[wasm-split] Support --no-placeholders option in multi-split (WebAssembly#7789)

This supports `--no-placeholders` option with `--multi-split`. Currently
`--multi-split` does not accept `--no-placeholders` option and set it to
true by default internally.

Because `--no-placeholders` is false by default and this has been the
case for the default two-way split, this means this PR is a breaking
change for multi-split users.

https://github.com/WebAssembly/binaryen/blob/8ad26cbaca2a3a8982789b25d9a279dc26e85f15/src/tools/wasm-split/split-options.cpp#L210-L218

But it will be inconsistent if we set the option by default to true in
case of two-way split and false in case of multi-split.

About the test:
- This changes `CHECK-A`, `CHECK-B`, and `CHECK-C` to `MOD1`, `MOD2`,
and `MOD3`
- Because previously `--no-placeholders` was the default, the
expectation lines for `PRIMARY` didn't have any placeholders. Now it
does.
- `***-OPTIONS` check lines now test both options, `--import-namespace`
and `--no-placeholders`.

Author: aheejin

CHANGELOG.md

@@ -15,6 +15,12 @@ full changeset diff at the end of each section.
 Current Trunk
 -------------
 
+ - `wasm-split`'s `--multi-split` mode now supports more options:
+   `--no-placeholders`, `--import-namespace`, `--emit-module-names`, and
+   `--emit-text`. Because `--no-placeholders` is false by default and until now
+   `--multi-split` didn't use placeholders at all, this is a breaking change. If
+   you want to continue to do multi-split without placeholders, you need to
+   explicitly specify `--no-placeholders`.
  - Add a `--string-lifting` pass that raises imported string operations and
    constants into stringref in Binaryen IR (which can then be fully optimized,
    and typically lowered back down with `--string-lowering`).

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

@@ -213,7 +213,7 @@ WasmSplitOptions::WasmSplitOptions()
       "Do not import placeholder functions. Calls to secondary functions will "
       "fail before the secondary module has been instantiated.",
       WasmSplitOption,
-      {Mode::Split},
+      {Mode::Split, Mode::MultiSplit},
       Options::Arguments::Zero,
       [&](Options* o, const std::string& argument) { usePlaceholders = false; })
     .add(

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

@@ -415,7 +415,7 @@ void multiSplitModule(const WasmSplitOptions& options) {
   }
 
   ModuleSplitting::Config config;
-  config.usePlaceholders = false;
+  config.usePlaceholders = options.usePlaceholders;
   config.importNamespace = options.importNamespace;
   config.minimizeNewExportNames = !options.passOptions.debugInfo;
   if (options.emitModuleNames && !wasm.name) {

test/lit/help/wasm-split.test

@@ -67,10 +67,10 @@
 ;; CHECK-NEXT:   --symbolmap                          [split] Write a symbol map file for each
 ;; CHECK-NEXT:                                        of the output modules.
 ;; CHECK-NEXT:
-;; CHECK-NEXT:   --no-placeholders                    [split] Do not import placeholder
-;; CHECK-NEXT:                                        functions. Calls to secondary functions
-;; CHECK-NEXT:                                        will fail before the secondary module has
-;; CHECK-NEXT:                                        been instantiated.
+;; CHECK-NEXT:   --no-placeholders                    [split, multi-split] Do not import
+;; CHECK-NEXT:                                        placeholder functions. Calls to secondary
+;; CHECK-NEXT:                                        functions will fail before the secondary
+;; CHECK-NEXT:                                        module has been instantiated.
 ;; CHECK-NEXT:
 ;; CHECK-NEXT:   --placeholdermap                     [split] Write a file mapping placeholder
 ;; CHECK-NEXT:                                        indices to the function names.