[llvm] [MachineSink] Extend loop sinking capability (PR #117247)

[Matt Arsenault via llvm-commits]

================
@@ -1574,83 +1644,136 @@ bool MachineSinking::hasStoreBetween(MachineBasicBlock *From,
   return HasAliasedStore;
 }
 
-/// Sink instructions into cycles if profitable. This especially tries to
-/// prevent register spills caused by register pressure if there is little to no
-/// overhead moving instructions into cycles.
-bool MachineSinking::SinkIntoCycle(MachineCycle *Cycle, MachineInstr &I) {
-  LLVM_DEBUG(dbgs() << "CycleSink: Finding sink block for: " << I);
-  MachineBasicBlock *Preheader = Cycle->getCyclePreheader();
-  assert(Preheader && "Cycle sink needs a preheader block");
-  MachineBasicBlock *SinkBlock = nullptr;
-  bool CanSink = true;
-  const MachineOperand &MO = I.getOperand(0);
-
-  for (MachineInstr &MI : MRI->use_instructions(MO.getReg())) {
-    LLVM_DEBUG(dbgs() << "CycleSink:   Analysing use: " << MI);
-    if (!Cycle->contains(MI.getParent())) {
-      LLVM_DEBUG(dbgs() << "CycleSink:   Use not in cycle, can't sink.\n");
-      CanSink = false;
-      break;
-    }
+bool MachineSinking::isDead(const MachineInstr *MI) const {
+  // Instructions without side-effects are dead iff they only define dead regs.
+  // This function is hot and this loop returns early in the common case,
+  // so only perform additional checks before this if absolutely necessary.
 
-    // FIXME: Come up with a proper cost model that estimates whether sinking
-    // the instruction (and thus possibly executing it on every cycle
-    // iteration) is more expensive than a register.
-    // For now assumes that copies are cheap and thus almost always worth it.
-    if (!MI.isCopy()) {
-      LLVM_DEBUG(dbgs() << "CycleSink:   Use is not a copy\n");
-      CanSink = false;
-      break;
-    }
-    if (!SinkBlock) {
-      SinkBlock = MI.getParent();
-      LLVM_DEBUG(dbgs() << "CycleSink:   Setting sink block to: "
-                        << printMBBReference(*SinkBlock) << "\n");
+  for (const MachineOperand &MO : MI->all_defs()) {
+    Register Reg = MO.getReg();
+    if (Reg.isPhysical())
+      return false;
+
+    if (MO.isDead()) {
+#ifndef NDEBUG
+      // Basic check on the register. All of them should be 'undef'.
+      for (auto &U : MRI->use_nodbg_operands(Reg))
+        assert(U.isUndef() && "'Undef' use on a 'dead' register is found!");
+#endif
       continue;
     }
-    SinkBlock = DT->findNearestCommonDominator(SinkBlock, MI.getParent());
-    if (!SinkBlock) {
-      LLVM_DEBUG(dbgs() << "CycleSink:   Can't find nearest dominator\n");
-      CanSink = false;
-      break;
-    }
-    LLVM_DEBUG(dbgs() << "CycleSink:   Setting nearest common dom block: "
-                      << printMBBReference(*SinkBlock) << "\n");
-  }
 
-  if (!CanSink) {
-    LLVM_DEBUG(dbgs() << "CycleSink: Can't sink instruction.\n");
-    return false;
-  }
-  if (!SinkBlock) {
-    LLVM_DEBUG(dbgs() << "CycleSink: Not sinking, can't find sink block.\n");
-    return false;
+    if (!(MRI->hasAtMostUserInstrs(Reg, 0)))
+      return false;
   }
-  if (SinkBlock == Preheader) {
-    LLVM_DEBUG(
-        dbgs() << "CycleSink: Not sinking, sink block is the preheader\n");
+
+  // Technically speaking inline asm without side effects and no defs can still
+  // be deleted. But there is so much bad inline asm code out there, we should
+  // let them be.
+  if (MI->isInlineAsm())
     return false;
-  }
-  if (SinkBlock->sizeWithoutDebugLargerThan(SinkLoadInstsPerBlockThreshold)) {
-    LLVM_DEBUG(
-        dbgs() << "CycleSink: Not Sinking, block too large to analyse.\n");
+
+  // FIXME: See issue #105950 for why LIFETIME markers are considered dead here.
+  if (MI->isLifetimeMarker())
+    return true;
+
+  // If there are no defs with uses, the instruction might be dead.
+  return MI->wouldBeTriviallyDead();
+}
+
+/// Aggressively sink instructions into cycles. This will aggressively try to
+/// sink all instructions in the top-most preheaders in an attempt to reduce RP.
+/// In particular, it will sink into multiple successor blocks without limits
+/// based on the amount of sinking, or the type of ops being sunk (so long as
+/// they are safe to sink).
+bool MachineSinking::aggressivelySinkIntoCycle(
+    MachineCycle *Cycle, MachineInstr &I,
+    DenseMap<std::pair<MachineInstr *, MachineBasicBlock *>, MachineInstr *>
+        &SunkInstrs) {
+  // TODO: support instructions with multiple defs
+  if (I.getNumDefs() > 1)
     return false;
+
+  LLVM_DEBUG(dbgs() << "AggressiveCycleSink: Finding sink block for: " << I);
+  MachineBasicBlock *Preheader = Cycle->getCyclePreheader();
+  assert(Preheader && "Cycle sink needs a preheader block");
+  SmallVector<std::pair<RegSubRegPair, MachineInstr *>> Uses;
+
+  MachineOperand &DefMO = I.getOperand(0);
+  for (MachineInstr &MI : MRI->use_instructions(DefMO.getReg())) {
+    Uses.push_back({{DefMO.getReg(), DefMO.getSubReg()}, &MI});
   }
 
-  LLVM_DEBUG(dbgs() << "CycleSink: Sinking instruction!\n");
-  SinkBlock->splice(SinkBlock->SkipPHIsAndLabels(SinkBlock->begin()), Preheader,
-                    I);
+  for (std::pair<RegSubRegPair, MachineInstr *> Entry : Uses) {
+    MachineInstr *MI = Entry.second;
+    LLVM_DEBUG(dbgs() << "AggressiveCycleSink:   Analysing use: " << MI);
+    if (MI->isPHI()) {
+      LLVM_DEBUG(
+          dbgs() << "AggressiveCycleSink:   Not attempting to sink for PHI.\n");
+      continue;
+    }
+    // We cannot sink before the prologue
+    if (TII->isBasicBlockPrologue(*MI) || MI->isPosition()) {
----------------
arsenm wrote:

```suggestion
    if (MI->isPosition() || TII->isBasicBlockPrologue(*MI)) {
```

https://github.com/llvm/llvm-project/pull/117247