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

[Michael Maitland 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();
----------------
michaelmaitland wrote:

I'm not really sure whether the Observer stuff is necessary or a nice to have, but there is an opportunity to do some Observer stuff here. It would be nice if the RISCV GISEL agreed on where and when it must be used.

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