wasm: Use Export section to initialise Name section

Works even if the preloaded Name section is missing.

compiler/gen_wasm/src/backend.rs

@@ -22,12 +22,11 @@ use crate::storage::{Storage, StoredValue, StoredValueKind};
 use crate::wasm_module::linking::{DataSymbol, LinkingSegment, WasmObjectSymbol};
 use crate::wasm_module::sections::{DataMode, DataSegment};
 use crate::wasm_module::{
-    code_builder, CodeBuilder, Export, ExportType, LocalId, Signature, SymInfo, ValueType,
-    WasmModule,
+    code_builder, CodeBuilder, ExportType, LocalId, Signature, SymInfo, ValueType, WasmModule,
 };
 use crate::{
-    copy_memory, round_up_to_alignment, CopyMemoryConfig, Env, DEBUG_LOG_SETTINGS, MEMORY_NAME,
-    PTR_SIZE, PTR_TYPE, STACK_POINTER_GLOBAL_ID, STACK_POINTER_NAME, TARGET_INFO,
+    copy_memory, round_up_to_alignment, CopyMemoryConfig, Env, DEBUG_LOG_SETTINGS, PTR_SIZE,
+    PTR_TYPE, TARGET_INFO,
 };
 
 pub struct WasmBackend<'a> {
@@ -62,22 +61,32 @@ impl<'a> WasmBackend<'a> {
         fn_index_offset: u32,
         helper_proc_gen: CodeGenHelp<'a>,
     ) -> Self {
-        module.export.append(Export {
-            name: MEMORY_NAME.as_bytes(),
-            ty: ExportType::Mem,
-            index: 0,
-        });
-        module.export.append(Export {
-            name: STACK_POINTER_NAME.as_bytes(),
-            ty: ExportType::Global,
-            index: STACK_POINTER_GLOBAL_ID,
-        });
+        // The preloaded builtins object file exports all functions, but the final app binary doesn't.
+        // Remove the function exports and use them to populate the Name section (debug info)
+        let platform_and_builtins_exports =
+            std::mem::replace(&mut module.export.exports, bumpalo::vec![in env.arena]);
+        let mut app_exports = Vec::with_capacity_in(32, env.arena);
+        for ex in platform_and_builtins_exports.into_iter() {
+            match ex.ty {
+                ExportType::Func => module.names.append_function(ex.index, ex.name),
+                _ => app_exports.push(ex),
+            }
+        }
 
         // The preloaded binary has a global to tell us where its data section ends
         // Note: We need this to account for zero data (.bss), which doesn't have an explicit DataSegment!
-        let data_end_idx = module.export.globals_lookup["__data_end".as_bytes()];
+        let data_end_name = "__data_end".as_bytes();
+        let data_end_idx = app_exports
+            .iter()
+            .find(|ex| ex.name == data_end_name)
+            .map(|ex| ex.index)
+            .unwrap_or_else(|| {
+                internal_error!("Preloaded Wasm binary must export global constant `__data_end`")
+            });
         let next_constant_addr = module.global.parse_u32_at_index(data_end_idx);
 
+        module.export.exports = app_exports;
+
         WasmBackend {
             env,
             interns,

compiler/gen_wasm/src/wasm_module/mod.rs

@@ -5,7 +5,7 @@ pub mod opcodes;
 pub mod sections;
 pub mod serialize;
 
-use bumpalo::Bump;
+use bumpalo::{collections::Vec, Bump};
 pub use code_builder::{Align, CodeBuilder, LocalId, ValueType, VmSymbolState};
 pub use linking::SymInfo;
 use roc_error_macros::internal_error;
@@ -144,7 +144,7 @@ impl<'a> WasmModule<'a> {
 
         let global = GlobalSection::preload(arena, bytes, &mut cursor);
 
-        let export = ExportSection::preload_globals(arena, bytes, &mut cursor);
+        let export = ExportSection::preload(arena, bytes, &mut cursor);
 
         let start = OpaqueSection::preload(SectionId::Start, arena, bytes, &mut cursor);
 
@@ -190,10 +190,18 @@ impl<'a> WasmModule<'a> {
         arena: &'a Bump,
         called_preload_fns: T,
     ) {
+        let exported_fn_iter = self
+            .export
+            .exports
+            .iter()
+            .filter(|ex| ex.ty == ExportType::Func)
+            .map(|ex| ex.index);
+        let function_indices = Vec::from_iter_in(exported_fn_iter, arena);
+
         self.code.remove_dead_preloads(
             arena,
             self.import.function_count,
-            &self.export.function_indices,
+            &function_indices,
             called_preload_fns,
         )
     }

compiler/gen_wasm/src/wasm_module/sections.rs

@@ -716,56 +716,35 @@ impl Serialize for Export<'_> {
 
 #[derive(Debug)]
 pub struct ExportSection<'a> {
-    pub count: u32,
-    pub bytes: Vec<'a, u8>,
-    /// List of exported functions to keep during dead-code-elimination
-    pub function_indices: Vec<'a, u32>,
-    /// name -> index
-    pub globals_lookup: MutMap<&'a [u8], u32>,
+    pub exports: Vec<'a, Export<'a>>,
 }
 
 impl<'a> ExportSection<'a> {
     const ID: SectionId = SectionId::Export;
 
-    pub fn append(&mut self, export: Export) {
-        export.serialize(&mut self.bytes);
-        self.count += 1;
-        if matches!(export.ty, ExportType::Func) {
-            self.function_indices.push(export.index);
-        }
+    pub fn append(&mut self, export: Export<'a>) {
+        self.exports.push(export);
     }
 
     pub fn size(&self) -> usize {
-        section_size(&self.bytes)
+        self.exports
+            .iter()
+            .map(|ex| ex.name.len() + 1 + MAX_SIZE_ENCODED_U32)
+            .sum()
     }
 
-    fn empty(arena: &'a Bump) -> Self {
-        ExportSection {
-            count: 0,
-            bytes: Vec::with_capacity_in(256, arena),
-            function_indices: Vec::with_capacity_in(4, arena),
-            globals_lookup: MutMap::default(),
-        }
-    }
-
-    /// Preload from object file. Keep only the Global exports, ignore the rest.
-    pub fn preload_globals(arena: &'a Bump, module_bytes: &[u8], cursor: &mut usize) -> Self {
+    /// Preload from object file.
+    pub fn preload(arena: &'a Bump, module_bytes: &[u8], cursor: &mut usize) -> Self {
         let (num_exports, body_bytes) = parse_section(Self::ID, module_bytes, cursor);
 
-        let mut export_section = ExportSection::empty(arena);
+        let mut export_section = ExportSection {
+            exports: Vec::with_capacity_in(num_exports as usize, arena),
+        };
 
         let mut body_cursor = 0;
-        for _ in 0..num_exports {
-            let export_start = body_cursor;
+        while body_cursor < body_bytes.len() {
             let export = Export::parse(arena, body_bytes, &mut body_cursor);
-            if matches!(export.ty, ExportType::Global) {
-                let global_bytes = &body_bytes[export_start..body_cursor];
-                export_section.bytes.extend_from_slice(global_bytes);
-                export_section.count += 1;
-                export_section
-                    .globals_lookup
-                    .insert(export.name, export.index);
-            }
+            export_section.exports.push(export);
         }
 
         export_section
@@ -774,10 +753,9 @@ impl<'a> ExportSection<'a> {
 
 impl<'a> Serialize for ExportSection<'a> {
     fn serialize<T: SerialBuffer>(&self, buffer: &mut T) {
-        if !self.bytes.is_empty() {
+        if !self.exports.is_empty() {
             let header_indices = write_section_header(buffer, Self::ID);
-            buffer.encode_u32(self.count);
-            buffer.append_slice(&self.bytes);
+            self.exports.serialize(buffer);
             update_section_size(buffer, header_indices);
         }
     }
@@ -1155,8 +1133,12 @@ impl<'a> NameSection<'a> {
     }
 
     pub fn parse(arena: &'a Bump, module_bytes: &[u8], cursor: &mut usize) -> Self {
+        // If we're already past the end of the preloaded file then there is no Name section
         if *cursor >= module_bytes.len() {
-            internal_error!("NameSection not found in preloaded object file");
+            return NameSection {
+                bytes: bumpalo::vec![in arena],
+                functions: MutMap::default(),
+            };
         }
 
         // Custom section ID