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

[Jessica Clarke via llvm-commits]

Roger Ferrer =?utf-8?q?Ibáñez?= <rofirrim at gmail.com>,Roger Ferrer
 Ibanez <roger.ferrer at bsc.es>,Roger Ferrer Ibanez <roger.ferrer at bsc.es>,Roger
 Ferrer Ibanez <roger.ferrer at bsc.es>
Message-ID:
In-Reply-To: <llvm.org/llvm/llvm-project/pull/96309 at github.com>


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@@ -7279,6 +7290,122 @@ 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();
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jrtc27 wrote:

Hoist this and the line below outside the lambda rather than recomputing each time?

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