[wasm-split] Remove unnecessary trampolines for ref.func initializers by aheejin · Pull Request #8443 · WebAssembly/binaryen

aheejin · 2026-03-10T09:38:38Z

When a global is exclusively used by a secondary module and thus moved to that module, and its initializer has a (ref.func $func), we used to create a trampoline and export it from the primary module in all cases, even in the case that the function is in the same secondary module. This now only avoids creating a trampoline when the function is already in the same secondary module.

To do this, we now skip scanning global initializers in indirectReferencesToSecondaryFunctions, and selectively create trampolines only when needed in shareImportableItems.

The running time of wasm-split hasn't really changed with this PR, compared to the previous PR #8442 (~25s range in acx_gallery).

#8441, #8442, and this PR combined reduce the size of the primary module of acx_gallery by 45.4%.

wasm-objdump -h result:

Before ([wasm-split] Split globals' transitive global dependencies #8442)

     Type start=0x0000000c end=0x00035d44 (size=0x00035d38) count: 11185
   Import start=0x00035d48 end=0x00132efc (size=0x000fd1b4) count: 32642
 Function start=0x00132f00 end=0x00145dac (size=0x00012eac) count: 62890
    Table start=0x00145daf end=0x001498ea (size=0x00003b3b) count: 2921
      Tag start=0x001498ec end=0x001498f0 (size=0x00000004) count: 1
   Global start=0x001498f4 end=0x00289e60 (size=0x0014056c) count: 47728
   Export start=0x00289e64 end=0x002e99c1 (size=0x0005fb5d) count: 35861
    Start start=0x002e99c3 end=0x002e99c5 (size=0x00000002) start: 828
     Elem start=0x002e99c9 end=0x0035380c (size=0x00069e43) count: 12303
DataCount start=0x0035380e end=0x0035380f (size=0x00000001) count: 1
     Code start=0x00353814 end=0x005830e5 (size=0x0022f8d1) count: 62890
     Data start=0x005830e9 end=0x005a2c76 (size=0x0001fb8d) count: 1

After (This PR)

     Type start=0x0000000c end=0x00035d38 (size=0x00035d2c) count: 11185
   Import start=0x00035d3c end=0x00132ef0 (size=0x000fd1b4) count: 32642
 Function start=0x00132ef4 end=0x001436cc (size=0x000107d8) count: 53001
    Table start=0x001436cf end=0x0014720a (size=0x00003b3b) count: 2921
      Tag start=0x0014720c end=0x00147210 (size=0x00000004) count: 1
   Global start=0x00147214 end=0x00287b75 (size=0x00140961) count: 47728
   Export start=0x00287b79 end=0x002d41ce (size=0x0004c655) count: 25972
    Start start=0x002d41d0 end=0x002d41d2 (size=0x00000002) start: 828
     Elem start=0x002d41d6 end=0x00336c36 (size=0x00062a60) count: 12303
DataCount start=0x00336c38 end=0x00336c39 (size=0x00000001) count: 1
     Code start=0x00336c3e end=0x0053dbdd (size=0x00206f9f) count: 53001
     Data start=0x0053dbe1 end=0x0055d76e (size=0x0001fb8d) count: 1

Fixes #7724.

When a global is exclusively used by a secondary module and thus moved to that module, and its initializer has a `(ref.func $func)`, we used to create a trampoline and export it from the primary module in all cases, even in the case that the function is in the same secondary module. This now moves those functions referred to by `ref.func`s to the secondary module, as long as they don't have uses anywhere else. To do this, we now skip scanning global initializers in `indirectReferencesToSecondaryFunctions`, and selectively create trampolines only when needed in `shareImportableItems`. The running time of `wasm-split` hasn't really changed with this PR, compared to the previous PR #8442 (~25s range in acx_gallery). #8441, #8442, and this PR combined reduce the size of the primary module by 46.6%. --- `wasm-objdump -h` result: - Before (#8442) ``` Type start=0x0000000c end=0x00035d44 (size=0x00035d38) count: 11185 Import start=0x00035d48 end=0x00132efc (size=0x000fd1b4) count: 32642 Function start=0x00132f00 end=0x00145dac (size=0x00012eac) count: 62890 Table start=0x00145daf end=0x001498ea (size=0x00003b3b) count: 2921 Tag start=0x001498ec end=0x001498f0 (size=0x00000004) count: 1 Global start=0x001498f4 end=0x00289e60 (size=0x0014056c) count: 47728 Export start=0x00289e65 end=0x004977fe (size=0x0020d999) count: 35861 Start start=0x00497800 end=0x00497802 (size=0x00000002) start: 828 Elem start=0x00497806 end=0x00501649 (size=0x00069e43) count: 12303 DataCount start=0x0050164b end=0x0050164c (size=0x00000001) count: 1 Code start=0x00501651 end=0x00730f22 (size=0x0022f8d1) count: 62890 Data start=0x00730f26 end=0x00750ab3 (size=0x0001fb8d) count: 1 ``` - After (This PR) ``` Type start=0x0000000c end=0x00035d38 (size=0x00035d2c) count: 11185 Import start=0x00035d3c end=0x00132ef0 (size=0x000fd1b4) count: 32642 Function start=0x00132ef4 end=0x001436cc (size=0x000107d8) count: 53001 Table start=0x001436cf end=0x0014720a (size=0x00003b3b) count: 2921 Tag start=0x0014720c end=0x00147210 (size=0x00000004) count: 1 Global start=0x00147214 end=0x00287b75 (size=0x00140961) count: 47728 Export start=0x00287b79 end=0x002e703f (size=0x0005f4c6) count: 25972 Start start=0x002e7041 end=0x002e7043 (size=0x00000002) start: 828 Elem start=0x002e7047 end=0x00349aa7 (size=0x00062a60) count: 12303 DataCount start=0x00349aa9 end=0x00349aaa (size=0x00000001) count: 1 Code start=0x00349aaf end=0x00550a4e (size=0x00206f9f) count: 53001 Data start=0x00550a52 end=0x005705df (size=0x0001fb8d) count: 1 ``` We can see while the size of the function and the code sections have decreased, the big gains come from the decrease of the export section, which can contain long function names.

aheejin · 2026-03-10T09:40:17Z

cc @biggs0125

When splitting a module, if non-function items (memories, tables, globals, tags) are exclusively used by a single secondary module, this moves them directly to that secondary module rather than exporting them from the primary module. When a global is moved, its initializer can contain `global.get` or `ref.func`s, creating dependences on other globals and functions. For now, this PR just exports all the dependences from the primary module to the secondary module. This will be improved by follow-up PRs. This PR reduces the size of the primary module for acx_gallery by 12.5%. Follow-up PRs will reduce it further. This also sadly increases wasm-split's running time on acx_gallery from 16.5s -> 24.7s, by 49%, due to more computations in `shareImportableItems`. --- The below is `wasm-objdump -h` result of the primary modules: - Before ``` Type start=0x0000000c end=0x00035e09 (size=0x00035dfd) count: 11192 Import start=0x00035e0e end=0x004bd669 (size=0x0048785b) count: 65720 Function start=0x004bd66d end=0x004d0519 (size=0x00012eac) count: 62890 Table start=0x004d051c end=0x004d4059 (size=0x00003b3d) count: 2921 Tag start=0x004d405b end=0x004d405f (size=0x00000004) count: 1 Global start=0x004d4063 end=0x00689ff8 (size=0x001b5f95) count: 80766 Export start=0x00689ffc end=0x0071b16c (size=0x00091170) count: 60877 Start start=0x0071b16e end=0x0071b170 (size=0x00000002) start: 828 Elem start=0x0071b174 end=0x00784fb9 (size=0x00069e45) count: 12303 DataCount start=0x00784fbb end=0x00784fbc (size=0x00000001) count: 1 Code start=0x00784fc1 end=0x009b4958 (size=0x0022f997) count: 62890 Data start=0x009b495c end=0x009d44e9 (size=0x0001fb8d) count: 1 ``` - After (This PR) ``` Type start=0x0000000c end=0x00035d44 (size=0x00035d38) count: 11185 Import start=0x00035d49 end=0x003faf6f (size=0x003c5226) count: 56805 Function start=0x003faf73 end=0x0040de1f (size=0x00012eac) count: 62890 Table start=0x0040de22 end=0x0041195d (size=0x00003b3b) count: 2921 Tag start=0x0041195f end=0x00411963 (size=0x00000004) count: 1 Global start=0x00411967 end=0x005541c5 (size=0x0014285e) count: 47771 Export start=0x005541c9 end=0x005dfc2c (size=0x0008ba63) count: 59077 Start start=0x005dfc2e end=0x005dfc30 (size=0x00000002) start: 828 Elem start=0x005dfc34 end=0x00649a77 (size=0x00069e43) count: 12303 DataCount start=0x00649a79 end=0x00649a7a (size=0x00000001) count: 1 Code start=0x00649a7f end=0x00879385 (size=0x0022f906) count: 62890 Data start=0x00879389 end=0x00898f16 (size=0x0001fb8d) count: 1 ``` Follow-ups: #8442 and #8443

…ffunc

aheejin · 2026-03-19T02:04:30Z

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

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

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


The test changes in this file was just caused by the order we create trampolines and are not really meaningful

…ffunc

tlively

LGTM % comments.

tlively · 2026-03-19T04:38:58Z

src/ir/module-splitting.cpp

+  // We shouldn't use collector.walkModuleCode here, because we don't want to
+  // walk on global initializers. At this point, all globals are still in the
+  // primary module, so if we walk on global initializers here, it will create
+  // unnecessary trampolines.


Suggested change

// We shouldn't use collector.walkModuleCode here, because we don't want to

// walk on global initializers. At this point, all globals are still in the

// primary module, so if we walk on global initializers here, it will create

// unnecessary trampolines.

// We shouldn't use collector.walkModuleCode here, because we don't want to

// walk global initializers. At this point, all globals are still in the

// primary module, so if we walk global initializers here, it will create

// unnecessary trampolines.

tlively · 2026-03-19T04:45:08Z

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

It would be good to add a test where another reference to the function in the primary module prevents it from being split out, even though the global is moved to the secondary module.

aheejin requested a review from tlively March 10, 2026 09:38

This was referenced Mar 10, 2026

[wasm-split] Move exclusive items to secondary modules #8441

Merged

[wasm-split] Split globals' transitive global dependencies #8442

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Comment fix

a0c54d9

aheejin added 4 commits March 19, 2026 01:27

Merge branch 'wasm_split_global_transitive' into wasm_split_global_re…

0d063f3

…ffunc

Merge branch 'wasm_split_global_transitive' into wasm_split_global_re…

c32c072

…ffunc

Merge branch 'wasm_split_global_transitive' into wasm_split_global_re…

6cdd238

…ffunc

Comment fix

6a823ef

aheejin changed the title ~~[wasm-split] Split globals' ref.func dependencies~~ [wasm-split] Remove unnecessary trampolines for ref.func initializers Mar 19, 2026

aheejin commented Mar 19, 2026

View reviewed changes

Merge branch 'wasm_split_global_transitive' into wasm_split_global_re…

f34a0d2

…ffunc

tlively approved these changes Mar 19, 2026

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[wasm-split] Remove unnecessary trampolines for ref.func initializers#8443

[wasm-split] Remove unnecessary trampolines for ref.func initializers#8443
aheejin wants to merge 7 commits intowasm_split_global_transitivefrom
wasm_split_global_reffunc

aheejin commented Mar 10, 2026 •

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