Create the wasm-bindgen-wasm-conventions crate

This tiny crate provides utilities for working with Wasm codegen
conventions (typically established by LLVM or lld) such as getting the shadow
stack pointer.

It also de-duplicates all the places in the codebase where we were implementing
these conventions in one-off ways.

crates/cli-support/src/lib.rs

@@ -8,6 +8,7 @@ use std::mem;
 use std::path::{Path, PathBuf};
 use std::str;
 use walrus::Module;
+use wasm_bindgen_wasm_conventions as wasm_conventions;
 
 mod anyref;
 mod decode;
@@ -278,18 +279,13 @@ impl Bindgen {
             }
         };
 
-        // Our multi-value xform relies on the presence of the stack pointer, so
-        // temporarily export it so that our many GC's don't remove it before
-        // the xform runs.
-        if self.multi_value {
-            // Assume that the first global is the shadow stack pointer, since that is
-            // what LLVM codegens.
-            match module.globals.iter().next() {
-                Some(g) if g.ty == walrus::ValType::I32 => {
-                    module.exports.add("__shadow_stack_pointer", g.id());
-                }
-                _ => {}
-            }
+        // Our threads and multi-value xforms rely on the presence of the stack
+        // pointer, so temporarily export it so that our many GC's don't remove
+        // it before the xform runs.
+        let mut exported_shadow_stack_pointer = false;
+        if self.multi_value || self.threads.is_enabled() {
+            wasm_conventions::export_shadow_stack_pointer(&mut module)?;
+            exported_shadow_stack_pointer = true;
         }
 
         // This isn't the hardest thing in the world too support but we
@@ -387,6 +383,17 @@ impl Bindgen {
             }
         }
 
+        // If we exported the shadow stack pointer earlier, remove it from the
+        // export set now.
+        if exported_shadow_stack_pointer {
+            wasm_conventions::unexport_shadow_stack_pointer(&mut module)?;
+            // The shadow stack pointer is potentially unused now, but since it
+            // most likely _is_ in use, we don't pay the cost of a full GC here
+            // just to remove one potentially unnecessary global.
+            //
+            // walrus::passes::gc::run(&mut module);
+        }
+
         Ok(Output {
             module,
             stem: stem.to_string(),

crates/cli-support/src/webidl/standard.rs

@@ -53,9 +53,10 @@ use crate::descriptor::VectorKind;
 use crate::webidl::{AuxExportKind, AuxImport, AuxValue, JsImport, JsImportName};
 use crate::webidl::{NonstandardIncoming, NonstandardOutgoing};
 use crate::webidl::{NonstandardWebidlSection, WasmBindgenAux};
-use failure::{bail, format_err, Error, ResultExt};
-use walrus::{GlobalId, MemoryId, Module};
+use failure::{bail, Error, ResultExt};
+use walrus::Module;
 use wasm_bindgen_multi_value_xform as multi_value_xform;
+use wasm_bindgen_wasm_conventions as wasm_conventions;
 use wasm_webidl_bindings::ast;
 
 pub fn add_multi_value(
@@ -79,8 +80,8 @@ pub fn add_multi_value(
         return Ok(());
     }
 
-    let memory = get_memory(module)?;
-    let shadow_stack_pointer = get_shadow_stack_pointer(module)?;
+    let shadow_stack_pointer = wasm_conventions::get_shadow_stack_pointer(module)?;
+    let memory = wasm_conventions::get_memory(module)?;
     multi_value_xform::run(module, memory, shadow_stack_pointer, &to_xform)?;
 
     // Finally, unset `return_via_outptr`, fix up its incoming bindings'
@@ -163,43 +164,6 @@ fn fixup_binding_argument_gets(incoming: &mut [NonstandardIncoming]) -> Result<(
     }
 }
 
-fn get_memory(module: &Module) -> Result<MemoryId, Error> {
-    let mut memories = module.memories.iter().map(|m| m.id());
-    let memory = memories.next();
-    if memories.next().is_some() {
-        bail!(
-            "expected a single memory, found multiple; multiple memories \
-             currently not supported"
-        );
-    }
-    memory.ok_or_else(|| {
-        format_err!(
-            "module does not have a memory; must have a memory \
-             to transform return pointers into Wasm multi-value"
-        )
-    })
-}
-
-// Get the `__shadow_stack_pointer` global that we stashed in an export early on
-// in the pipeline.
-fn get_shadow_stack_pointer(module: &mut Module) -> Result<GlobalId, Error> {
-    let (g, e) = module
-        .exports
-        .iter()
-        .find(|e| e.name == "__shadow_stack_pointer")
-        .map(|e| {
-            let g = match e.item {
-                walrus::ExportItem::Global(g) => g,
-                _ => unreachable!(),
-            };
-            (g, e.id())
-        })
-        .ok_or_else(|| format_err!("module does not have a shadow stack pointer"))?;
-
-    module.exports.delete(e);
-    Ok(g)
-}
-
 pub fn add_section(
     module: &mut Module,
     aux: &WasmBindgenAux,