Add a --constant-arrays-to-data pass by gkdn · Pull Request #9274 · WebAssembly/binaryen

gkdn · 2026-10-17T04:35:21Z

Follow-up to #9248: a late pass that converts array.new_fixed of constant
numeric values in function bodies to array.new_data when that is smaller
(array.new_fixed needs a full *.const per element, a data segment stores raw
bytes).

Since array.new_fixed is a constant expression that other optimizations
understand, the default pipeline runs this only in the last set of opts
(lastOpts, #9238).

Global initializers are left alone, as array.new_data is not a constant
expression there.
Converted payloads go into one new passive segment. Identical payloads are
stored once, and a payload identical to an existing, never-dropped passive
segment reuses that segment.
A group of identical payloads is converted only if the estimated binary size
is smaller, and the new segment is only added if the savings that depend on it
pay for its header.
The segment is never dropped and all reads are in bounds, so the result
cannot trap.

Note that this does not make it safe to raise the 64-element cap in
OptimizeInstructions: the conversion is not always reversible. A larger
array.new_fixed can be hoisted into a global initializer, where it cannot be
turned back into array.new_data. On a large J2Wasm binary, removing the cap
moved the remaining big tables into globals and grew the binary (+0.9 KB raw,
+1.4 KB gzipped), which this pass cannot recover.

Convert array.new_fixed of constant numeric values in function bodies to array.new_data when that is smaller in the binary. array.new_fixed needs a full *.const per element, while a data segment stores raw bytes. As array.new_fixed is a constant expression that other optimizations understand, this is only done in the last set of default optimizations (lastOpts). All converted payloads go into a single new passive segment. Identical payloads are stored once, and payloads that already appear in the new segment or in an existing passive segment that is never dropped reuse those bytes.

* Copy debug locations to the new array.new_data. * Use FindAllPointers and FindAll in the parallel analysis (which now also collects the dropped segments), Builder::makeDataSegment, and the LEB writers to measure sizes. * Only add the new segment if the conversions that use it pay for its header, and still reuse existing segments otherwise. * Reuse existing segments only if they are identical to the payload, to avoid a quadratic substring search (left as a TODO, along with array.new_elem). * Check all module items in the lit tests (--all-items), including the new segment's contents and debug locations.

* Move the size estimates into helpers: getArrayNewFixedSize, getArrayNewDataSize and getNewSegmentOverhead. * Split run() into findCandidates, groupByPayload, decide and replace. * Build payloads as std::vector<char> (like DataSegment::data) with getBits and memcpy, as the interpreter does, instead of casting. No functional change.

* Order the pass top-down: run, then each step in the order it is called, with the helpers of planConversions after it. * Rename Group to PayloadGroup, Decision to Conversion, decide to planConversions, replace to applyConversions, and addSegment to addNewSegment. Store the array in Candidate instead of a get() method. * Plan without modifying the module: the new segment's contents are gathered in newSegmentData, and run() adds the segment. * Comment the non-obvious parts: why unreachable, empty and non-numeric arrays are skipped, why dropped segments cannot be reused, why the result cannot trap, pointer stability of the candidates, and why the new segment's cost is checked after all groups. No functional change.

gemini-code-assist

Code Review

This pull request introduces the --constant-arrays-to-data optimization pass, which converts array.new_fixed instructions of constant numeric values into array.new_data from passive data segments when it reduces binary size. The pass is integrated into the end of the default optimization pipeline. The review feedback suggests avoiding registering the pass if the bulk-memory feature is disabled, and recommends replacing functional-style casts with explicit static_cast calls to avoid potential narrowing warnings on 64-bit platforms and conform to modern C++ practices.

gemini-code-assist · 2026-10-17T04:37:54Z

+  if (options.lastOpts && wasm->features.hasGC() &&
+      (options.optimizeLevel >= 2 || options.shrinkLevel >= 1)) {


Since the constant-arrays-to-data pass requires the bulk-memory feature (as checked in ConstantArraysToData::run), we can avoid adding the pass to the runner entirely if bulk-memory is not enabled. This saves a small amount of overhead.

Suggested change

if (options.lastOpts && wasm->features.hasGC() &&

(options.optimizeLevel >= 2 || options.shrinkLevel >= 1)) {

if (options.lastOpts && wasm->features.hasGC() && wasm->features.hasBulkMemory() &&

(options.optimizeLevel >= 2 || options.shrinkLevel >= 1)) {

gemini-code-assist · 2026-10-17T04:37:54Z

+// The size of an array.new_fixed of constants, apart from the prefix, opcode
+// and type index, which are the same in array.new_data.
+size_t getArrayNewFixedSize(ArrayNewFixed* curr) {
+  size_t size = getWrittenSize(U32LEB(curr->values.size()));


On 64-bit platforms, curr->values.size() is of type size_t (64-bit), whereas U32LEB expects a uint32_t (32-bit). This can trigger narrowing or conversion warnings on some compilers (especially with strict warning settings or -Wconversion). It is safer to explicitly cast it using static_cast<uint32_t> per the Google C++ Style Guide.

Suggested change

size_t size = getWrittenSize(U32LEB(curr->values.size()));

size_t size = getWrittenSize(U32LEB(static_cast<uint32_t>(curr->values.size())));

gemini-code-assist · 2026-10-17T04:37:55Z

+// The size of a new passive data segment, apart from its data.
+size_t getNewSegmentOverhead(const Module& module, size_t dataSize) {
+  // The flags and the size of the data.
+  size_t size = 1 + getWrittenSize(U32LEB(dataSize));


On 64-bit platforms, dataSize is of type size_t (64-bit), whereas U32LEB expects a uint32_t (32-bit). This can trigger narrowing or conversion warnings on some compilers. It is safer to explicitly cast it using static_cast<uint32_t> per the Google C++ Style Guide.

Suggested change

size_t size = 1 + getWrittenSize(U32LEB(dataSize));

size_t size = 1 + getWrittenSize(U32LEB(static_cast<uint32_t>(dataSize)));

gemini-code-assist · 2026-10-17T04:37:55Z

+  return getWrittenSize(U32LEB(segment)) + 1 +
+         getWrittenSize(S32LEB(int32_t(offset))) + 1 +
+         getWrittenSize(S32LEB(int32_t(size)));


Using C-style or functional-style casts like int32_t(offset) is discouraged in modern C++. It is safer and more idiomatic to use static_cast<int32_t> per the Google C++ Style Guide.

Suggested change

return getWrittenSize(U32LEB(segment)) + 1 +

getWrittenSize(S32LEB(int32_t(offset))) + 1 +

getWrittenSize(S32LEB(int32_t(size)));

return getWrittenSize(U32LEB(segment)) + 1 +

getWrittenSize(S32LEB(static_cast<int32_t>(offset))) + 1 +

getWrittenSize(S32LEB(static_cast<int32_t>(size)));

gemini-code-assist · 2026-10-17T04:37:55Z

+      auto savings = getCodeSavings(group, newSegmentIndex, offset) -
+                     int64_t(group.payload.size());


Using C-style or functional-style casts like int64_t(group.payload.size()) is discouraged in modern C++. It is safer and more idiomatic to use static_cast<int64_t> per the Google C++ Style Guide.

Suggested change

auto savings = getCodeSavings(group, newSegmentIndex, offset) -

int64_t(group.payload.size());

auto savings = getCodeSavings(group, newSegmentIndex, offset) -

static_cast<int64_t>(group.payload.size());

gemini-code-assist · 2026-10-17T04:37:55Z

+        newSegmentSavings <=
+          int64_t(getNewSegmentOverhead(*module, newSegmentData.size()))) {


Using C-style or functional-style casts like int64_t(...) is discouraged in modern C++. It is safer and more idiomatic to use static_cast<int64_t> per the Google C++ Style Guide.

Suggested change

newSegmentSavings <=

int64_t(getNewSegmentOverhead(*module, newSegmentData.size()))) {

newSegmentSavings <=

static_cast<int64_t>(getNewSegmentOverhead(*module, newSegmentData.size()))) {

gemini-code-assist · 2026-10-17T04:37:55Z

+      savings +=
+        int64_t(getArrayNewFixedSize(array)) -
+        int64_t(getArrayNewDataSize(segment, offset, array->values.size()));


Using C-style or functional-style casts like int64_t(...) is discouraged in modern C++. It is safer and more idiomatic to use static_cast<int64_t> per the Google C++ Style Guide.

Suggested change

savings +=

int64_t(getArrayNewFixedSize(array)) -

int64_t(getArrayNewDataSize(segment, offset, array->values.size()));

savings +=

static_cast<int64_t>(getArrayNewFixedSize(array)) -

static_cast<int64_t>(getArrayNewDataSize(segment, offset, array->values.size()));

gemini-code-assist · 2026-10-17T04:37:55Z

+          builder.makeConst(int32_t(conversion.offset)),
+          builder.makeConst(int32_t(array->values.size())));


Using C-style or functional-style casts like int32_t(...) is discouraged in modern C++. It is safer and more idiomatic to use static_cast<int32_t> per the Google C++ Style Guide.

Suggested change

builder.makeConst(int32_t(conversion.offset)),

builder.makeConst(int32_t(array->values.size())));

builder.makeConst(static_cast<int32_t>(conversion.offset)),

builder.makeConst(static_cast<int32_t>(array->values.size())));

gkdn added 4 commits October 16, 2026 17:16

gkdn requested a review from a team as a code owner October 17, 2026 04:35

gkdn requested review from tlively and removed request for a team October 17, 2026 04:35

gemini-code-assist Bot reviewed Oct 17, 2026

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Add a --constant-arrays-to-data pass - #9274

Add a --constant-arrays-to-data pass#9274
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gkdn:constant-arrays-to-data

gkdn commented Oct 17, 2026 •

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		if (options.lastOpts && wasm->features.hasGC() &&
		(options.optimizeLevel >= 2 \|\| options.shrinkLevel >= 1)) {

	size_t size = getWrittenSize(U32LEB(curr->values.size()));
	size_t size = getWrittenSize(U32LEB(static_cast<uint32_t>(curr->values.size())));

	size_t size = 1 + getWrittenSize(U32LEB(dataSize));
	size_t size = 1 + getWrittenSize(U32LEB(static_cast<uint32_t>(dataSize)));

		auto savings = getCodeSavings(group, newSegmentIndex, offset) -
		int64_t(group.payload.size());

		newSegmentSavings <=
		int64_t(getNewSegmentOverhead(*module, newSegmentData.size()))) {

		builder.makeConst(int32_t(conversion.offset)),
		builder.makeConst(int32_t(array->values.size())));

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gkdn commented Oct 17, 2026 •

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