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wasm2c: Save memory base/size locally as a perf optimization #2804

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4af896a
wasm2c: Use segue only for single unshared default page-size memories
shravanrn Aug 7, 2026
4ed201d
wasm2c: Save memory base locally as a perf optimization
shravanrn Aug 7, 2026
70b728f
wasm2c: Save memory size locally as a perf optimization
shravanrn Aug 15, 2026
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110 changes: 80 additions & 30 deletions src/c-writer.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -414,7 +414,10 @@ class CWriter {
void WriteElemInitializers();
void WriteFuncRefWrappers();
void WriteElemTableInit(bool, const ElemSegment*, const Table*);
bool IsSingleUnsharedDefault32Memory();
bool IsSingleUnsharedMemory();
void WriteLocalMemoryBaseSizeDeclaration();
void RefreshLocalMemorySize();
void InstallSegueBase(Memory* memory, bool save_old_value);
void RestoreSegueBase();
void WriteExports(CWriterPhase);
Expand Down Expand Up @@ -1593,8 +1596,8 @@ void CWriter::WriteSourceTop() {
Write(s_source_includes);
Write(Newline(), "#include \"", header_name_, "\"", Newline());

if (IsSingleUnsharedMemory()) {
Write("#define IS_SINGLE_UNSHARED_MEMORY 1", Newline());
if (IsSingleUnsharedDefault32Memory()) {
Write("#define IS_SINGLE_UNSHARED_DEFAULT32_MEMORY 1", Newline());
}

Write(s_source_declarations, Newline());
Expand Down Expand Up @@ -2599,9 +2602,41 @@ void CWriter::WriteElemTableInit(bool active_initialization,
Write(");", Newline());
}

bool CWriter::IsSingleUnsharedMemory() {
bool CWriter::IsSingleUnsharedDefault32Memory() {
return module_->memories.size() == 1 &&
!module_->memories[0]->page_limits.is_shared;
!module_->memories[0]->page_limits.is_shared &&
module_->memories[0]->page_size == WABT_DEFAULT_PAGE_SIZE &&
!module_->memories[0]->page_limits.is_64;
}

void CWriter::WriteLocalMemoryBaseSizeDeclaration() {
Write("uint8_t* const wasm_rt_local_memory_base = ");
if (IsSingleUnsharedDefault32Memory()) {
const Memory* memory = module_->memories[0];
Write("(", ExternalInstancePtr(ModuleFieldType::Memory, memory->name),
")->data");
} else {
Write("NULL");
}
Write(";", Newline(), "uint64_t wasm_rt_local_memory_size = ");
if (IsSingleUnsharedDefault32Memory()) {
const Memory* memory = module_->memories[0];
Write("(", ExternalInstancePtr(ModuleFieldType::Memory, memory->name),
")->size");
} else {
Write("0");
}
Write(";", Newline(), "(void)wasm_rt_local_memory_base;", Newline(),
"(void)wasm_rt_local_memory_size;", Newline());
}

void CWriter::RefreshLocalMemorySize() {
if (IsSingleUnsharedDefault32Memory()) {
const Memory* memory = module_->memories[0];
Write("wasm_rt_local_memory_size = (",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name),
")->size;", Newline());
}
}

void CWriter::InstallSegueBase(Memory* memory, bool save_old_value) {
Expand Down Expand Up @@ -2703,7 +2738,7 @@ void CWriter::WriteExports(CWriterPhase kind) {
switch (export_->kind) {
case ExternalKind::Func: {
Write(OpenBrace());
if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
InstallSegueBase(module_->memories[0], true /* save_old_value */);
}
auto num_results = func_->GetNumResults();
Expand All @@ -2722,7 +2757,7 @@ void CWriter::WriteExports(CWriterPhase kind) {
Write("instance");
}
WriteParamSymbols(index_to_name);
if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
RestoreSegueBase();
}
if (num_results > 0) {
Expand Down Expand Up @@ -2826,7 +2861,7 @@ void CWriter::WriteInit() {
}
if (!module_->memories.empty()) {
Write("init_memories(instance);", Newline());
if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
InstallSegueBase(module_->memories[0], true /* save_old_value */);
}
}
Expand All @@ -2850,7 +2885,7 @@ void CWriter::WriteInit() {
Write(Newline());
}

if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
RestoreSegueBase();
}
Write(CloseBrace(), Newline());
Expand Down Expand Up @@ -3175,6 +3210,7 @@ void CWriter::Write(const Func& func) {
GlobalName(ModuleFieldType::Func, func.name), "(");
WriteParamsAndLocals();
Write("FUNC_PROLOGUE;", Newline());
WriteLocalMemoryBaseSizeDeclaration();

size_t stack_vars_section = func_sections_.size() - 1;
PushFuncSection();
Expand Down Expand Up @@ -3249,6 +3285,7 @@ void CWriter::WriteTailCallee(const Func& func) {
Write(" ", OpenBrace());
WriteTailCallAsserts(func.decl.sig);
Write(ModuleInstanceTypeName(), "* instance = *instance_ptr;", Newline());
WriteLocalMemoryBaseSizeDeclaration();

std::vector<std::string> index_to_name;
MakeTypeBindingReverseMapping(func.GetNumParamsAndLocals(), func.bindings,
Expand Down Expand Up @@ -3417,6 +3454,7 @@ size_t CWriter::BeginTry(const Block& block) {

void CWriter::WriteTryCatch(const TryExpr& tryexpr) {
const size_t mark = BeginTry(tryexpr.block);
RefreshLocalMemorySize();

/* exception has been thrown -- do we catch it? */

Expand Down Expand Up @@ -3563,6 +3601,7 @@ void CWriter::WriteTryDelegate(const TryExpr& tryexpr) {

void CWriter::Write(const TryTableExpr& try_table_expr) {
const size_t mark = BeginTry(try_table_expr.block);
RefreshLocalMemorySize();

/* exception has been thrown -- do we catch it? */

Expand Down Expand Up @@ -3717,6 +3756,7 @@ void CWriter::Write(const ExprList& exprs) {
Write(StackVar(num_params - i - 1));
}
Write(");", Newline());
RefreshLocalMemorySize();
DropTypes(num_params);
PushTypes(func.decl.sig.result_types);
if (num_results > 1) {
Expand Down Expand Up @@ -3753,9 +3793,10 @@ void CWriter::Write(const ExprList& exprs) {
Write(", ", StackVar(num_params - i));
}
Write(");", Newline());
if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
InstallSegueBase(module_->memories[0], false /* save_old_value */);
}
RefreshLocalMemorySize();
DropTypes(num_params + 1);
PushTypes(decl.sig.result_types);
if (num_results > 1) {
Expand Down Expand Up @@ -4081,9 +4122,10 @@ void CWriter::Write(const ExprList& exprs) {
Write(StackVar(0), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ",
StackVar(0), ");", Newline());
if (IsSingleUnsharedMemory()) {
if (IsSingleUnsharedDefault32Memory()) {
InstallSegueBase(module_->memories[0], false /* save_old_value */);
}
RefreshLocalMemorySize();
break;
}

Expand Down Expand Up @@ -5373,8 +5415,9 @@ void CWriter::Write(const LoadExpr& expr) {
func = GetMemoryAPIString(*memory, func);

Type result_type = expr.opcode.GetResultType();
Write(StackVar(0, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(0, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(0, memory, expr.offset);
Write(");", Newline());
DropTypes(1);
Expand Down Expand Up @@ -5404,8 +5447,8 @@ void CWriter::Write(const StoreExpr& expr) {
Memory* memory = module_->memories[module_->GetMemoryIndex(expr.memidx)];
func = GetMemoryAPIString(*memory, func);

Write(func, "(", ExternalInstancePtr(ModuleFieldType::Memory, memory->name),
", ");
Write(func, "(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(1, memory, expr.offset);
Write(", ", StackVar(0), ");", Newline());
DropTypes(2);
Expand Down Expand Up @@ -5863,8 +5906,9 @@ void CWriter::Write(const SimdLoadLaneExpr& expr) {
// clang-format on
Memory* memory = module_->memories[module_->GetMemoryIndex(expr.memidx)];
Type result_type = expr.opcode.GetResultType();
Write(StackVar(1, result_type), " = ", func, expr.val, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(1, result_type), " = ", func, expr.val,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(1, memory, expr.offset);
Write(", ", StackVar(0));
Write(");", Newline());
Expand All @@ -5887,8 +5931,9 @@ void CWriter::Write(const SimdStoreLaneExpr& expr) {
// clang-format on
Memory* memory = module_->memories[module_->GetMemoryIndex(expr.memidx)];

Write(func, expr.val, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(func, expr.val,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(1, memory, expr.offset);
Write(", ", StackVar(0));
Write(");", Newline());
Expand Down Expand Up @@ -5930,8 +5975,9 @@ void CWriter::Write(const LoadSplatExpr& expr) {
}
// clang-format on
Type result_type = expr.opcode.GetResultType();
Write(StackVar(0, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(0, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(0, memory, expr.offset);
Write(");", Newline());

Expand All @@ -5953,8 +5999,9 @@ void CWriter::Write(const LoadZeroExpr& expr) {
// clang-format on

Type result_type = expr.opcode.GetResultType();
Write(StackVar(0, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(0, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(0, memory, expr.offset);
Write(");", Newline());

Expand Down Expand Up @@ -5983,8 +6030,9 @@ void CWriter::Write(const AtomicLoadExpr& expr) {
func = GetMemoryAPIString(*memory, func);

Type result_type = expr.opcode.GetResultType();
Write(StackVar(0, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(0, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(0, memory, expr.offset);
Write(");", Newline());
DropTypes(1);
Expand All @@ -6011,8 +6059,8 @@ void CWriter::Write(const AtomicStoreExpr& expr) {
Memory* memory = module_->memories[module_->GetMemoryIndex(expr.memidx)];
func = GetMemoryAPIString(*memory, func);

Write(func, "(", ExternalInstancePtr(ModuleFieldType::Memory, memory->name),
", ");
Write(func, "(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(1, memory, expr.offset);
Write(", ", StackVar(0), ");", Newline());
DropTypes(2);
Expand Down Expand Up @@ -6074,8 +6122,9 @@ void CWriter::Write(const AtomicRmwExpr& expr) {

Type result_type = expr.opcode.GetResultType();

Write(StackVar(1, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(1, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(1, memory, expr.offset);
Write(", ", StackVar(0), ");", Newline());
DropTypes(2);
Expand Down Expand Up @@ -6103,8 +6152,9 @@ void CWriter::Write(const AtomicRmwCmpxchgExpr& expr) {

Type result_type = expr.opcode.GetResultType();

Write(StackVar(2, result_type), " = ", func, "(",
ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
Write(StackVar(2, result_type), " = ", func,
"(wasm_rt_local_memory_base, wasm_rt_local_memory_size, ");
Write(ExternalInstancePtr(ModuleFieldType::Memory, memory->name), ", ");
WriteMemoryAddress(2, memory, expr.offset);
Write(", ", StackVar(1), ", ", StackVar(0), ");", Newline());
DropTypes(3);
Expand Down
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