Add stack-pointer argument to post-emscripten pass. (#2823)

This allows emscripten to statically set the initial value of the
stack pointer.

Should allow use to avoid doing it dynamically at startup:
emscripten-core/emscripten#11031

Author: sbc100

src/passes/PostEmscripten.cpp

@@ -28,8 +28,11 @@
 #include <pass.h>
 #include <shared-constants.h>
 #include <wasm-builder.h>
+#include <wasm-emscripten.h>
 #include <wasm.h>
 
+#define DEBUG_TYPE "post-emscripten"
+
 namespace wasm {
 
 namespace {
@@ -75,6 +78,24 @@ struct OptimizeCalls : public WalkerPass<PostWalker<OptimizeCalls>> {
 
 struct PostEmscripten : public Pass {
   void run(PassRunner* runner, Module* module) override {
+    // Apply the stack pointer, if it was provided.  This is needed here
+    // because the emscripten JS compiler can add static data allocations that
+    // come before the stack.
+    auto stackPtrStr =
+      runner->options.getArgumentOrDefault("stack-pointer", "");
+    if (stackPtrStr != "") {
+      Global* stackPointer = getStackPointerGlobal(*module);
+      BYN_TRACE("stack_pointer: " << stackPtrStr << "\n");
+      if (stackPointer && !stackPointer->imported()) {
+        auto stackPtr = std::stoi(stackPtrStr);
+        auto oldValue = stackPointer->init->cast<Const>()->value;
+        BYN_TRACE("updating __stack_pointer: " << oldValue.geti32() << " -> "
+                                               << stackPtr << "\n");
+        stackPointer->init =
+          Builder(*module).makeConst(Literal(int32_t(stackPtr)));
+      }
+    }
+
     // Apply the sbrk ptr, if it was provided.
     auto sbrkPtrStr =
       runner->options.getArgumentOrDefault("emscripten-sbrk-ptr", "");

src/wasm-emscripten.h

@@ -23,6 +23,8 @@
 
 namespace wasm {
 
+Global* getStackPointerGlobal(Module& wasm);
+
 // Class which modifies a wasm module for use with emscripten. Generates
 // runtime functions and emits metadata.
 class EmscriptenGlueGenerator {
@@ -78,7 +80,6 @@ class EmscriptenGlueGenerator {
   // so far.
   std::unordered_set<Signature> sigs;
 
-  Global* getStackPointerGlobal();
   Expression* generateLoadStackPointer();
   Expression* generateStoreStackPointer(Function* func, Expression* value);
   void generateDynCallThunk(Signature sig);

src/wasm/wasm-emscripten.cpp

@@ -65,7 +65,7 @@ bool isExported(Module& wasm, Name name) {
   return false;
 }
 
-Global* EmscriptenGlueGenerator::getStackPointerGlobal() {
+Global* getStackPointerGlobal(Module& wasm) {
   // Assumption: The stack pointer is either imported as __stack_pointer or
   // its the first non-imported and non-exported global.
   // TODO(sbc): Find a better way to discover the stack pointer.  Perhaps the
@@ -92,7 +92,7 @@ Expression* EmscriptenGlueGenerator::generateLoadStackPointer() {
       /* ptr    =*/builder.makeConst(Literal(0)),
       /* type   =*/Type::i32);
   }
-  Global* stackPointer = getStackPointerGlobal();
+  Global* stackPointer = getStackPointerGlobal(wasm);
   if (!stackPointer) {
     Fatal() << "stack pointer global not found";
   }
@@ -142,7 +142,7 @@ EmscriptenGlueGenerator::generateStoreStackPointer(Function* func,
       /* value  =*/value,
       /* type   =*/Type::i32);
   }
-  Global* stackPointer = getStackPointerGlobal();
+  Global* stackPointer = getStackPointerGlobal(wasm);
   if (!stackPointer) {
     Fatal() << "stack pointer global not found";
   }
@@ -530,7 +530,7 @@ struct RemoveStackPointer : public PostWalker<RemoveStackPointer> {
 // __stack_pointer and initializes it from an immutable global instead.
 // For -shared builds we instead call replaceStackPointerGlobal.
 void EmscriptenGlueGenerator::internalizeStackPointerGlobal() {
-  Global* stackPointer = getStackPointerGlobal();
+  Global* stackPointer = getStackPointerGlobal(wasm);
   if (!stackPointer || !stackPointer->imported() || !stackPointer->mutable_) {
     return;
   }
@@ -552,7 +552,7 @@ void EmscriptenGlueGenerator::internalizeStackPointerGlobal() {
 }
 
 void EmscriptenGlueGenerator::replaceStackPointerGlobal() {
-  Global* stackPointer = getStackPointerGlobal();
+  Global* stackPointer = getStackPointerGlobal(wasm);
   if (!stackPointer) {
     return;
   }
@@ -606,7 +606,7 @@ struct StackLimitEnforcer : public WalkerPass<PostWalker<StackLimitEnforcer>> {
 };
 
 void EmscriptenGlueGenerator::enforceStackLimit() {
-  Global* stackPointer = getStackPointerGlobal();
+  Global* stackPointer = getStackPointerGlobal(wasm);
   if (!stackPointer) {
     return;
   }

test/passes/[email protected]

@@ -0,0 +1,4 @@
+(module
+ (import "env" "not_stack" (global $import$global0 i32))
+ (global $should_be_stack_pointer (mut i32) (i32.const 1234))
+)

test/passes/[email protected]

@@ -0,0 +1,4 @@
+(module
+  (import "env" "not_stack" (global i32))
+  (global $should_be_stack_pointer (mut i32) (i32.const 2))
+)