[wasm-split] Pre-create trampolines for global initializers by aheejin · Pull Request #8542 · WebAssembly/binaryen

aheejin · 2026-03-29T11:13:27Z

Before #8443, we scanned ref.funcs in global initizliers early in indirectReferencesToSecondaryFunctions and created trampolines for them and replaced ref.func $funcs with ref.func $trampoline_func if func was set to move to a secondary module. But in case the global containing ref.func $trampoline_func also ends up moving to the same secondary module, creating trampoline and using it was not necessary, because the global can simply use ref.func $func because func is in the same secondary module.

To fix this, in #8443, we postponed creating trampolines for ref.funcs in global initializers until shareImportableItems. This had a problem, because we end up creating new trampolines late in shareImportableItems. But trampolines were designed to go through indirectCallsToSecondaryFunctions and setupTablePatching, so those late trampolines were invalid, like

(func $trampoline_foo
  (call $foo)
)

when foo was in a secondary module. This was supposed to be converted to a call_indirect in indirectCallsToSecondaryFunctions and the table elements were supposed to set up in setupTablePatching.

To fix the issue, this PR pre-creates trampolines for ref.funcs in all global initializers in indirectReferencesToSecondaryFunctions, before indirectCallsToSecondaryFunctions and setupTablePatching, but doesn't make ref.funcs reference them just yet. We make ref.funcs in global initializers reference the trampolines only when it is decided they are not moving to the same secondary module, in shareImportableItems.

But this can leave unused trampolines in the primary module. So in cleanupUnusedTrampolines, we remove unused trampolines.

This increases acx_gallery code size only by 0.6%. If we didn't clean up the unnecessary trampolines, the increase was 3.6%. This only removes unused trampoline functions and not placeholder imports and elements yet, but they don't affect acx_gallery because it uses --no-placeholders. The running time hasn't meaningfully changed.

Fixes #8510. (Both tests fail for the same reason)

Before WebAssembly#8443, we scanned `ref.func`s in global initizliers early in `indirectReferencesToSecondaryFunctions` and created trampolines for them and replaced `ref.func $func`s with `ref.func $trampoline_func` if `func` was set to move to a secondary module. But in case the global containing `ref.func $trampoline_func` also ends up moving to the same secondary module, creating trampoline and using it was not necessary, because the global can simply use `ref.func $func` because `func` is in the same secondary module. To fix this, in WebAssembly#8443, we postponed creating trampolines for `ref.func`s in global initializers until `shareImportableItems`. This had a problem, because we end up creating new trampolines late in `shareImportableItems`. But trampolines were designed to go through `indirectCallsToSecondaryFunctions` and `setupTablePatching`, so those late trampolines were invalid, like ```wast (func $trampoline_foo (call $foo) ) ``` when `foo` is in a secondary module. This was supposed to be converted to a `call_indirect` in `indirectCallsToSecondaryFunctions` and the table elements were supposed to set up in `setupTablePatching`. --- To fix the issue, this PR pre-creates trampolines for `ref.func`s in all global initializers in `indirectReferencesToSecondaryFunctions`, before `indirectCallsToSecondaryFunctions` and `setupTablePatching`, but doesn't make `ref.func`s reference them just yet. We make `ref.func`s in global initializers reference the trampolines only when it is decided they are not moving to the same secondary module, in `shareImportableItems`. But this can leave unused trampolines in the primary module. So in `cleanupUnusedTrampolines`, we remove unused trampolines. This increases acx_gallery code size only by 0.6%. If we didn't clean up the unnecessary trampolines, the increase was 3.6%. This only removes unused trampoline functions and not placeholder imports and elements yet, but they don't affect acx_gallery because it uses `--no-placeholders`. Fixes WebAssembly#8510. (Both tests fail for the same reason)

aheejin · 2026-03-29T11:15:46Z

test/lit/wasm-split/global-reffunc.wast

@@ -0,0 +1,51 @@
+;; RUN: wasm-split %s -all -g -o1 %t.1.wasm -o2 %t.2.wasm --keep-funcs=keep


This file was renamed from global-funcref.wast‎, and the expectations were rewritten. Previously I tried to put each line of expectation above the corresponding code line, but it's not easy when we have many other things like imports, exports, and trampolines.

aheejin · 2026-03-29T11:16:15Z

src/ir/module-splitting.cpp

+  // Generate the call and the function. We generate a direct call here, but
+  // this will be converted to a call_indirect in
+  // indirectCallsToSecondaryFunctions.


Drive-by comment improvement. Not related to this PR

aheejin · 2026-03-29T11:16:57Z

test/lit/wasm-split/ref.func.wast

 ;; SECONDARY:      (import "primary" "prime" (func $prime (exact (type $0))))

- ;; SECONDARY:      (elem $0 (i32.const 0) $second-in-table $second)
+ ;; SECONDARY:      (elem $0 (i32.const 0) $second $second-in-table)


This depends on the trampoline creating order. Not meaningful

tlively · 2026-03-30T15:49:12Z

src/ir/module-splitting.cpp

    exportImportCalledPrimaryFunctions();
    setupTablePatching();
    shareImportableItems();
+    cleanupUnusedTrampolines();


It would be good to update the big comment at the top of the file to include this new step.

tlively · 2026-03-30T16:08:37Z

src/ir/module-splitting.cpp

+  // primary module, it means it was created for a global initializer but that
+  // global ended up moving to the same secondary module as the function
+  // referenced in the global initializer's ref.func. We can remove them here.
+  // TODO Remove placeholders and table elements created by unused trampolines


This fix conservatively creates extra trampolines, but then we have to clean them up afterwards. What would it look like to fix the bug by instead moving the globals to secondary modules earlier or being more precise about which globals need to use trampolines in their initializers? That way we wouldn't have to clean up afterwards.

aheejin requested a review from tlively March 29, 2026 11:13

aheejin requested a review from a team as a code owner March 29, 2026 11:13

aheejin commented Mar 29, 2026

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aheejin removed the request for review from a team March 29, 2026 11:22

tlively reviewed Mar 30, 2026

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[wasm-split] Pre-create trampolines for global initializers#8542

[wasm-split] Pre-create trampolines for global initializers#8542
aheejin wants to merge 1 commit intoWebAssembly:mainfrom
aheejin:wasm_split_fuzz_fix

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		@@ -0,0 +1,51 @@
		;; RUN: wasm-split %s -all -g -o1 %t.1.wasm -o2 %t.2.wasm --keep-funcs=keep

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aheejin commented Mar 29, 2026 •

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