// Branch analysis.
bool WebAssemblyInstrInfo::analyzeBranch(MachineBasicBlock &MBB,
MachineBasicBlock *&TBB,
MachineBasicBlock *&FBB,
SmallVectorImpl<MachineOperand> &Cond,
bool /*AllowModify*/) const {
bool HaveCond = false;
for (MachineInstr &MI : MBB.terminators()) {
switch (MI.getOpcode()) {
default:
// Unhandled instruction; bail out.
return true;
case WebAssembly::BR_IF:
if (HaveCond)
return true;
// If we're running after CFGStackify, we can't optimize further.
if (!MI.getOperand(0).isMBB())
return true;
Cond.push_back(MachineOperand::CreateImm(true));
Cond.push_back(MI.getOperand(1));
TBB = MI.getOperand(0).getMBB();
HaveCond = true;
break;
case WebAssembly::BR_UNLESS:
if (HaveCond)
return true;
// If we're running after CFGStackify, we can't optimize further.
if (!MI.getOperand(0).isMBB())
return true;
Cond.push_back(MachineOperand::CreateImm(false));
Cond.push_back(MI.getOperand(1));
TBB = MI.getOperand(0).getMBB();
HaveCond = true;
break;
case WebAssembly::BR:
// If we're running after CFGStackify, we can't optimize further.
if (!MI.getOperand(0).isMBB())
return true;
if (!HaveCond)
TBB = MI.getOperand(0).getMBB();
else
FBB = MI.getOperand(0).getMBB();
break;
}
if (MI.isBarrier())
break;
}
return false;
}
void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB,
RegScavenger *RS) {
// Get rid of the easy cases first.
if (!Save)
Save = &MBB;
else
Save = MDT->findNearestCommonDominator(Save, &MBB);
if (!Save) {
LLVM_DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
return;
}
if (!Restore)
Restore = &MBB;
else if (MPDT->getNode(&MBB)) // If the block is not in the post dom tree, it
// means the block never returns. If that's the
// case, we don't want to call
// `findNearestCommonDominator`, which will
// return `Restore`.
Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
else
Restore = nullptr; // Abort, we can't find a restore point in this case.
// Make sure we would be able to insert the restore code before the
// terminator.
if (Restore == &MBB) {
for (const MachineInstr &Terminator : MBB.terminators()) {
if (!useOrDefCSROrFI(Terminator, RS))
continue;
// One of the terminator needs to happen before the restore point.
if (MBB.succ_empty()) {
Restore = nullptr; // Abort, we can't find a restore point in this case.
break;
}
// Look for a restore point that post-dominates all the successors.
// The immediate post-dominator is what we are looking for.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
break;
}
}
if (!Restore) {
LLVM_DEBUG(
dbgs() << "Restore point needs to be spanned on several blocks\n");
return;
}
// Make sure Save and Restore are suitable for shrink-wrapping:
// 1. all path from Save needs to lead to Restore before exiting.
// 2. all path to Restore needs to go through Save from Entry.
// We achieve that by making sure that:
// A. Save dominates Restore.
// B. Restore post-dominates Save.
// C. Save and Restore are in the same loop.
bool SaveDominatesRestore = false;
bool RestorePostDominatesSave = false;
while (Save && Restore &&
(!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
!(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
// Post-dominance is not enough in loops to ensure that all uses/defs
// are after the prologue and before the epilogue at runtime.
// E.g.,
// while(1) {
// Save
// Restore
// if (...)
// break;
// use/def CSRs
// }
// All the uses/defs of CSRs are dominated by Save and post-dominated
// by Restore. However, the CSRs uses are still reachable after
// Restore and before Save are executed.
//
// For now, just push the restore/save points outside of loops.
// FIXME: Refine the criteria to still find interesting cases
// for loops.
MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
// Fix (A).
if (!SaveDominatesRestore) {
Save = MDT->findNearestCommonDominator(Save, Restore);
continue;
}
// Fix (B).
if (!RestorePostDominatesSave)
Restore = MPDT->findNearestCommonDominator(Restore, Save);
// Fix (C).
if (Save && Restore &&
(MLI->getLoopFor(Save) || MLI->getLoopFor(Restore))) {
if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore)) {
// Push Save outside of this loop if immediate dominator is different
// from save block. If immediate dominator is not different, bail out.
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
if (!Save)
break;
} else {
// If the loop does not exit, there is no point in looking
// for a post-dominator outside the loop.
SmallVector<MachineBasicBlock*, 4> ExitBlocks;
MLI->getLoopFor(Restore)->getExitingBlocks(ExitBlocks);
// Push Restore outside of this loop.
// Look for the immediate post-dominator of the loop exits.
MachineBasicBlock *IPdom = Restore;
for (MachineBasicBlock *LoopExitBB: ExitBlocks) {
IPdom = FindIDom<>(*IPdom, LoopExitBB->successors(), *MPDT);
if (!IPdom)
break;
}
// If the immediate post-dominator is not in a less nested loop,
// then we are stuck in a program with an infinite loop.
// In that case, we will not find a safe point, hence, bail out.
if (IPdom && MLI->getLoopDepth(IPdom) < MLI->getLoopDepth(Restore))
Restore = IPdom;
else {
Restore = nullptr;
break;
}
}
}
}
}
void ShrinkWrap::updateSaveRestorePoints(MachineBasicBlock &MBB) {
// Get rid of the easy cases first.
if (!Save)
Save = &MBB;
else
Save = MDT->findNearestCommonDominator(Save, &MBB);
if (!Save) {
DEBUG(dbgs() << "Found a block that is not reachable from Entry\n");
return;
}
if (!Restore)
Restore = &MBB;
else
Restore = MPDT->findNearestCommonDominator(Restore, &MBB);
// Make sure we would be able to insert the restore code before the
// terminator.
if (Restore == &MBB) {
for (const MachineInstr &Terminator : MBB.terminators()) {
if (!useOrDefCSROrFI(Terminator))
continue;
// One of the terminator needs to happen before the restore point.
if (MBB.succ_empty()) {
Restore = nullptr;
break;
}
// Look for a restore point that post-dominates all the successors.
// The immediate post-dominator is what we are looking for.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
break;
}
}
if (!Restore) {
DEBUG(dbgs() << "Restore point needs to be spanned on several blocks\n");
return;
}
// Make sure Save and Restore are suitable for shrink-wrapping:
// 1. all path from Save needs to lead to Restore before exiting.
// 2. all path to Restore needs to go through Save from Entry.
// We achieve that by making sure that:
// A. Save dominates Restore.
// B. Restore post-dominates Save.
// C. Save and Restore are in the same loop.
bool SaveDominatesRestore = false;
bool RestorePostDominatesSave = false;
while (Save && Restore &&
(!(SaveDominatesRestore = MDT->dominates(Save, Restore)) ||
!(RestorePostDominatesSave = MPDT->dominates(Restore, Save)) ||
MLI->getLoopFor(Save) != MLI->getLoopFor(Restore))) {
// Fix (A).
if (!SaveDominatesRestore) {
Save = MDT->findNearestCommonDominator(Save, Restore);
continue;
}
// Fix (B).
if (!RestorePostDominatesSave)
Restore = MPDT->findNearestCommonDominator(Restore, Save);
// Fix (C).
if (Save && Restore && Save != Restore &&
MLI->getLoopFor(Save) != MLI->getLoopFor(Restore)) {
if (MLI->getLoopDepth(Save) > MLI->getLoopDepth(Restore))
// Push Save outside of this loop.
Save = FindIDom<>(*Save, Save->predecessors(), *MDT);
else
// Push Restore outside of this loop.
Restore = FindIDom<>(*Restore, Restore->successors(), *MPDT);
}
}
}
