Where in the V8 source does the automatic cast for BinaryOperation occour?
I stumbled again in the good old '12' + 2 = '122'
I wanted to deeply understand what happens here, so my first thesis was that
Maybe Javascript casts the right operand to the type of the first one and
then operates, like so:'12' + String(2) = '122'all good...
But no, because 12 + '2' = '122' too; So the engine's magic is clearly favoring to concat over casting to number.
My second thesis was then
Maybe the engine enumerates all operands and looks for an "operator override", similar to C#? And then favor executing that over doing the self-magic thing?
My confusion got even weirder when I realized that also '5' * '8' = 40, it casts both operands to Number and does the operation.
The only way I could possibly really understand that was to read the V8 code directly from GitHub
The farther I could track down was at v8/src/parsing/parser-base.h line 2865
// We have a "normal" binary operation.
x = factory()->NewBinaryOperation(op, x, y, pos);
if (op == Token::OR || op == Token::AND) {
impl()->RecordBinaryOperationSourceRange(x, right_range);
}
From here I got lost, because I couldn't find where this factory() is coming from.
Long story short, where does the JavaScript "type Magic" come from in the V8 Engine Source code?
v8
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
add a comment |
I stumbled again in the good old '12' + 2 = '122'
I wanted to deeply understand what happens here, so my first thesis was that
Maybe Javascript casts the right operand to the type of the first one and
then operates, like so:'12' + String(2) = '122'all good...
But no, because 12 + '2' = '122' too; So the engine's magic is clearly favoring to concat over casting to number.
My second thesis was then
Maybe the engine enumerates all operands and looks for an "operator override", similar to C#? And then favor executing that over doing the self-magic thing?
My confusion got even weirder when I realized that also '5' * '8' = 40, it casts both operands to Number and does the operation.
The only way I could possibly really understand that was to read the V8 code directly from GitHub
The farther I could track down was at v8/src/parsing/parser-base.h line 2865
// We have a "normal" binary operation.
x = factory()->NewBinaryOperation(op, x, y, pos);
if (op == Token::OR || op == Token::AND) {
impl()->RecordBinaryOperationSourceRange(x, right_range);
}
From here I got lost, because I couldn't find where this factory() is coming from.
Long story short, where does the JavaScript "type Magic" come from in the V8 Engine Source code?
v8
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
add a comment |
I stumbled again in the good old '12' + 2 = '122'
I wanted to deeply understand what happens here, so my first thesis was that
Maybe Javascript casts the right operand to the type of the first one and
then operates, like so:'12' + String(2) = '122'all good...
But no, because 12 + '2' = '122' too; So the engine's magic is clearly favoring to concat over casting to number.
My second thesis was then
Maybe the engine enumerates all operands and looks for an "operator override", similar to C#? And then favor executing that over doing the self-magic thing?
My confusion got even weirder when I realized that also '5' * '8' = 40, it casts both operands to Number and does the operation.
The only way I could possibly really understand that was to read the V8 code directly from GitHub
The farther I could track down was at v8/src/parsing/parser-base.h line 2865
// We have a "normal" binary operation.
x = factory()->NewBinaryOperation(op, x, y, pos);
if (op == Token::OR || op == Token::AND) {
impl()->RecordBinaryOperationSourceRange(x, right_range);
}
From here I got lost, because I couldn't find where this factory() is coming from.
Long story short, where does the JavaScript "type Magic" come from in the V8 Engine Source code?
v8
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
I stumbled again in the good old '12' + 2 = '122'
I wanted to deeply understand what happens here, so my first thesis was that
Maybe Javascript casts the right operand to the type of the first one and
then operates, like so:'12' + String(2) = '122'all good...
But no, because 12 + '2' = '122' too; So the engine's magic is clearly favoring to concat over casting to number.
My second thesis was then
Maybe the engine enumerates all operands and looks for an "operator override", similar to C#? And then favor executing that over doing the self-magic thing?
My confusion got even weirder when I realized that also '5' * '8' = 40, it casts both operands to Number and does the operation.
The only way I could possibly really understand that was to read the V8 code directly from GitHub
The farther I could track down was at v8/src/parsing/parser-base.h line 2865
// We have a "normal" binary operation.
x = factory()->NewBinaryOperation(op, x, y, pos);
if (op == Token::OR || op == Token::AND) {
impl()->RecordBinaryOperationSourceRange(x, right_range);
}
From here I got lost, because I couldn't find where this factory() is coming from.
Long story short, where does the JavaScript "type Magic" come from in the V8 Engine Source code?
v8
v8
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
asked Nov 24 '18 at 1:35
FelypeFelype
1,77811327
1,77811327
add a comment |
add a comment |
1 Answer
1
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V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
add a comment |
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1 Answer
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1 Answer
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active
oldest
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active
oldest
votes
V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
add a comment |
V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
add a comment |
V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
V8 developer here.
There are several fast paths for various cases of addition and other operations in V8. If you want to study a canonical (slow, but complete) version, you can look for Object::Add in src/objects.cc.
That said, the source of truth here is not any given engine's implementation, but the JavaScript specification. What the + operator is supposed to do is defined here: https://tc39.github.io/ecma262/#sec-addition-operator-plus.
Any engine's implementation either does precisely that, or something that from the outside is indistinguishable from that -- otherwise it's a bug. It's not a coincidence that the implementation of Object::Add reads almost exactly like the spec ;-)
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
answered Nov 24 '18 at 2:33
jmrkjmrk
6,203828
6,203828
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
add a comment |
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
Awesome, this is precisely what I was looking for, tyvm.
– Felype
Nov 24 '18 at 3:21
add a comment |
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