[wasm-split] Export/Import only necessary elements #8221

aheejin · 2026-01-20T17:03:44Z

In ModuleSplitter::shareImportableItems, we now check if each module element is used by a secondary module and export & import it only when it is necessary. This removes the need to run RemoveUnusedModuleElements pass for secondary modules at the end, and also removes unnecessary exports from the primary module in case they are not used in any secondary modules.

This reduces the running time on a 'acx_gallery' reproducer provided by @biggs0125 before from 78.3s to 16.4s in average on my machine, reducing it by around 79%.

In that 'acx_gallery' program, this PR reduces the size of the primary module by 13% and its export section by 62.1%, and reduces the all combined module size (primary + secondaries) by 7.4%.

Detailed analysis for 'acx_gallery', a case where we split a module into 301 (1 primary + 300 secondary) modules:

Before this PR:
Time: 78.3s
Task breakdown:

Task                                          Total Time (ms)      Percentage
---------------------------------------------------------------------------
shareImportableItems                               34472.0000          49.90%
removeUnusedSecondaryElements                      24720.2000          35.78%
moveSecondaryFunctions                              4938.0500           7.15%
writeModule_secondary                               2892.7603           4.19%
writeModule_primary                                  897.3630           1.30%
exportImportCalledPrimaryFunctions                   667.9780           0.97%
indirectReferencesToSecondaryFunctions               201.9830           0.29%
indirectCallsToSecondaryFunctions                    133.2190           0.19%
classifyFunctions                                    118.3200           0.17%
setupTablePatching                                    42.6402           0.06%
thunkExportedSecondaryFunctions                        0.9307           0.00%
initExportedPrimaryFuncs                               0.8008           0.00%
---------------------------------------------------------------------------

After this PR:
Time: 16.4s
Task breakdown:

Task                                          Total Time (ms)      Percentage
---------------------------------------------------------------------------
moveSecondaryFunctions                              5341.1700          43.94%
writeModule_secondary                               2960.8319          24.36%
shareImportableItems                                1568.6400          12.91%
writeModule_primary                                  765.7240           6.30%
exportImportCalledPrimaryFunctions                   762.9670           6.28%
indirectReferencesToSecondaryFunctions               362.3300           2.98%
classifyFunctions                                    204.4540           1.68%
indirectCallsToSecondaryFunctions                    139.2110           1.15%
setupTablePatching                                    45.2461           0.37%
thunkExportedSecondaryFunctions                        3.0373           0.02%
initExportedPrimaryFuncs                               0.9982           0.01%
---------------------------------------------------------------------------
Overall Total                                      12154.6095         100.00%

We can see that shareImportableItems, which took up the largest share, has been reduced significantly, and removeUnusedSecondaryElements, which was the second largest, is not necessary anymore.

Code size

Primary module's export section size: -62.1%
With the name section present
- Primary module size: -8%
- All combined module size: -4.4%
With the name section stripped from the result
- Primary module size: -13%
- All combined module size: -7.4%

@biggs0125

In `ModuleSplitter::shareImportableItems`, we now check if each module element is used by a secondary module and export & import it only when it is necessary. This removes the need to run RemoveUnusedModuleElements pass for secondary modules at the end, and also removes unnecessary exports from the primary module in case they are not used in any secondary modules. This reduces the running time on a 'acx_gallery' reproducer provided by @biggs0125 before from 78.3s to 16.4s in average on my machine, reducing it by around 79%. In that 'acx_gallery' program, this PR reduces the size of the primary module by 8% and reduces the all combined module size (primary + secondaries) by 3.7%. --- Detailed analysis for 'acx_gallery', a case where we split a module into 301 (1 primary + 300 secondary) modules: - Before this PR: Time: 78.3s Task breakdown: ``` Task Total Time (ms) Percentage --------------------------------------------------------------------------- shareImportableItems 34472.0000 49.90% removeUnusedSecondaryElements 24720.2000 35.78% moveSecondaryFunctions 4938.0500 7.15% writeModule_secondary 2892.7603 4.19% writeModule_primary 897.3630 1.30% exportImportCalledPrimaryFunctions 667.9780 0.97% indirectReferencesToSecondaryFunctions 201.9830 0.29% indirectCallsToSecondaryFunctions 133.2190 0.19% classifyFunctions 118.3200 0.17% setupTablePatching 42.6402 0.06% thunkExportedSecondaryFunctions 0.9307 0.00% initExportedPrimaryFuncs 0.8008 0.00% --------------------------------------------------------------------------- Overall Total ``` - After this PR: Time: 16.4s Task breakdown: ``` Task Total Time (ms) Percentage --------------------------------------------------------------------------- moveSecondaryFunctions 5341.1700 43.94% writeModule_secondary 2960.8319 24.36% shareImportableItems 1568.6400 12.91% writeModule_primary 765.7240 6.30% exportImportCalledPrimaryFunctions 762.9670 6.28% indirectReferencesToSecondaryFunctions 362.3300 2.98% classifyFunctions 204.4540 1.68% indirectCallsToSecondaryFunctions 139.2110 1.15% setupTablePatching 45.2461 0.37% thunkExportedSecondaryFunctions 3.0373 0.02% initExportedPrimaryFuncs 0.9982 0.01% --------------------------------------------------------------------------- Overall Total 12154.6095 100.00% ``` We can see that `shareImportableItems`, which took up the largest share, has been reduced significantly, and `removeUnusedSecondaryElements`, which was the second largest, is not necessary anymore.

tlively · 2026-01-20T18:25:31Z

src/ir/module-splitting.cpp

+
+#define DELEGATE_FIELD_NAME_KIND(id, field, kind)                              \
+  if (cast->field.is()) {                                                      \
+    if (kind == ModuleItemKind::Global) {                                      \


A switch here might be better than an if-else chain. If nothing else, it would give us a compiler error if we ever add a new ModuleItemKind.

Done: 8642d46

tlively · 2026-01-20T18:29:16Z

src/ir/module-splitting.cpp

+    NameCollector collector(used);
+    for (auto& global : secondary.globals) {
+      if (!global->imported()) {
+        collector.walk(global->init);
+      }
+    }
+    for (auto& segment : secondary.dataSegments) {
+      used.memories.insert(segment->memory);
+      if (segment->offset) {
+        collector.walk(segment->offset);
+      }
+    }
+    for (auto& segment : secondary.elementSegments) {
+      if (segment->table.is()) {
+        used.tables.insert(segment->table);
+      }
+      if (segment->offset) {
+        collector.walk(segment->offset);
+      }
+      for (auto* item : segment->data) {
+        collector.walk(item);
+      }
+    }


All of this except checking for tables and memories in active segments could be replaced with a call to collector.walkModuleCode().

Done: 8b7850a
And added a missing check e859242

tlively · 2026-01-20T18:30:30Z

src/ir/module-splitting.cpp


    for (auto& table : primary.tables) {
      auto secondaryTable = secondary.getTableOrNull(table->name);
+      if (!secondaryTable && !used.tables.count(table->name)) {


Why is this condition different from the others? Maybe worth a comment.

Done: 5e7b376

aheejin · 2026-01-24T04:12:56Z

test/example/module-splitting.txt

+ (type $0 (func))
+ (import "primary" "%a" (func $0 (exact (type $0))))
+ (import "primary" "%c" (func $1 (exact (type $0))))
+ (import "primary" "%d" (func $2 (exact (type $0))))
+ (import "primary" "already_exported" (func $3 (exact (type $0))))
+ (import "primary" "%e" (func $4 (exact (type $0))))
+ (import "primary" "%f" (func $5 (exact (type $0))))
+ (import "primary" "%g" (func $6 (exact (type $0))))
+ (import "primary" "%b" (func $7 (exact (type $0))))
+ (import "primary" "%h" (func $8 (exact (type $0))))
+ (import "primary" "%i" (func $9 (exact (type $0))))
+ (func $call (type $0)
+  (call $0)
+  (call $1)
+  (call $2)
+  (call $3)
+  (call $4)
+  (call $5)
+  (call $6)
+  (call $7)
+  (call $8)
+  (call $9)
+ )


There are a few test cases in this file that have new functions in secondary modules. This is because we don't run RemoveUnusedModuleElements on secondaries anymore. In practice this wouldn't be a problem because if a function is not used anywhere it would have been optimized away before splitting.

tlively

Looks great!

This reverts commit 51e3151.

aheejin · 2026-01-26T23:05:56Z

Sorry, I pushed code for time measuring by mistake. Reverted.

aheejin · 2026-01-27T09:45:35Z

Updated more code size stats in the PR description.
(I also restored some incorrect time measurements: it's ~80% reduction, not 90%)

@biggs0125

`moveSecondaryFunctions`, despite the name, actually copies functions to secondary modules and removes them from the primary module. This prevents this overhead by actually moving the functions to secondary modules. This reduces the running time on acx_gallery (provided by @biggs0125) by 31%. --- - Before this PR: Time: 16.4s Task breakdown: ``` Task Total Time (ms) Percentage --------------------------------------------------------------------------- moveSecondaryFunctions 5341.1700 43.94% writeModule_secondary 2960.8319 24.36% shareImportableItems 1568.6400 12.91% writeModule_primary 765.7240 6.30% exportImportCalledPrimaryFunctions 762.9670 6.28% indirectReferencesToSecondaryFunctions 362.3300 2.98% classifyFunctions 204.4540 1.68% indirectCallsToSecondaryFunctions 139.2110 1.15% setupTablePatching 45.2461 0.37% thunkExportedSecondaryFunctions 3.0373 0.02% initExportedPrimaryFuncs 0.9982 0.01% --------------------------------------------------------------------------- Overall Total 12154.6095 100.00% ``` - After this PR: Time: 11.3s ``` Task Total Time (ms) Percentage --------------------------------------------------------------------------- writeModule_secondary 2908.4728 43.52% shareImportableItems 1562.7800 23.39% exportImportCalledPrimaryFunctions 786.6140 11.77% writeModule_primary 751.3420 11.24% indirectReferencesToSecondaryFunctions 250.7260 3.75% indirectCallsToSecondaryFunctions 149.7940 2.24% classifyFunctions 126.4520 1.89% moveSecondaryFunctions 106.0680 1.59% setupTablePatching 38.9292 0.58% initExportedPrimaryFuncs 0.8816 0.01% thunkExportedSecondaryFunctions 0.4937 0.01% --------------------------------------------------------------------------- Overall Total 6682.5533 100.00% ``` We can see the time spent in `moveSecondaryFunctions`, which used to take nearly half of the running time (after WebAssembly#8221), is now negligible.

aheejin requested a review from tlively January 20, 2026 17:03

tlively reviewed Jan 20, 2026

View reviewed changes

aheejin added 6 commits January 23, 2026 05:15

Merge branch 'main' into wasm_split_share_needed

6fdbc5a

Use switch-case

8642d46

Use walkModuleCode()

8b7850a

Add missing memory.is() check

e859242

Table export comments

5e7b376

Example test update

8f80464

aheejin commented Jan 24, 2026

View reviewed changes

Merge branch 'main' into wasm_split_share_needed

ee78dd7

tlively approved these changes Jan 24, 2026

View reviewed changes

aheejin added 2 commits January 26, 2026 22:35

Measure time

51e3151

Revert "Measure time"

794a530

This reverts commit 51e3151.

aheejin merged commit d9e6ed3 into WebAssembly:main Jan 27, 2026
17 checks passed

aheejin deleted the wasm_split_share_needed branch January 27, 2026 09:46

This was referenced Jan 28, 2026

[wasm-split] "Move" secondary functions #8244

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[wasm-split] Export/Import only necessary elements #8221

[wasm-split] Export/Import only necessary elements #8221

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[wasm-split] Export/Import only necessary elements #8221

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