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[wasm-split] Split immutable globals #9064

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[wasm-split] Split immutable globals
aheejin Aug 29, 2026
b40df24
clang-format
aheejin Sep 1, 2026
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94 changes: 85 additions & 9 deletions src/ir/module-splitting.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -336,6 +336,7 @@ struct OwnershipTracker {
struct ItemInfo {
UsedNames* owner = nullptr;
SmallVector<Module*, 2> usingSecondaries;
bool usedByPrimary = false;
};

std::unordered_map<Name, ItemInfo> tables;
Expand Down Expand Up @@ -376,6 +377,9 @@ struct OwnershipTracker {
// used by the primary module or multiple secondary modules, the primary
// module is the owner.
auto [it, inserted] = (this->*mapField).insert({name, ItemInfo{owner, {}}});
if (owner == &primaryUsed) {
it->second.usedByPrimary = true;
}
Module* secondary = nullptr;
if (owner != &primaryUsed) {
size_t index = owner - secondaryUsed.data();
Expand Down Expand Up @@ -420,13 +424,30 @@ struct OwnershipTracker {
}

bool isUnused(Name name, const std::unordered_map<Name, ItemInfo>& map) {
return getOwner(name, map) == nullptr;
auto it = map.find(name);
if (it != map.end()) {
return !it->second.usedByPrimary && it->second.usingSecondaries.empty();
}
return true;
}

bool isUsedByPrimary(Name name,
const std::unordered_map<Name, ItemInfo>& map) {
auto it = map.find(name);
if (it != map.end()) {
return it->second.usedByPrimary;
}
return false;
}

bool usedBySingleSecondary(Name name,
const std::unordered_map<Name, ItemInfo>& map) {
auto* owner = getOwner(name, map);
return owner != nullptr && owner != &primaryUsed;
auto it = map.find(name);
if (it != map.end()) {
return !it->second.usedByPrimary &&
it->second.usingSecondaries.size() == 1;
}
return false;
}
};

Expand Down Expand Up @@ -1047,9 +1068,39 @@ void ModuleSplitter::computeUsedNames() {
if (!global->init) {
continue;
}
if (UsedNames* owner = tracker.getOwner(global->name, tracker.globals)) {
for (auto* get : FindAll<GlobalGet>(global->init).list) {
tracker.insert<Global>(get->name, owner);
auto gets = FindAll<GlobalGet>(global->init).list;
if (gets.empty()) {
continue;
}

// In case of mutable globals, we cannot have multiple copies. Compute the
// 'owner' of the global and insert its dependent globals there.
if (global->mutable_) {
if (UsedNames* owner = tracker.getOwner(global->name, tracker.globals)) {

@tlively tlively Sep 2, 2026

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It would be nice if we could have tracker.getOwner<Global>(global->name). This might be possible using struct specialization to create a type-level mapping between module item types (Global, Memory, etc) and OwnershipTracker member pointers. (This mechanism could also potentially replace the macros in OwnershipTracker::insert.)

for (auto* get : gets) {
tracker.insert<Global>(get->name, owner);
}
}
continue;
}

// In case of immutable globals, we can have multiple copies of it. To
// reduce the primary module size, we just copy the globals to all secondary
// modules using them. So here we insert the global in all using modules.
if (tracker.isUsedByPrimary(global->name, tracker.globals)) {
for (auto* get : gets) {
tracker.insert<Global>(get->name, &primaryUsed);
}
}
auto& usingSecs =
tracker.getUsingSecondaries(global->name, tracker.globals);
for (size_t i = 0; i < secondaries.size(); ++i) {
Module* sec = secondaries[i].get();
if (std::find(usingSecs.begin(), usingSecs.end(), sec) !=
usingSecs.end()) {
Comment on lines +1097 to +1100

@tlively tlively Sep 2, 2026

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This looks like it's doing "for each secondary module mi, if usingSecs contains mi, then mark all the referenced globals as used in mi." Why do we need the std::find here? Can we not just do something like this?

for i in usingSecs:
  for get in gets:
    tracker.insert(get->name, &secondaryUsed[i]);

for (auto* get : gets) {
tracker.insert<Global>(get->name, &secondaryUsed[i]);
}
}
}
}
Expand All @@ -1063,8 +1114,8 @@ void ModuleSplitter::shareImportableItems() {
// 2. If an item is used by only a single secondary module, move the item to
// that secondary module. If an item is used by multiple modules (including
// the primary and secondary modules), export the item from the primary and
// import it from the using secondary modules.

// import it from the using secondary modules. (except for immutable
// globals, which we just copy to using secondary modules)
auto shareElements = [&](auto& elements,
auto& trackerElements,
auto copyElement,
Expand All @@ -1073,18 +1124,43 @@ void ModuleSplitter::shareImportableItems() {
ExternalKind kind = ExternalKind::Invalid) {
std::unordered_set<Name> elementsToRemove;
for (auto& element : elements) {
using T = std::remove_pointer_t<decltype(element.get())>;
bool isImmutableGlobal = false;
if constexpr (std::is_same_v<T, Global>) {
if (!element->mutable_) {
isImmutableGlobal = true;
}
}

if (tracker.isUnused(element->name, trackerElements)) {
// If this element is not used anywhere, just remove it
elementsToRemove.insert(element->name);
} else if (tracker.usedBySingleSecondary(element->name,
trackerElements)) {
// If this element is used in a single secondary module, move it to the
// secondary module
auto* secondary =
tracker.getUsingSecondaries(element->name, trackerElements)[0];
copyElement(element.get(), *secondary);
elementsToRemove.insert(element->name);
} else if (isImmutableGlobal) {
// If this element is an immutable global, copy it to all using
// secondary modules to reduce the primary module size, because we can
// have multiple copies. If it is not used in the primary module, remove
// it from there.
if (!tracker.isUsedByPrimary(element->name, trackerElements)) {
elementsToRemove.insert(element->name);
}
for (auto* secondary :
tracker.getUsingSecondaries(element->name, trackerElements)) {
copyElement(element.get(), *secondary);
}
} else {
// If this element is used by multiple modules (primary + multiple
// secondaries or just multiple secondaries), export them from the
// primary module and import them from using secondary modules.
// We only import and export Importables, i.e., we don't do this for
// segments.
using T = std::remove_pointer_t<decltype(element.get())>;
if constexpr (std::is_base_of_v<Importable, T>) {
for (auto* secondary :
tracker.getUsingSecondaries(element->name, trackerElements)) {
Expand Down
4 changes: 1 addition & 3 deletions test/lit/wasm-split/dispatch-table-base-global-used-elsewhere.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -25,8 +25,6 @@

;; PRIMARY: (export "table" (table $table))

;; PRIMARY: (export "global" (global $base))

;; PRIMARY: (export "keep" (func $keep))

;; PRIMARY: (func $keep
Expand All @@ -42,7 +40,7 @@
)
;; SECONDARY: (import "primary" "table" (table $table 2 2 funcref))

;; SECONDARY: (import "primary" "global" (global $base i32))
;; SECONDARY: (import "env" "base" (global $base i32))

;; SECONDARY: (import "primary" "keep" (func $keep))

Expand Down
3 changes: 1 addition & 2 deletions test/lit/wasm-split/passive-deps.wast
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Expand Up @@ -22,15 +22,14 @@
;; PRIMARY-NEXT: (type $0 (func))
;; PRIMARY-NEXT: (global $g funcref (ref.null nofunc))
;; PRIMARY-NEXT: (elem $passive-elem funcref (item (global.get $g)))
;; PRIMARY-NEXT: (export "global" (global $g))
;; PRIMARY-NEXT: (func $keep (type $0)
;; PRIMARY-NEXT: (elem.drop $passive-elem)
;; PRIMARY-NEXT: )
;; PRIMARY-NEXT: )

;; SECONDARY: (module
;; SECONDARY-NEXT: (type $0 (func))
;; SECONDARY-NEXT: (import "primary" "global" (global $g funcref))
;; SECONDARY-NEXT: (global $g funcref (ref.null nofunc))
;; SECONDARY-NEXT: (func $split (type $0)
;; SECONDARY-NEXT: (drop
;; SECONDARY-NEXT: (global.get $g)
Expand Down
17 changes: 9 additions & 8 deletions test/lit/wasm-split/split-module-items.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -5,8 +5,10 @@
;; Check that
;; 1. Items only used in the primary module stay in the primary module
;; 2. Items only used in the secondary module are moved to the secondary module
;; 3. Items used in both modules are exported from the primary and imported from
;; the secondary module
;; 3. Items (that are not immutable globals) used in both modules are exported
;; from the primary and imported from the secondary module
;; 3-1. Immutable globals used in both modules stay in the primary module and
;; also are copied to the secondary module

(module
(rec
Expand Down Expand Up @@ -78,24 +80,23 @@
;; PRIMARY-NEXT: (export "memory_1" (memory $shared-memory))
;; PRIMARY-NEXT: (export "table" (table $keep-table2))
;; PRIMARY-NEXT: (export "table_3" (table $shared-table))
;; PRIMARY-NEXT: (export "global" (global $shared-immutable-global))
;; PRIMARY-NEXT: (export "global_5" (global $shared-mutable-global))
;; PRIMARY-NEXT: (export "global" (global $shared-mutable-global))
;; PRIMARY-NEXT: (export "tag" (tag $shared-tag))
;; PRIMARY-NEXT: (export "keep" (func $keep))
;; PRIMARY-NEXT: (export "table_8" (table $3))
;; PRIMARY-NEXT: (export "table_7" (table $3))

;; SECONDARY: (import "primary" "memory" (memory $keep-memory2 1 1))
;; SECONDARY-NEXT: (import "primary" "memory_1" (memory $shared-memory 1 1))
;; SECONDARY-NEXT: (import "primary" "table" (table $keep-table2 1 1 (ref null $2)))
;; SECONDARY-NEXT: (import "primary" "table_3" (table $shared-table 1 1 funcref))
;; SECONDARY-NEXT: (import "primary" "table_8" (table $timport$2 1 funcref))
;; SECONDARY-NEXT: (import "primary" "global" (global $shared-immutable-global i32))
;; SECONDARY-NEXT: (import "primary" "global_5" (global $shared-mutable-global (mut i32)))
;; SECONDARY-NEXT: (import "primary" "table_7" (table $timport$2 1 funcref))
;; SECONDARY-NEXT: (import "primary" "global" (global $shared-mutable-global (mut i32)))
;; SECONDARY-NEXT: (import "primary" "keep" (func $keep (exact (param i32) (result i32))))
;; SECONDARY-NEXT: (import "primary" "tag" (tag $shared-tag (type $1) (param i32)))

;; SECONDARY: (global $split-immutable-global i32 (i32.const 20))
;; SECONDARY-NEXT: (global $split-mutable-global (mut i32) (i32.const 20))
;; SECONDARY-NEXT: (global $shared-immutable-global i32 (i32.const 20))
;; SECONDARY-NEXT: (memory $split-memory 1 1)
;; SECONDARY-NEXT: (data $split-data (memory $split-memory) (i32.const 0) "a")
;; SECONDARY-NEXT: (table $split-table 1 1 funcref)
Expand Down
16 changes: 8 additions & 8 deletions test/lit/wasm-split/transitive-globals.wast
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Original file line number Diff line number Diff line change
Expand Up @@ -3,13 +3,14 @@
;; RUN: wasm-dis %t.2.wasm | filecheck %s --check-prefix SECONDARY

;; Check that transitive dependencies in global initializers are correctly
;; analyzed and moved to the secondary module.
;; analyzed and copied to the secondary module.

(module
;; There are two dependency chains: $a->$b->$c and $d->$e->$f. While all of
;; $a, $b, and $c can be moved to the secondary module because all f them are
;; used only there, $e is used in the primary module, preventing $e and $f
;; from being moved to the secondary module.
;; There are two dependency chains: $a->$b->$c and $d->$e->$f. Because these
;; are immutable globals, globals are copied to whichever module they are
;; used. The secondary module uses $a and $d, so it will have all globals
;; copied to it. The primary module only uses $e, so it will have $e and its
;; dependency $f.

(global $c i32 (i32.const 42))
(global $b i32 (global.get $c))
Expand All @@ -29,7 +30,6 @@
)
)

;; Exclusively uses $a and $d, causing them to move to the secondary module
(func $split
(drop
(global.get $a)
Expand All @@ -44,7 +44,6 @@
;; PRIMARY-NEXT: (type $0 (func))
;; PRIMARY-NEXT: (global $f i32 (i32.const 42))
;; PRIMARY-NEXT: (global $e i32 (global.get $f))
;; PRIMARY-NEXT: (export "global" (global $e))
;; PRIMARY-NEXT: (func $keep
;; PRIMARY-NEXT: (drop
;; PRIMARY-NEXT: (global.get $e)
Expand All @@ -54,10 +53,11 @@

;; SECONDARY: (module
;; SECONDARY-NEXT: (type $0 (func))
;; SECONDARY-NEXT: (import "primary" "global" (global $e i32))
;; SECONDARY-NEXT: (global $c i32 (i32.const 42))
;; SECONDARY-NEXT: (global $b i32 (global.get $c))
;; SECONDARY-NEXT: (global $a i32 (global.get $b))
;; SECONDARY-NEXT: (global $f i32 (i32.const 42))
;; SECONDARY-NEXT: (global $e i32 (global.get $f))
;; SECONDARY-NEXT: (global $d i32 (global.get $e))
;; SECONDARY-NEXT: (func $split
;; SECONDARY-NEXT: (drop
Expand Down
9 changes: 5 additions & 4 deletions test/lit/wasm-split/transitive-globals2.wast
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Expand Up @@ -3,8 +3,9 @@
;; RUN: wasm-dis -all %t.2.wasm | filecheck %s --check-prefix SECONDARY

;; The dependence chain is $g4->$g3->$g2->$g1, and because $g4 is used in the
;; primary module, all four globals should end up in the primary module. Only
;; $g2 needs to be exported to the secondary module, not $g1.
;; primary module, all four globals should end up in the primary module. The
;; secondary module uses $g2, so it will have $g2 and $g1 it depends on.
;; (Immmutable globals are copied to whichever modules they are used.)

(module
(global $g1 i32 (i32.const 42))
Expand All @@ -27,7 +28,6 @@
;; PRIMARY-NEXT: (global $g2 i32 (global.get $g1))
;; PRIMARY-NEXT: (global $g3 i32 (global.get $g2))
;; PRIMARY-NEXT: (global $g4 i32 (global.get $g3))
;; PRIMARY-NEXT: (export "global" (global $g2))
;; PRIMARY-NEXT: (func $keep (type $0)
;; PRIMARY-NEXT: (drop
;; PRIMARY-NEXT: (global.get $g4)
Expand All @@ -37,7 +37,8 @@

;; SECONDARY: (module
;; SECONDARY-NEXT: (type $0 (func))
;; SECONDARY-NEXT: (import "primary" "global" (global $g2 i32))
;; SECONDARY-NEXT: (global $g1 i32 (i32.const 42))
;; SECONDARY-NEXT: (global $g2 i32 (global.get $g1))
;; SECONDARY-NEXT: (func $split (type $0)
;; SECONDARY-NEXT: (drop
;; SECONDARY-NEXT: (global.get $g2)
Expand Down
14 changes: 7 additions & 7 deletions test/lit/wasm-split/transitive-immutable-globals-multi.wast
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Expand Up @@ -3,8 +3,9 @@
;; RUN: wasm-dis -all %t1.wasm | filecheck %s --check-prefix SECONDARY1
;; RUN: wasm-dis -all %t2.wasm | filecheck %s --check-prefix SECONDARY2

;; Because global $e is used in both module1 ($split1) and module2 ($split2), $e
;; will be exported / imported, but we don't need to export $f.
;; In case of immutable globals, they are copied to whichever modules they are
;; used. $e will be copied to both module1 ($split1) and module2 ($split2), and
;; this will also cause $f to be copied to both modules.

(module
(global $f i32 (i32.const 42))
Expand All @@ -25,17 +26,15 @@

;; PRIMARY: (module
;; PRIMARY-NEXT: (type $0 (func))
;; PRIMARY-NEXT: (global $f i32 (i32.const 42))
;; PRIMARY-NEXT: (global $e i32 (global.get $f))
;; PRIMARY-NEXT: (export "global" (global $e))
;; PRIMARY-NEXT: (func $keep (type $0)
;; PRIMARY-NEXT: (nop)
;; PRIMARY-NEXT: )
;; PRIMARY-NEXT: )

;; SECONDARY1: (module
;; SECONDARY1-NEXT: (type $0 (func))
;; SECONDARY1-NEXT: (import "primary" "global" (global $e i32))
;; SECONDARY1-NEXT: (global $f i32 (i32.const 42))
;; SECONDARY1-NEXT: (global $e i32 (global.get $f))
;; SECONDARY1-NEXT: (func $split1 (type $0)
;; SECONDARY1-NEXT: (drop
;; SECONDARY1-NEXT: (global.get $e)
Expand All @@ -45,7 +44,8 @@

;; SECONDARY2: (module
;; SECONDARY2-NEXT: (type $0 (func))
;; SECONDARY2-NEXT: (import "primary" "global" (global $e i32))
;; SECONDARY2-NEXT: (global $f i32 (i32.const 42))
;; SECONDARY2-NEXT: (global $e i32 (global.get $f))
;; SECONDARY2-NEXT: (func $split2 (type $0)
;; SECONDARY2-NEXT: (drop
;; SECONDARY2-NEXT: (global.get $e)
Expand Down
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