Import memory instead of defining/exporting it when using emscripten glue (#777)

The emscripten JS module code creates the memory using the native wasm
APIs, and imports that into the wasm module.

3 files changed, 33 insertions, 18 deletions

diff --git a/src/tools/s2wasm.cpp b/src/tools/s2wasm.cpp
index 8c36a0d58..ef5d276d3 100644
--- a/src/tools/s2wasm.cpp
+++ b/src/tools/s2wasm.cpp
@@ -132,7 +132,8 @@ int main(int argc, const char *argv[]) {
   if (options.debug) std::cerr << "Global base " << globalBase << '\n';
 
   Linker linker(globalBase, stackAllocation, initialMem, maxMem,
-                ignoreUnknownSymbols, startFunction, options.debug);
+                generateEmscriptenGlue, ignoreUnknownSymbols, startFunction,
+                options.debug);
 
   S2WasmBuilder mainbuilder(input.c_str(), options.debug);
   linker.linkObject(mainbuilder);
diff --git a/src/wasm-linker.cpp b/src/wasm-linker.cpp
index 014dfe6a9..56ec03851 100644
--- a/src/wasm-linker.cpp
+++ b/src/wasm-linker.cpp
@@ -107,14 +107,25 @@ void Linker::layout() {
   } else {
     out.wasm.memory.initial = initialMem / Memory::kPageSize;
   }
+  out.wasm.memory.exists = true;
 
   if (userMaxMemory) out.wasm.memory.max = userMaxMemory / Memory::kPageSize;
 
-  auto memoryExport = make_unique<Export>();
-  memoryExport->name = MEMORY;
-  memoryExport->value = Name::fromInt(0);
-  memoryExport->kind = ExternalKind::Memory;
-  out.wasm.addExport(memoryExport.release());
+  if (importMemory) {
+    auto memoryImport = make_unique<Import>();
+    memoryImport->name = MEMORY;
+    memoryImport->module = ENV;
+    memoryImport->base = MEMORY;
+    memoryImport->kind = ExternalKind::Memory;
+    out.wasm.memory.imported = true;
+    out.wasm.addImport(memoryImport.release());
+  } else {
+    auto memoryExport = make_unique<Export>();
+    memoryExport->name = MEMORY;
+    memoryExport->value = Name::fromInt(0);
+    memoryExport->kind = ExternalKind::Memory;
+    out.wasm.addExport(memoryExport.release());
+  }
 
   // XXX For now, export all functions marked .globl.
   for (Name name : out.globls) exportFunction(name, false);
diff --git a/src/wasm-linker.h b/src/wasm-linker.h
index 4a0e1e0b9..3660b3038 100644
--- a/src/wasm-linker.h
+++ b/src/wasm-linker.h
@@ -193,18 +193,18 @@ class LinkerObject {
 // applying the relocations, resulting in an executable wasm module.
 class Linker {
  public:
-  Linker(Address globalBase, Address stackAllocation,
-         Address userInitialMemory, Address userMaxMemory,
-         bool ignoreUnknownSymbols, Name startFunction,
-         bool debug) :
-      ignoreUnknownSymbols(ignoreUnknownSymbols),
-      startFunction(startFunction),
-      globalBase(globalBase),
-      nextStatic(globalBase),
-      userInitialMemory(userInitialMemory),
-      userMaxMemory(userMaxMemory),
-      stackAllocation(stackAllocation),
-      debug(debug) {
+  Linker(Address globalBase, Address stackAllocation, Address userInitialMemory,
+         Address userMaxMemory, bool importMemory, bool ignoreUnknownSymbols,
+         Name startFunction, bool debug)
+      : ignoreUnknownSymbols(ignoreUnknownSymbols),
+        startFunction(startFunction),
+        globalBase(globalBase),
+        nextStatic(globalBase),
+        userInitialMemory(userInitialMemory),
+        userMaxMemory(userMaxMemory),
+        importMemory(importMemory),
+        stackAllocation(stackAllocation),
+        debug(debug) {
     if (userMaxMemory && userMaxMemory < userInitialMemory) {
       Fatal() << "Specified max memory " << userMaxMemory <<
           " is < specified initial memory " << userInitialMemory;
@@ -217,6 +217,7 @@ class Linker {
       Fatal() << "Specified initial memory " << userInitialMemory <<
           " is not a multiple of 64k";
     }
+
     // Don't allow anything to be allocated at address 0
     if (globalBase == 0) nextStatic = 1;
 
@@ -312,6 +313,8 @@ class Linker {
   Address userInitialMemory; // Initial memory size (in bytes) specified by the user.
   Address userMaxMemory; // Max memory size (in bytes) specified by the user.
   //(after linking, this is rounded and set on the wasm object in pages)
+  bool importMemory;  // Whether the memory should be imported instead of
+                      // defined.
   Address stackAllocation;
   bool debug;