wasm-emscripten-finalize: Internalize mutable __stack_pointer import (#2213)

I'm working on a change to lld that will cause `-pie` binaries to
import __stack_pointer, just like -shared do already.  Because we
don't yet support mutable globals everywhere this change will
internalize the import and create a new immutable import that is used
to initialize the internal one.

This change is part of the fix for:
https://github.com/emscripten-core/emscripten/issues/8915

1 files changed, 26 insertions, 0 deletions

diff --git a/src/wasm/wasm-emscripten.cpp b/src/wasm/wasm-emscripten.cpp
index 00a3eb6b7..8463354e8 100644
--- a/src/wasm/wasm-emscripten.cpp
+++ b/src/wasm/wasm-emscripten.cpp
@@ -397,6 +397,32 @@ private:
   Global* stackPointer;
 };
 
+// lld can sometimes produce a build with an imported mutable __stack_pointer
+// (i.e.  when linking with -fpie).  This method internalizes the
+// __stack_pointer and initializes it from an immutable global instead.
+// For -shared builds we instead call replaceStackPointerGlobal.
+void EmscriptenGlueGenerator::internalizeStackPointerGlobal() {
+  Global* stackPointer = getStackPointerGlobal();
+  if (!stackPointer || !stackPointer->imported() || !stackPointer->mutable_) {
+    return;
+  }
+
+  Name internalName = stackPointer->name;
+  Name externalName = internalName.c_str() + std::string("_import");
+
+  // Rename the imported global, and make it immutable
+  stackPointer->name = externalName;
+  stackPointer->mutable_ = false;
+  wasm.updateMaps();
+
+  // Create a new global with the old name that is not imported.
+  Builder builder(wasm);
+  auto* init = builder.makeGlobalGet(externalName, stackPointer->type);
+  auto* sp = builder.makeGlobal(
+    internalName, stackPointer->type, init, Builder::Mutable);
+  wasm.addGlobal(sp);
+}
+
 void EmscriptenGlueGenerator::replaceStackPointerGlobal() {
   Global* stackPointer = getStackPointerGlobal();
   if (!stackPointer) {