[llvm] [RISCV] Implement trampolines for rv64 (PR #96309)

[Pengcheng Wang via llvm-commits]

================
@@ -7278,6 +7289,135 @@ SDValue RISCVTargetLowering::emitFlushICache(SelectionDAG &DAG, SDValue InChain,
   return CallResult.second;
 }
 
+SDValue RISCVTargetLowering::lowerINIT_TRAMPOLINE(SDValue Op,
+                                                  SelectionDAG &DAG) const {
+  if (!Subtarget.is64Bit())
+    llvm::report_fatal_error("Trampolines only implemented for RV64");
+
+  // Create an MCCodeEmitter to encode instructions.
+  TargetLoweringObjectFile *TLO = getTargetMachine().getObjFileLowering();
+  assert(TLO);
+  MCContext &MCCtx = TLO->getContext();
+
+  std::unique_ptr<MCCodeEmitter> CodeEmitter(
+      createRISCVMCCodeEmitter(*getTargetMachine().getMCInstrInfo(), MCCtx));
+
+  SDValue Root = Op.getOperand(0);
+  SDValue Trmp = Op.getOperand(1); // trampoline
+  SDLoc dl(Op);
+
+  const Value *TrmpAddr = cast<SrcValueSDNode>(Op.getOperand(4))->getValue();
+
+  // We store in the trampoline buffer the following instructions and data.
+  // Offset:
+  //      0: auipc   t2, 0
+  //      4: ld      t0, 24(t2)
+  //      8: ld      t2, 16(t2)
+  //     12: jalr    t0
+  //     16: <StaticChainOffset>
+  //     24: <FunctionAddressOffset>
+  //     32:
+
+  constexpr unsigned StaticChainOffset = 16;
+  constexpr unsigned FunctionAddressOffset = 24;
+
+  auto GetEncoding = [&](const MCInst &MC) {
+    SmallVector<char, 4> CB;
+    SmallVector<MCFixup> Fixups;
+    const MCSubtargetInfo *STI = getTargetMachine().getMCSubtargetInfo();
+    assert(STI);
+    CodeEmitter->encodeInstruction(MC, CB, Fixups, *STI);
+    assert(CB.size() == 4);
+    assert(Fixups.empty());
+    uint32_t Encoding = support::endian::read32le(CB.data());
+    return Encoding;
+  };
+
+  SDValue OutChains[6];
+  SDValue Addr = Trmp;
+
+  // auipc t2, 0
+  // Loads the current PC into t2.
+  MCInst AUIPC_X7_0_Inst =
+      MCInstBuilder(RISCV::AUIPC).addReg(RISCV::X7).addImm(0);
+  uint32_t AUIPC_X7_0 = GetEncoding(AUIPC_X7_0_Inst);
+  OutChains[0] =
+      DAG.getTruncStore(Root, dl, DAG.getConstant(AUIPC_X7_0, dl, MVT::i64),
+                        Addr, MachinePointerInfo(TrmpAddr), MVT::i32);
+
+  // ld t0, 24(t2)
+  // Loads the function address into t0. Note that we are using offsets
+  // pc-relative to the first instruction of the trampoline.
+  MCInst LD_X5_TargetFunctionOffset_Inst =
+      MCInstBuilder(RISCV::LD).addReg(RISCV::X5).addReg(RISCV::X7).addImm(24);
+  uint32_t LD_X5_TargetFunctionOffset =
+      GetEncoding(LD_X5_TargetFunctionOffset_Inst);
+  Addr = DAG.getNode(ISD::ADD, dl, MVT::i64, Trmp,
+                     DAG.getConstant(4, dl, MVT::i64));
+  OutChains[1] = DAG.getTruncStore(
+      Root, dl, DAG.getConstant(LD_X5_TargetFunctionOffset, dl, MVT::i64), Addr,
+      MachinePointerInfo(TrmpAddr, 4), MVT::i32);
+
+  // ld t2, 16(t2)
+  // Load the value of the static chain.
+  MCInst LD_X7_StaticChainOffset_Inst =
+      MCInstBuilder(RISCV::LD).addReg(RISCV::X7).addReg(RISCV::X7).addImm(16);
----------------
wangpc-pp wrote:

```suggestion
      MCInstBuilder(RISCV::LD).addReg(RISCV::X7).addReg(RISCV::X7).addImm(StaticChainOffset);
```

https://github.com/llvm/llvm-project/pull/96309