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Implement 'pack(n)' option #34

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Implement 'pack(n)' option #34

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b8844ab
Implement 'pack(n)' option
enlightenedmouse Jan 22, 2019
d9394a4
Removes ES2016 specific Array method.
enlightenedmouse Jan 22, 2019
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37 changes: 28 additions & 9 deletions lib/struct.js
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Original file line number Diff line number Diff line change
Expand Up @@ -122,10 +122,14 @@ function Struct () {
StructType.size = 0
StructType.alignment = 0
StructType.indirection = 1
StructType.isPacked = opt.packed ? Boolean(opt.packed) : false
StructType.pack = typeof(opt.pack) === "number" ? opt.pack : opt.packed ? 1 : 0;
StructType.get = get
StructType.set = set

if( StructType.pack ) {
assert([1,2,4,8,16].indexOf(StructType.pack)>=0,'pack value must be 1, 2, 4, 8, or 16')
}

// Read the fields list and apply all the fields to the struct
// TODO: Better arg handling... (maybe look at ES6 binary data API?)
var arg = arguments[0]
Expand Down Expand Up @@ -247,13 +251,17 @@ function recalc (struct) {
if (type.indirection > 1) {
alignment = ref.alignof.pointer
}
if (struct.isPacked) {
struct.alignment = Math.min(struct.alignment || alignment, alignment)
} else {
struct.alignment = Math.max(struct.alignment, alignment)
}
// the alignment of the structure will be as that of its largest member
struct.alignment = Math.max(struct.alignment, alignment)
})

if( struct.pack ) {
// if packed, the alignment of the structure will be that of the n-byte
// boundary or the alignment of the largest member of the structure; whichever
// is smaller
struct.alignment = Math.min(struct.alignment, struct.pack);
}

// second loop through sets the `offset` property on each "field"
// object, and sets the `struct.size` as we go along
fieldNames.forEach(function (name) {
Expand All @@ -276,12 +284,19 @@ function recalc (struct) {
function addType (type) {
var offset = struct.size
var align = type.indirection === 1 ? type.alignment : ref.alignof.pointer
var padding = struct.isPacked ? 0 : (align - (offset % align)) % align
if( struct.pack ) {
// #pragma pack(n)
// https://docs.microsoft.com/en-us/cpp/preprocessor/pack?view=vs-2017
// The alignment of a member will be on a boundary that is either a multiple
// of n or a multiple of the size of the member, whichever is smaller
align = Math.min(struct.pack, align);
}
var padding = (align - (offset % align)) % align
var size = type.indirection === 1 ? type.size : ref.sizeof.pointer

offset += padding

if (!struct.isPacked) {
if (!struct.pack) {
assert.equal(offset % align, 0, "offset should align")
}

Expand All @@ -292,7 +307,11 @@ function recalc (struct) {
return offset
}

// any final padding?
// http://www.catb.org/esr/structure-packing/#_structure_alignment_and_padding
// The stride address of a structure is the first address following the structure
// data that has the same alignment as the structure. The general rule of
// trailing structure padding is that the structure has trailing padding out to
// its stride address
var left = struct.size % struct.alignment
if (left > 0) {
debug('additional padding to the end of struct:', struct.alignment - left)
Expand Down
17 changes: 17 additions & 0 deletions test/struct.js
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Expand Up @@ -377,6 +377,23 @@ describe('Struct', function () {
test19.defineProperty('next', ref.refType(test19));
test(test19, 19);

var test20 = Struct({
a: 'char',
p: ref.refType('void')
}, {packed: true})
test(test20, 20)

var test21 = Struct({
a: 'char',
p: ref.refType('void')
})
test(test21, 21)

var test22 = Struct({
a: 'char',
p: ref.refType('void')
},{pack: 2})
test(test22, 22)
})

describe('packed struct', function () {
Expand Down
15 changes: 15 additions & 0 deletions test/struct_tests.cc
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Expand Up @@ -157,6 +157,12 @@ typedef struct _test21 {
void *p;
} test21;

#pragma pack(2)
typedef struct _test22 {
char a;
void *p;
} test22;

void Initialize(v8::Handle<v8::Object> target) {
Nan::HandleScope scope;

Expand Down Expand Up @@ -289,9 +295,18 @@ void Initialize(v8::Handle<v8::Object> target) {

target->Set(Nan::New<v8::String>("test20 sizeof").ToLocalChecked(), Nan::New<v8::Number>(sizeof(test20)));
target->Set(Nan::New<v8::String>("test20 alignof").ToLocalChecked(), Nan::New<v8::Number>(__alignof__(test20)));
target->Set(Nan::New<v8::String>("test20 offsetof a").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test20, a)));
target->Set(Nan::New<v8::String>("test20 offsetof p").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test20, p)));

target->Set(Nan::New<v8::String>("test21 sizeof").ToLocalChecked(), Nan::New<v8::Number>(sizeof(test21)));
target->Set(Nan::New<v8::String>("test21 alignof").ToLocalChecked(), Nan::New<v8::Number>(__alignof__(test21)));
target->Set(Nan::New<v8::String>("test21 offsetof a").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test21, a)));
target->Set(Nan::New<v8::String>("test21 offsetof p").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test21, p)));

target->Set(Nan::New<v8::String>("test22 sizeof").ToLocalChecked(), Nan::New<v8::Number>(sizeof(test22)));
target->Set(Nan::New<v8::String>("test22 alignof").ToLocalChecked(), Nan::New<v8::Number>(__alignof__(test22)));
target->Set(Nan::New<v8::String>("test22 offsetof a").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test22, a)));
target->Set(Nan::New<v8::String>("test22 offsetof p").ToLocalChecked(), Nan::New<v8::Number>(offsetof(test22, p)));
}

} // anonymous namespace
Expand Down