[llvm] [MachineSink] Add option for aggressive loop sinking (PR #117247)

[Matt Arsenault via llvm-commits]

================
@@ -1574,6 +1615,149 @@ bool MachineSinking::hasStoreBetween(MachineBasicBlock *From,
   return HasAliasedStore;
 }
 
+/// Copy paste from DeadMachineInstructionElimImpl
+
+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.
+  for (const MachineOperand &MO : MI->all_defs()) {
+    Register Reg = MO.getReg();
+    if (Reg.isPhysical()) {
+      return false;
+    } else {
+      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;
+      }
+      for (const MachineInstr &Use : MRI->use_nodbg_instructions(Reg)) {
+        if (&Use != MI)
+          // This def has a non-debug use. Don't delete the instruction!
+          return false;
+      }
+    }
+  }
+
+  // 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;
+
+  // 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<MachineInstr *,
+             std::list<std::pair<MachineBasicBlock *, MachineInstr *>>>
+        SunkInstrs) {
+  LLVM_DEBUG(dbgs() << "AggressiveCycleSink: Finding sink block for: " << I);
+  MachineBasicBlock *Preheader = Cycle->getCyclePreheader();
+  assert(Preheader && "Cycle sink needs a preheader block");
+  SmallVector<std::pair<MachineOperand, MachineInstr *>> Uses;
+  // TODO: support instructions with multiple defs
+  if (I.getNumDefs() > 1)
+    return false;
+
+  MachineOperand DefMO = I.getOperand(0);
+  for (MachineInstr &MI : MRI->use_instructions(DefMO.getReg())) {
+    Uses.push_back({DefMO, &MI});
+  }
+
+  for (std::pair<MachineOperand, 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()) {
+      LLVM_DEBUG(dbgs() << "AggressiveCycleSink:   Use is BasicBlock prologue, "
+                           "can't sink.\n");
+      continue;
+    }
+    if (!Cycle->contains(MI->getParent())) {
+      LLVM_DEBUG(
+          dbgs() << "AggressiveCycleSink:   Use not in cycle, can't sink.\n");
+      continue;
+    }
+
+    MachineBasicBlock *SinkBlock = MI->getParent();
+    MachineInstr *NewMI = nullptr;
+
+    // Check for the case in which we have already sunk a copy of this
+    // instruction into the user block.
+    if (SunkInstrs.contains(&I)) {
+      auto SunkBlocks = SunkInstrs[&I];
----------------
arsenm wrote:

Combine these into one map find 

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