachineBasicBlock *Header = nullptr;
bool IsBranchedTo = false;
int MBBNumber = MBB.getNumber();
if (!IsBranchedTo)
return;
+ assert(&MBB != &MF.front() && "Header blocks shouldn't have predecessors");
+ MachineBasicBlock *LayoutPred = &*prev(MachineFunction::iterator(&MBB));
+
+ // If the nearest common dominator is inside a more deeply nested context,
+ // walk out to the nearest scope which isn't more deeply nested.
+ for (MachineFunction::iterator I(LayoutPred), E(Header); I != E; --I) {
+ if (MachineBasicBlock *ScopeTop = ScopeTops[I->getNumber()]) {
+ if (ScopeTop->getNumber() > Header->getNumber()) {
+ // Skip over an intervening scope.
+ I = next(MachineFunction::iterator(ScopeTop));
+ } else {
+ // We found a scope level at an appropriate depth.
+ Header = ScopeTop;
+ break;
+ }
+ }
+ }
+
+ // If there's a loop which ends just before MBB which contains Header, we can
+ // reuse its label instead of inserting a new BLOCK.
+ for (MachineLoop *Loop = MLI.getLoopFor(LayoutPred);
+ Loop && Loop->contains(LayoutPred); Loop = Loop->getParentLoop())
+ if (Loop && LoopBottom(Loop) == LayoutPred && Loop->contains(Header))
+ return;
+
+ // Decide where in Header to put the BLOCK.
MachineBasicBlock::iterator InsertPos;
MachineLoop *HeaderLoop = MLI.getLoopFor(Header);
- unsigned MBBLoopDepth = GetLoopDepth(MLI.getLoopFor(&MBB));
- unsigned HeaderLoopDepth = GetLoopDepth(HeaderLoop);
- if (HeaderLoopDepth > MBBLoopDepth) {
- // The nearest common dominating point is more deeply nested. Insert the
- // BLOCK just above the LOOP.
- for (unsigned i = 0; i < HeaderLoopDepth - 1 - MBBLoopDepth; ++i)
- HeaderLoop = HeaderLoop->getParentLoop();
- Header = HeaderLoop->getHeader();
+ if (HeaderLoop && MBB.getNumber() > LoopBottom(HeaderLoop)->getNumber()) {
+ // Header is the header of a loop that does not lexically contain MBB, so
+ // the BLOCK needs to be above the LOOP.
InsertPos = Header->begin();
- // Don't insert a BLOCK if we can reuse a loop exit label though.
- if (InsertPos != Header->end() &&
- InsertPos->getOpcode() == WebAssembly::LOOP &&
- InsertPos->getOperand(0).getMBB() == &MBB)
- return;
} else {
- // Insert the BLOCK as late in the block as we can, but before any existing
- // BLOCKs.
+ // Otherwise, insert the BLOCK as late in Header as we can, but before any
+ // existing BLOCKs.
InsertPos = Header->getFirstTerminator();
while (InsertPos != Header->begin() &&
- std::prev(InsertPos)->getOpcode() == WebAssembly::BLOCK)
+ prev(InsertPos)->getOpcode() == WebAssembly::BLOCK)
--InsertPos;
}
+ // Add the BLOCK.
BuildMI(*Header, InsertPos, DebugLoc(), TII.get(WebAssembly::BLOCK))
.addMBB(&MBB);
+
+ // Track the farthest-spanning scope that ends at this point.
+ int Number = MBB.getNumber();
+ if (!ScopeTops[Number] ||
+ ScopeTops[Number]->getNumber() > Header->getNumber())
+ ScopeTops[Number] = Header;
+}
+
+/// Insert a LOOP marker for a loop starting at MBB (if it's a loop header).
+static void PlaceLoopMarker(MachineBasicBlock &MBB, MachineFunction &MF,
+ SmallVectorImpl<MachineBasicBlock *> &ScopeTops,
+ const WebAssemblyInstrInfo &TII,
+ const MachineLoopInfo &MLI) {
+ MachineLoop *Loop = MLI.getLoopFor(&MBB);
+ if (!Loop || Loop->getHeader() != &MBB)
+ return;
+
+ // The operand of a LOOP is the first block after the loop. If the loop is the
+ // bottom of the function, insert a dummy block at the end.
+ MachineBasicBlock *Bottom = LoopBottom(Loop);
+ auto Iter = next(MachineFunction::iterator(Bottom));
+ if (Iter == MF.end()) {
+ MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
+ // Give it a fake predecessor so that AsmPrinter prints its label.
+ Label->addSuccessor(Label);
+ MF.push_back(Label);
+ Iter = next(MachineFunction::iterator(Bottom));
+ }
+ MachineBasicBlock *AfterLoop = &*Iter;
+ BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
+ .addMBB(AfterLoop);
+
+ // Emit a special no-op telling the asm printer that we need a label to close
+ // the loop scope, even though the destination is only reachable by
+ // fallthrough.
+ if (!Bottom->back().isBarrier())
+ BuildMI(*Bottom, Bottom->end(), DebugLoc(), TII.get(WebAssembly::LOOP_END));
+
+ assert((!ScopeTops[AfterLoop->getNumber()] ||
+ ScopeTops[AfterLoop->getNumber()]->getNumber() < MBB.getNumber()) &&
+ "With RPO we should visit the outer-most loop for a block first.");
+ if (!ScopeTops[AfterLoop->getNumber()])
+ ScopeTops[AfterLoop->getNumber()] = &MBB;
}
/// Insert LOOP and BLOCK markers at appropriate places.
static void PlaceMarkers(MachineFunction &MF, const MachineLoopInfo &MLI,
const WebAssemblyInstrInfo &TII,
MachineDominatorTree &MDT) {
+ // For each block whose label represents the end of a scope, record the block
+ // which holds the beginning of the scope. This will allow us to quickly skip
+ // over scoped regions when walking blocks. We allocate one more than the
+ // number of blocks in the function to accommodate for the possible fake block
+ // we may insert at the end.
+ SmallVector<MachineBasicBlock *, 8> ScopeTops(MF.getNumBlockIDs() + 1);
+
for (auto &MBB : MF) {
// Place the LOOP for MBB if MBB is the header of a loop.
- if (MachineLoop *Loop = MLI.getLoopFor(&MBB))
- if (Loop->getHeader() == &MBB) {
- // The operand of a LOOP is the first block after the loop. If the loop
- // is the bottom of the function, insert a dummy block at the end.
- MachineBasicBlock *Bottom = Loop->getBottomBlock();
- auto Iter = next(MachineFunction::iterator(Bottom));
- if (Iter == MF.end()) {
- MachineBasicBlock *Label = MF.CreateMachineBasicBlock();
- // Give it a fake predecessor so that AsmPrinter prints its label.
- Label->addSuccessor(Label);
- MF.push_back(Label);
- Iter = next(MachineFunction::iterator(Bottom));
- }
- BuildMI(MBB, MBB.begin(), DebugLoc(), TII.get(WebAssembly::LOOP))
- .addMBB(&*Iter);
-
- // Emit a special no-op telling the asm printer that we need a label
- // to close the loop scope, even though the destination is only
- // reachable by fallthrough.
- if (!Bottom->back().isBarrier())
- BuildMI(*Bottom, Bottom->end(), DebugLoc(),
- TII.get(WebAssembly::LOOP_END));
- }
+ PlaceLoopMarker(MBB, MF, ScopeTops, TII, MLI);
// Place the BLOCK for MBB if MBB is branched to from above.
- PlaceBlockMarkers(MBB, TII, MDT, MLI);
+ PlaceBlockMarker(MBB, MF, ScopeTops, TII, MLI, MDT);
}
}
PlaceMarkers(MF, MLI, TII, MDT);
#ifndef NDEBUG
- // Verify that block and loop beginnings and endings are in FIFO order, and
+ // Verify that block and loop beginnings and endings are in LIFO order, and
// that all references to blocks are to blocks on the stack at the point of
// the reference.
SmallVector<std::pair<MachineBasicBlock *, bool>, 0> Stack;
Stack.push_back(std::make_pair(MI.getOperand(0).getMBB(), false));
break;
default:
+ // Verify that all referenced blocks are in scope. A reference to a
+ // block with a negative number is invalid, but can happen with inline
+ // asm, so we shouldn't assert on it, but instead let CodeGen properly
+ // fail on it.
for (const MachineOperand &MO : MI.explicit_operands())
- if (MO.isMBB())
+ if (MO.isMBB() && MO.getMBB()->getNumber() >= 0)
assert(IsOnStack(Stack, MO.getMBB()));
break;
}
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