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feat: Add memory.dataUTF8() built-in function #2974

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231750d
Add `memory.dataUTF8()` built-in function
geraintluff Jan 13, 2026
7f1f5a8
Add to `NOTICE`
geraintluff Jan 13, 2026
90eab32
Add `utf16ToUtf8()` helper, instead of trying to use `TextEncoder`
geraintluff Jan 15, 2026
5d54b3c
Trim UTF8 buffer to size after encoding
geraintluff Jan 15, 2026
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1 change: 1 addition & 0 deletions NOTICE
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Expand Up @@ -62,6 +62,7 @@ under the licensing terms detailed in LICENSE:
* Kam Chehresa <kaz.che@gmail.com>
* Mopsgamer <79159094+Mopsgamer@users.noreply.github.com>
* EDM115 <github@edm115.dev>
* Geraint Luff <luffgd@gmail.com>

Portions of this software are derived from third-party works licensed under
the following terms:
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73 changes: 73 additions & 0 deletions src/builtins.ts
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Expand Up @@ -749,6 +749,7 @@ export namespace BuiltinNames {
export const memory_copy = "~lib/memory/memory.copy";
export const memory_fill = "~lib/memory/memory.fill";
export const memory_data = "~lib/memory/memory.data";
export const memory_dataUTF8 = "~lib/memory/memory.dataUTF8";

// std/typedarray.ts
export const Int8Array = "~lib/typedarray/Int8Array";
Expand Down Expand Up @@ -3491,6 +3492,78 @@ function builtin_memory_data(ctx: BuiltinFunctionContext): ExpressionRef {
}
builtinFunctions.set(BuiltinNames.memory_data, builtin_memory_data);

function utf16ToUtf8(str: string) : Uint8Array {
let result = new Uint8Array(str.length*3); // max possible length is 1.5x the UTF16 encoding
let utf8Length: i32 = 0; // track actual encoded length
for (let i: i32 = 0; i < str.length; ++i) {
// UTF16 decode
let codePoint: u32 = str.charCodeAt(i);
if (codePoint >= 0xD800 && codePoint < 0xDC00) {
// could be the first half of a surrogate pair (if)
let codePoint2: u32 = str.charCodeAt(i + 1);
if (i + 1 < str.length && codePoint2 >= 0xDC00 && codePoint2 < 0xE000) {
// valid surrogate pair - combine to get the code-point
codePoint = ((codePoint&0x3FF)<<10) + (codePoint2&0x3FF) + 0x10000;
++i;
}
}
// UTF8 encode
if (codePoint < 0x0080) {
result[utf8Length++] = codePoint;
} else if (codePoint < 0x0800) {
result[utf8Length++] = 0xC0 + (codePoint>>6);
result[utf8Length++] = 0x80 + (codePoint&0x3F);
} else if (codePoint < 0x10000) {
result[utf8Length++] = 0xE0 + ((codePoint>>12)&0x0F);
result[utf8Length++] = 0x80 + ((codePoint>>6)&0x3F);
result[utf8Length++] = 0x80 + (codePoint&0x3F);
} else {
result[utf8Length++] = 0xF0 + ((codePoint>>18)&0x07);
result[utf8Length++] = 0x80 + ((codePoint>>12)&0x3F);
result[utf8Length++] = 0x80 + ((codePoint>>6)&0x3F);
result[utf8Length++] = 0x80 + (codePoint&0x3F);
}
}
let trimmed = new Uint8Array(utf8Length);
for (let i: i32 = 0; i < utf8Length; ++i) trimmed[i] = result[i];
return trimmed;
Comment on lines +3527 to +3529
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@MaxGraey MaxGraey Apr 6, 2026

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Suggested change
let trimmed = new Uint8Array(utf8Length);
for (let i: i32 = 0; i < utf8Length; ++i) trimmed[i] = result[i];
return trimmed;
return result.subarray(0, utf8Length);

}

// memory.dataUTF8(value) -> usize
function builtin_memory_dataUTF8(ctx: BuiltinFunctionContext): ExpressionRef {
let compiler = ctx.compiler;
let module = compiler.module;
if (
checkTypeAbsent(ctx) |
checkArgsRequired(ctx, 1)
) return module.unreachable();
let operands = ctx.operands;
let usizeType = compiler.options.usizeType;
let offset: i64;
let arg0 = operands[0];
if (!arg0.isLiteralKind(LiteralKind.String)) {
compiler.error(
DiagnosticCode.String_literal_expected,
arg0.range
);
return module.unreachable();
}
let str = (<StringLiteralExpression>arg0).value;
let array : Uint8Array = utf16ToUtf8(str);
let arrayNullTerminated = new Uint8Array(array.length + 1);
arrayNullTerminated.set(array);
offset = compiler.addAlignedMemorySegment(arrayNullTerminated, 1).offset;
// FIXME: what if recompiles happen? recompiles are bad.
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@geraintluff geraintluff Jan 13, 2026

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This FIXME is copied from the memory.data<T> implementation just above. Tbh I don't understand it, but if it's a problem there, then it's a problem here.

compiler.currentType = usizeType;
if (usizeType == Type.usize32) {
assert(!i64_high(offset));
return module.i32(i64_low(offset));
} else {
return module.i64(i64_low(offset), i64_high(offset));
}
}
builtinFunctions.set(BuiltinNames.memory_dataUTF8, builtin_memory_dataUTF8);

// === GC =====================================================================================

function builtin_i31_new(ctx: BuiltinFunctionContext): ExpressionRef {
Expand Down
2 changes: 2 additions & 0 deletions std/assembly/index.d.ts
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Expand Up @@ -1798,6 +1798,8 @@ declare namespace memory {
export function data(size: i32, align?: i32): usize;
/** Gets a pointer to a pre-initialized static chunk of memory. Alignment defaults to the size of `T`. Arguments must be compile-time constants. */
export function data<T>(values: T[], align?: i32): usize;
/** Gets a pointer to a pre-initialized static chunk of memory containing null-terminated UTF8. Value must be a compile-time constant. */
export function dataUTF8(value: string): usize;
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@MaxGraey MaxGraey Apr 6, 2026 •
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I think this API could be improved and instead dataUTF8 use more generalized text:

  enum TextEncoding { UTF8, UTF16 }

  declare namespace memory {
    ...
    export function text(value: string, encoding?: TextEncoding, nullTerminated?: bool): usize;
  }


export namespace atomic {
/** Performs a wait operation on a 32-bit integer value in memory suspending this agent if the condition is met. */
Expand Down
5 changes: 5 additions & 0 deletions std/assembly/memory.ts
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Expand Up @@ -76,6 +76,11 @@ export namespace memory {
// @ts-ignore: decorator
@builtin
export declare function data<T>(size: T, align?: i32): usize;

/** Gets a pointer to a null-terminated UTF8 constant in static memory. */
// @ts-ignore: decorator
@builtin
export declare function dataUTF8(str : string): usize;
}

// @ts-ignore: decorator
Expand Down
128 changes: 125 additions & 3 deletions tests/compiler/memory.debug.wat
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Expand Up @@ -4,9 +4,9 @@
(type $2 (func (param i32 i32 i32 i32)))
(import "env" "abort" (func $~lib/builtins/abort (param i32 i32 i32 i32)))
(global $memory/ptr (mut i32) (i32.const 80))
(global $~lib/memory/__data_end i32 (i32.const 212))
(global $~lib/memory/__stack_pointer (mut i32) (i32.const 32980))
(global $~lib/memory/__heap_base i32 (i32.const 32980))
(global $~lib/memory/__data_end i32 (i32.const 220))
(global $~lib/memory/__stack_pointer (mut i32) (i32.const 32988))
(global $~lib/memory/__heap_base i32 (i32.const 32988))
(memory $0 1)
(data $0 (i32.const 16) "\00\00\00\00")
(data $1 (i32.const 28) ",\00\00\00\00\00\00\00\00\00\00\00\02\00\00\00\12\00\00\00m\00e\00m\00o\00r\00y\00.\00t\00s\00\00\00\00\00\00\00\00\00\00\00")
Expand All @@ -33,6 +33,8 @@
(data $22 (i32.const 206) "\01")
(data $23 (i32.const 207) "\01")
(data $24 (i32.const 208) "\01")
(data $25 (i32.const 209) ":)\00")
(data $26 (i32.const 212) "\f0\9f\90\8c\00")
(table $0 1 1 funcref)
(elem $0 (i32.const 1))
(export "memory" (memory $0))
Expand Down Expand Up @@ -475,6 +477,126 @@
call $~lib/builtins/abort
unreachable
end
i32.const 209
global.set $memory/ptr
global.get $memory/ptr
i32.load8_u
i32.const 58
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 66
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 1
i32.add
i32.load8_u
i32.const 41
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 67
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 2
i32.add
i32.load8_u
i32.const 0
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 68
i32.const 1
call $~lib/builtins/abort
unreachable
end
i32.const 212
global.set $memory/ptr
global.get $memory/ptr
i32.load8_u
i32.const 240
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 71
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 1
i32.add
i32.load8_u
i32.const 159
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 72
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 2
i32.add
i32.load8_u
i32.const 144
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 73
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 3
i32.add
i32.load8_u
i32.const 140
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 74
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.const 4
i32.add
i32.load8_u
i32.const 0
i32.eq
i32.eqz
if
i32.const 0
i32.const 48
i32.const 75
i32.const 1
call $~lib/builtins/abort
unreachable
end
)
(func $~start
call $start:memory
Expand Down
98 changes: 98 additions & 0 deletions tests/compiler/memory.release.wat
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Expand Up @@ -14,6 +14,8 @@
(data $22 (i32.const 1214) "\01")
(data $23 (i32.const 1215) "\01")
(data $24 (i32.const 1216) "\01")
(data $25 (i32.const 1217) ":)")
(data $26 (i32.const 1220) "\f0\9f\90\8c")
(export "memory" (memory $0))
(start $~start)
(func $start:memory
Expand Down Expand Up @@ -256,6 +258,102 @@
global.set $memory/ptr
i32.const 1215
global.set $memory/ptr
i32.const 1217
global.set $memory/ptr
i32.const 1217
i32.load8_u
i32.const 58
i32.ne
if
i32.const 0
i32.const 1056
i32.const 66
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=1
i32.const 41
i32.ne
if
i32.const 0
i32.const 1056
i32.const 67
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=2
if
i32.const 0
i32.const 1056
i32.const 68
i32.const 1
call $~lib/builtins/abort
unreachable
end
i32.const 1220
global.set $memory/ptr
i32.const 1220
i32.load8_u
i32.const 240
i32.ne
if
i32.const 0
i32.const 1056
i32.const 71
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=1
i32.const 159
i32.ne
if
i32.const 0
i32.const 1056
i32.const 72
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=2
i32.const 144
i32.ne
if
i32.const 0
i32.const 1056
i32.const 73
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=3
i32.const 140
i32.ne
if
i32.const 0
i32.const 1056
i32.const 74
i32.const 1
call $~lib/builtins/abort
unreachable
end
global.get $memory/ptr
i32.load8_u offset=4
if
i32.const 0
i32.const 1056
i32.const 75
i32.const 1
call $~lib/builtins/abort
unreachable
end
)
(func $~start
call $start:memory
Expand Down
14 changes: 14 additions & 0 deletions tests/compiler/memory.ts
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Expand Up @@ -59,3 +59,17 @@ assert(ptr + 4 == (ptr = memory.data<u8>([1], 4)));
assert(ptr + 2 == (ptr = memory.data<u8>([1], 2)));
assert(ptr + 1 == (ptr = memory.data<u8>([1], 1)));
assert(ptr + 1 == memory.data<u8>([1], 16));

// Should correctly encode strings to UTF-8

ptr = memory.dataUTF8(":)");
assert(load<u8>(ptr) == 0x3A);
assert(load<u8>(ptr + 1) == 0x29);
assert(load<u8>(ptr + 2) == 0);

ptr = memory.dataUTF8("🐌");
assert(load<u8>(ptr) == 0xF0);
assert(load<u8>(ptr + 1) == 0x9F);
assert(load<u8>(ptr + 2) == 0x90);
assert(load<u8>(ptr + 3) == 0x8C);
assert(load<u8>(ptr + 4) == 0x00);