[WebAssembly] Initial WebAssembly backend

This WebAssembly backend is just a skeleton at this time and is not yet
functional.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241022 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/WebAssembly/WebAssemblyISelDAGToDAG.cpp

@@ -0,0 +1,73 @@
+//- WebAssemblyISelDAGToDAG.cpp - A dag to dag inst selector for WebAssembly -//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+///
+/// \file
+/// \brief This file defines an instruction selector for the WebAssembly target.
+///
+//===----------------------------------------------------------------------===//
+
+#include "WebAssembly.h"
+#include "MCTargetDesc/WebAssemblyMCTargetDesc.h"
+#include "WebAssemblyTargetMachine.h"
+#include "llvm/CodeGen/SelectionDAGISel.h"
+#include "llvm/IR/Function.h" // To access function attributes.
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/MathExtras.h"
+#include "llvm/Support/raw_ostream.h"
+using namespace llvm;
+
+#define DEBUG_TYPE "wasm-isel"
+
+//===--------------------------------------------------------------------===//
+/// WebAssembly-specific code to select WebAssembly machine instructions for
+/// SelectionDAG operations.
+///
+namespace {
+class WebAssemblyDAGToDAGISel final : public SelectionDAGISel {
+  /// Keep a pointer to the WebAssemblySubtarget around so that we can make the
+  /// right decision when generating code for different targets.
+  const WebAssemblySubtarget *Subtarget;
+
+  bool ForCodeSize;
+
+public:
+  WebAssemblyDAGToDAGISel(WebAssemblyTargetMachine &tm,
+                          CodeGenOpt::Level OptLevel)
+      : SelectionDAGISel(tm, OptLevel), Subtarget(nullptr), ForCodeSize(false) {
+  }
+
+  const char *getPassName() const override {
+    return "WebAssembly Instruction Selection";
+  }
+
+  bool runOnMachineFunction(MachineFunction &MF) override {
+    ForCodeSize =
+        MF.getFunction()->hasFnAttribute(Attribute::OptimizeForSize) ||
+        MF.getFunction()->hasFnAttribute(Attribute::MinSize);
+    Subtarget = &MF.getSubtarget<WebAssemblySubtarget>();
+    return SelectionDAGISel::runOnMachineFunction(MF);
+  }
+
+  SDNode *Select(SDNode *Node) override;
+
+private:
+  // add select functions here...
+};
+} // end anonymous namespace
+
+SDNode *WebAssemblyDAGToDAGISel::Select(SDNode *Node) {
+  llvm_unreachable("TODO: implement Select");
+}
+
+/// This pass converts a legalized DAG into a WebAssembly-specific DAG, ready
+/// for instruction scheduling.
+FunctionPass *llvm::createWebAssemblyISelDag(WebAssemblyTargetMachine &TM,
+                                             CodeGenOpt::Level OptLevel) {
+  return new WebAssemblyDAGToDAGISel(TM, OptLevel);
+}