[llvm] [RISCV][GISel] Move G_BRJT expansion to legalization (PR #73711)

[Craig Topper via llvm-commits]

================
@@ -317,6 +318,62 @@ bool RISCVLegalizerInfo::legalizeShlAshrLshr(
   return true;
 }
 
+bool RISCVLegalizerInfo::legalizeBRJT(MachineInstr &MI,
+                                      MachineIRBuilder &MIRBuilder) const {
+  MachineRegisterInfo &MRI = *MIRBuilder.getMRI();
+  auto &MF = *MI.getParent()->getParent();
+  const MachineJumpTableInfo *MJTI = MF.getJumpTableInfo();
+  unsigned EntrySize = MJTI->getEntrySize(MF.getDataLayout());
+
+  Register PtrReg = MI.getOperand(0).getReg();
+  LLT PtrTy = MRI.getType(PtrReg);
+  Register IndexReg = MI.getOperand(2).getReg();
+  LLT IndexTy = MRI.getType(IndexReg);
+
+  MachineInstrBuilder Index;
+  if (isPowerOf2_32(EntrySize)) {
+    auto ShiftAmt = MIRBuilder.buildConstant(IndexTy, Log2_32(EntrySize));
+    Index = MIRBuilder.buildShl(IndexTy, IndexReg, ShiftAmt);
+  } else
+    return false;
+
+  auto Addr = MIRBuilder.buildPtrAdd(PtrTy, PtrReg, Index);
+
+  MachineMemOperand *MMO = MF.getMachineMemOperand(
+      MachinePointerInfo::getJumpTable(MF), MachineMemOperand::MOLoad,
+      EntrySize, Align(MJTI->getEntryAlignment(MF.getDataLayout())));
+
+  Register TargetReg;
+  switch (MJTI->getEntryKind()) {
+  default:
+    llvm_unreachable("Unexpected jumptable entry kind");
+  case MachineJumpTableInfo::EK_LabelDifference32: {
+    // For PIC, the sequence is:
+    // BRIND(load(Jumptable + index) + RelocBase)
+    // RelocBase can be JumpTable, GOT or some sort of global base.
+    auto Load = MIRBuilder.buildLoadInstr(
+        TargetOpcode::G_LOAD, LLT::scalar(EntrySize * 8), Addr, *MMO);
+    Load = MIRBuilder.buildSExtOrTrunc(IndexTy, Load);
+    TargetReg = MIRBuilder.buildPtrAdd(PtrTy, PtrReg, Load).getReg(0);
+    break;
+  }
+  case MachineJumpTableInfo::EK_Custom32: {
+    auto Load = MIRBuilder.buildLoadInstr(TargetOpcode::G_SEXTLOAD, IndexTy,
+                                          Addr, *MMO);
+    TargetReg = MIRBuilder.buildIntToPtr(PtrTy, Load).getReg(0);
+    break;
+  }
+  case MachineJumpTableInfo::EK_BlockAddress:
+    TargetReg = MIRBuilder.buildLoad(PtrTy, Addr, *MMO).getReg(0);
+    break;
+  }
+
+  MIRBuilder.buildBrIndirect(TargetReg);
+
+  MI.eraseFromParent();
----------------
topperc wrote:

MIRBuilder tells the observer when a new instruction is created. We need to tell the observer when an instruction is modified in place which doesn't happen here. What were you thinking we need to tell the observer about?

https://github.com/llvm/llvm-project/pull/73711