Canonical Copy Propagation

[plotfi]

Summary

This is a new technique I thought of today, idea is to fold copies of vregs to vregs. Probably will support some cases where there are physregs too.

[lebedev.ri]

1. Please always upload all patches with full context
2. Test?

[plotfi]

Ah yes, thanks. Test and full context included now.

PL

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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐

[bogner]

lib/CodeGen/MIRCanonicalizerPass.cpp

318

Capitalize variables and clang-format, please.

319

for (MachineInstr &MI : MBB->instrs()), maybe?

320–321

Do we need to be careful about register classes here to preserve semantics?

324

Better to write "MachineInstr *" rather than auto here, it's not much longer and the clarity is worth it

339–347

Is the RenameMOs vector necessary? Why not just setReg in the loop over uses? Also I believe we can use range-for with MRI.uses() here to simplify.

test/CodeGen/MIR/AArch64/mirCanonCopyCopyProp.mir

109–111 ↗

(On Diff #141518)

I feel like this test can be made much shorter. Can't we do this with just two or three instructions?

[plotfi]

Justin gave me a LGTM in Person.

lib/CodeGen/MIRCanonicalizerPass.cpp

   return Changed;
 }
 
+bool propagateLocalCopies(MachineBasicBlock *MBB) {
+  bool Changed = false;
+  MachineRegisterInfo &MRI = MBB->getParent()->getRegInfo();
+
+  std::vector<MachineInstr*> copies;
+  for (auto II = MBB->begin(), IE = MBB->end(); II != IE; ++II) {
+    if (II->isCopy())
+      copies.push_back(&*II);
+  }
+
+  for (auto MI : copies) {
+
+    if (!MI->getOperand(0).isReg())
+      continue;
+    if (!MI->getOperand(1).isReg())
+      continue;
+
+    const unsigned Dst = MI->getOperand(0).getReg();
+    const unsigned Src = MI->getOperand(1).getReg();
+
+    if (!TargetRegisterInfo::isVirtualRegister(Dst))
+      continue;
+    if (!TargetRegisterInfo::isVirtualRegister(Src))
+      continue;
+
+    std::vector<MachineOperand *> RenameMOs;
+    for (auto UI = MRI.use_begin(Dst); UI != MRI.use_end(); ++UI) {
+      RenameMOs.push_back(&*UI);
+    }
+
+    for (auto *MO : RenameMOs) {
+      Changed = true;
+      MO->setReg(Src);
+    }
+
+    MI->eraseFromParent();
+  }
+
+  return Changed;
+}
+
 /// Here we find our candidates. What makes an interesting candidate?
 /// An candidate for a canonicalization tree root is normally any kind of
 /// instruction that causes side effects such as a store to memory or a copy to

   bbNames.push_back(MBB->getName());
   DEBUG(dbgs() << "\n\n NEW BASIC BLOCK: " << MBB->getName() << "\n\n";);
 
+  DEBUG(dbgs() << "MBB Before Canonical Copy Propagation:\n"; MBB->dump(););
+  Changed |= propagateLocalCopies(MBB);
+  DEBUG(dbgs() << "MBB After Canonical Copy Propagation:\n"; MBB->dump(););
+
   DEBUG(dbgs() << "MBB Before Scheduling:\n"; MBB->dump(););
   unsigned IdempotentInstCount = 0;
   Changed |= rescheduleCanonically(IdempotentInstCount, MBB);