wasm: Improve dead code elimination to handle indirect calls

compiler/gen_wasm/src/wasm_module/dead_code.rs

@@ -39,16 +39,16 @@ pub struct PreloadsCallGraph<'a> {
 }
 
 impl<'a> PreloadsCallGraph<'a> {
-    pub fn new(arena: &'a Bump, import_fn_count: u32, fn_count: u32) -> Self {
-        let num_preloads = (import_fn_count + fn_count) as usize;
+    pub fn new(arena: &'a Bump, import_fn_count: usize, fn_count: usize) -> Self {
+        let num_preloads = import_fn_count + fn_count;
 
         let mut code_offsets = Vec::with_capacity_in(num_preloads, arena);
         let calls = Vec::with_capacity_in(2 * num_preloads, arena);
         let mut calls_offsets = Vec::with_capacity_in(1 + num_preloads, arena);
 
         // Imported functions have zero code length and no calls
-        code_offsets.extend(std::iter::repeat(0).take(import_fn_count as usize));
-        calls_offsets.extend(std::iter::repeat(0).take(import_fn_count as usize));
+        code_offsets.extend(std::iter::repeat(0).take(import_fn_count));
+        calls_offsets.extend(std::iter::repeat(0).take(import_fn_count));
 
         PreloadsCallGraph {
             num_preloads,
@@ -65,11 +65,18 @@ impl<'a> PreloadsCallGraph<'a> {
 /// use this backend without a linker.
 pub fn parse_preloads_call_graph<'a>(
     arena: &'a Bump,
-    fn_count: u32,
     code_section_body: &[u8],
-    import_fn_count: u32,
+    import_signatures: &[u32],
+    function_signatures: &[u32],
+    indirect_callees: &[u32],
 ) -> PreloadsCallGraph<'a> {
-    let mut call_graph = PreloadsCallGraph::new(arena, import_fn_count, fn_count);
+    let mut call_graph =
+        PreloadsCallGraph::new(arena, import_signatures.len(), function_signatures.len());
+
+    let mut signatures =
+        Vec::with_capacity_in(import_signatures.len() + function_signatures.len(), arena);
+    signatures.extend_from_slice(import_signatures);
+    signatures.extend_from_slice(function_signatures);
 
     // Iterate over the bytes of the Code section
     let mut cursor: usize = 0;
@@ -88,13 +95,23 @@ pub fn parse_preloads_call_graph<'a>(
             cursor += 1; // ValueType
         }
 
-        // Parse `call` instructions and skip over all other instructions
+        // Parse `call` and `call_indirect` instructions, skip over everything else
         while cursor < func_end {
             let opcode_byte: u8 = code_section_body[cursor];
             if opcode_byte == OpCode::CALL as u8 {
                 cursor += 1;
                 let call_index = parse_u32_or_panic(code_section_body, &mut cursor);
                 call_graph.calls.push(call_index as u32);
+            } else if opcode_byte == OpCode::CALLINDIRECT as u8 {
+                cursor += 1;
+                // Insert all indirect callees with a matching type signature
+                let sig = parse_u32_or_panic(code_section_body, &mut cursor);
+                call_graph.calls.extend(
+                    indirect_callees
+                        .iter()
+                        .filter(|f| signatures[**f as usize] == sig),
+                );
+                u32::skip_bytes(code_section_body, &mut cursor); // table_idx
             } else {
                 OpCode::skip_bytes(code_section_body, &mut cursor);
             }

compiler/gen_wasm/src/wasm_module/mod.rs

@@ -133,18 +133,35 @@ impl<'a> WasmModule<'a> {
         let mut types = TypeSection::preload(arena, bytes, &mut cursor);
         types.parse_offsets();
 
-        let import = ImportSection::preload(arena, bytes, &mut cursor);
+        let mut import = ImportSection::preload(arena, bytes, &mut cursor);
+        let import_signatures = import.parse(arena);
+
         let function = FunctionSection::preload(arena, bytes, &mut cursor);
+        let function_signatures = function.parse(arena);
+
         let table = OpaqueSection::preload(SectionId::Table, arena, bytes, &mut cursor);
+
         let memory = MemorySection::preload(arena, bytes, &mut cursor);
+
         let global = GlobalSection::preload(arena, bytes, &mut cursor);
 
         ExportSection::skip_bytes(bytes, &mut cursor);
+
         let export = ExportSection::empty(arena);
 
         let start = OpaqueSection::preload(SectionId::Start, arena, bytes, &mut cursor);
+
         let element = ElementSection::preload(arena, bytes, &mut cursor);
-        let code = CodeSection::preload(arena, bytes, &mut cursor, import.function_count);
+        let indirect_callees = element.indirect_callees(arena);
+
+        let code = CodeSection::preload(
+            arena,
+            bytes,
+            &mut cursor,
+            &import_signatures,
+            &function_signatures,
+            &indirect_callees,
+        );
 
         let data = DataSection::preload(arena, bytes, &mut cursor);
 

compiler/gen_wasm/src/wasm_module/sections.rs

@@ -377,8 +377,8 @@ impl<'a> ImportSection<'a> {
         self.count += 1;
     }
 
-    fn update_function_count(&mut self) {
-        let mut f_count = 0;
+    pub fn parse(&mut self, arena: &'a Bump) -> Vec<'a, u32> {
+        let mut fn_signatures = bumpalo::vec![in arena];
         let mut cursor = 0;
         while cursor < self.bytes.len() {
             String::skip_bytes(&self.bytes, &mut cursor);
@@ -389,8 +389,7 @@ impl<'a> ImportSection<'a> {
 
             match type_id {
                 ImportTypeId::Func => {
-                    f_count += 1;
-                    u32::skip_bytes(&self.bytes, &mut cursor);
+                    fn_signatures.push(parse_u32_or_panic(&self.bytes, &mut cursor));
                 }
                 ImportTypeId::Table => {
                     TableType::skip_bytes(&self.bytes, &mut cursor);
@@ -404,17 +403,16 @@ impl<'a> ImportSection<'a> {
             }
         }
 
-        self.function_count = f_count;
+        self.function_count = fn_signatures.len() as u32;
+        fn_signatures
     }
 
     pub fn from_count_and_bytes(count: u32, bytes: Vec<'a, u8>) -> Self {
-        let mut created = ImportSection {
+        ImportSection {
             bytes,
             count,
             function_count: 0,
-        };
-        created.update_function_count();
-        created
+        }
     }
 }
 
@@ -442,6 +440,16 @@ impl<'a> FunctionSection<'a> {
         self.bytes.encode_u32(sig_id);
         self.count += 1;
     }
+
+    pub fn parse(&self, arena: &'a Bump) -> Vec<'a, u32> {
+        let count = self.count as usize;
+        let mut signatures = Vec::with_capacity_in(count, arena);
+        let mut cursor = 0;
+        for _ in 0..count {
+            signatures.push(parse_u32_or_panic(&self.bytes, &mut cursor));
+        }
+        signatures
+    }
 }
 
 section_impl!(FunctionSection, SectionId::Function);
@@ -887,6 +895,18 @@ impl<'a> ElementSection<'a> {
     pub fn size(&self) -> usize {
         self.segments.iter().map(|seg| seg.size()).sum()
     }
+
+    pub fn indirect_callees(&self, arena: &'a Bump) -> Vec<'a, u32> {
+        let mut result = bumpalo::vec![in arena];
+        for segment in self.segments.iter() {
+            if let ElementSegment::ActiveImplicitTableIndex { fn_indices, .. } = segment {
+                result.extend_from_slice(fn_indices);
+            } else {
+                internal_error!("Unsupported ElementSegment {:?}", self)
+            }
+        }
+        result
+    }
 }
 
 impl<'a> Serialize for ElementSection<'a> {
@@ -940,14 +960,21 @@ impl<'a> CodeSection<'a> {
         arena: &'a Bump,
         module_bytes: &[u8],
         cursor: &mut usize,
-        import_fn_count: u32,
+        import_signatures: &[u32],
+        function_signatures: &[u32],
+        indirect_callees: &[u32],
     ) -> Self {
         let (preloaded_count, initial_bytes) = parse_section(SectionId::Code, module_bytes, cursor);
         let preloaded_bytes = arena.alloc_slice_copy(initial_bytes);
 
         // TODO: Try to move this call_graph preparation to platform build time
-        let dead_code_metadata =
-            parse_preloads_call_graph(arena, preloaded_count, initial_bytes, import_fn_count);
+        let dead_code_metadata = parse_preloads_call_graph(
+            arena,
+            initial_bytes,
+            import_signatures,
+            function_signatures,
+            indirect_callees,
+        );
 
         CodeSection {
             preloaded_count,