[wasm-split] Do not split out the start function

src/tools/wasm-split/wasm-split.cpp

@@ -242,6 +242,33 @@ void setCommonSplitConfigs(ModuleSplitting::Config& config,
   }
 }
 
+// Returns whether it is valid to split a function out from the main module.
+bool canSplitFunc(Function* func,
+                  const Module& wasm,
+                  const WasmSplitOptions& options) {
+  if (!func) {
+    if (!options.quiet) {
+      std::cerr << "warning: function " << func->name << " does not exist\n";
+    }
+    return false;
+  }
+  if (func->imported()) {
+    if (!options.quiet) {
+      std::cerr << "warning: cannot split out imported function " << func->name
+                << "\n";
+    }
+    return false;
+  }
+  if (func->name == wasm.start) {
+    if (!options.quiet) {
+      std::cerr << "warning: cannot split out start function " << func->name
+                << "\n";
+    }
+    return false;
+  }
+  return true;
+}
+
 void splitModule(const WasmSplitOptions& options) {
   Module wasm;
   parseInput(wasm, options);
@@ -283,17 +310,7 @@ void splitModule(const WasmSplitOptions& options) {
   // Use the explicitly provided `splitFuncs`.
   for (auto& func : options.splitFuncs) {
     auto* function = wasm.getFunctionOrNull(func);
-    if (!function) {
-      if (!options.quiet) {
-        std::cerr << "warning: function " << func << " does not exist\n";
-      }
-      continue;
-    }
-    if (function->imported()) {
-      if (!options.quiet) {
-        std::cerr << "warning: cannot split out imported function " << func
-                  << "\n";
-      }
+    if (!canSplitFunc(function, wasm, options)) {
       continue;
     }
     if (!options.quiet && options.keepFuncs.contains(func)) {
@@ -435,16 +452,16 @@ void multiSplitModule(const WasmSplitOptions& options) {
       continue;
     }
     assert(currFuncs);
+    if (!canSplitFunc(wasm.getFunctionOrNull(name), wasm, options)) {
+      continue;
+    }
     currFuncs->insert(name);
     auto [it, inserted] = funcModules.insert({name, currModule});
     if (!inserted && it->second != currModule) {
       Fatal() << "Function " << name << "cannot be assigned to module "
               << currModule << "; it is already assigned to module "
               << it->second << '\n';
     }
-    if (inserted && !options.quiet && !wasm.getFunctionOrNull(name)) {
-      std::cerr << "warning: Function " << name << " does not exist\n";
-    }
   }
 
   if (options.emitModuleNames && !wasm.name) {

test/lit/wasm-split/multi-split-start.wast

@@ -0,0 +1,86 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: wasm-split -all -g --multi-split %s --manifest %s.manifest --out-prefix=%t -o %t.wasm
+;; RUN: wasm-dis %t.wasm | filecheck %s --check-prefix=PRIMARY
+;; RUN: wasm-dis %t1.wasm | filecheck %s --check-prefix=MOD1
+;; RUN: wasm-dis %t2.wasm | filecheck %s --check-prefix=MOD2
+;; RUN: wasm-dis %t3.wasm | filecheck %s --check-prefix=MOD3
+
+;; The start function, $C, cannot be split it, and will remain in the primary.
+
+(module
+ ;; PRIMARY:      (type $0 (func))
+
+ ;; PRIMARY:      (import "placeholder.2" "0" (func $placeholder_0))
+
+ ;; PRIMARY:      (table $0 1 funcref)
+
+ ;; PRIMARY:      (elem $0 (i32.const 0) $placeholder_0)
+
+ ;; PRIMARY:      (export "table" (table $0))
+
+ ;; PRIMARY:      (export "C" (func $C))
+
+ ;; PRIMARY:      (start $C)
+ (start $C)
+
+ ;; MOD1:      (type $0 (func))
+
+ ;; MOD1:      (import "primary" "table" (table $timport$0 1 funcref))
+
+ ;; MOD1:      (import "primary" "C" (func $C (exact)))
+
+ ;; MOD1:      (func $A
+ ;; MOD1-NEXT:  (call $A)
+ ;; MOD1-NEXT:  (call_indirect (type $0)
+ ;; MOD1-NEXT:   (i32.const 0)
+ ;; MOD1-NEXT:  )
+ ;; MOD1-NEXT:  (call $C)
+ ;; MOD1-NEXT: )
+ (func $A
+  (call $A)
+  (call $B)
+  (call $C)
+ )
+
+ ;; MOD2:      (type $0 (func))
+
+ ;; MOD2:      (import "primary" "table" (table $timport$0 1 funcref))
+
+ ;; MOD2:      (elem $0 (i32.const 0) $B)
+
+ ;; MOD2:      (func $B
+ ;; MOD2-NEXT:  (drop
+ ;; MOD2-NEXT:   (i32.const 42)
+ ;; MOD2-NEXT:  )
+ ;; MOD2-NEXT: )
+ (func $B
+  (drop
+   (i32.const 42)
+  )
+ )
+
+ ;; PRIMARY:      (func $C
+ ;; PRIMARY-NEXT:  (drop
+ ;; PRIMARY-NEXT:   (i32.const 1337)
+ ;; PRIMARY-NEXT:  )
+ ;; PRIMARY-NEXT: )
+ (func $C
+  (drop
+   (i32.const 1337)
+  )
+ )
+
+ ;; MOD3:      (type $0 (func))
+
+ ;; MOD3:      (func $D
+ ;; MOD3-NEXT:  (drop
+ ;; MOD3-NEXT:   (i32.const 999999)
+ ;; MOD3-NEXT:  )
+ ;; MOD3-NEXT: )
+ (func $D
+  (drop
+   (i32.const 999999)
+  )
+ )
+)

test/lit/wasm-split/multi-split-start.wast.manifest

@@ -0,0 +1,9 @@
+1:
+A
+
+2:
+B
+
+3:
+C
+D

test/lit/wasm-split/start.wast

@@ -0,0 +1,28 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
+
+;; RUN: wasm-split %s --split-funcs=start_func,other -o1 %t.1.wasm -o2 %t.2.wasm -g 2>&1
+;; RUN: wasm-dis -all %t.1.wasm | filecheck %s --check-prefix PRIMARY
+;; RUN: wasm-dis -all %t.2.wasm | filecheck %s --check-prefix SECONDARY
+
+;; Do not error on trying to split out the start function. It cannot be
+;; split out, keep it in the primary module.
+
+(module
+  ;; PRIMARY:      (type $0 (func))
+
+  ;; PRIMARY:      (start $start_func)
+
+  ;; PRIMARY:      (func $start_func (type $0)
+  ;; PRIMARY-NEXT: )
+  (func $start_func)
+
+  (start $start_func)
+
+  ;; SECONDARY:      (type $0 (func))
+
+  ;; SECONDARY:      (func $other (type $0)
+  ;; SECONDARY-NEXT: )
+  (func $other)
+)
+
+