5 ms·

My guess is that this is what JIT people asked for.

by kanox 6y ago

amelius 6y ago
Don't sufficiently advanced compilers infer what the real type of a variable is, in the most important cases?
- chrisseaton 6y ago
  Many important cases are polymorphic so have to be able to handle both.
- robocat 6y ago
  First comment on question: “The JavaScript engine has to do this operation (which is called ToInt32 in the spec) whenver you apply a bitwise operator to a number and at various other times (unless the engine has been able to maintain the number as an integer as an optimization, but in many cases it cannot). – T.J. Crowder” Edit: From https://www.ecma-international.org/ecma-262/5.1/#sec-9.5 https://www.ecma-international.org/ecma-262/5.1/#sec-9.5 9.5 ToInt32: (Signed 32 Bit Integer) The abstract operation ToInt32 converts its argument to one of 2 integer values in the range −2³¹ through 2³¹−1, inclusive. This abstract operation functions as follows: Let number be the result of calling ToNumber on the input argument. If number is NaN, +0, −0, +∞, or −∞, return +0. Let posInt be sign(number) * floor(abs(number)). Let int32bit be posInt modulo 2³²; that is, a finite integer value k of Number type with positive sign and less than 2³² in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 2³². If int32bit is greater than or equal to 2³¹, return int32bit − 2³², otherwise return int32bit. NOTE Given the above definition of ToInt32: The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged. ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.) ToInt32 maps −0 to +0.
  - domenicd 6y ago
    Remember not to refer to outdated specs; the modern version is at https://tc39.es/ecma262/#sec-toint32 https://tc39.es/ecma262/#sec-toint32 . The changes look editorial modernizations (i.e., I don't think there have been any bugfixes to this low-level operation in the 9 years since ES 5.1 was published), but it's better to be safe than sorry, and build the right habits.
- olliej 6y ago
  We do, but there are still times when a double -> int conversion is necessary, this is true in every other language as well. The real problem is that JS inherited the x86 behavior, so everyone has to match that. The default ARM behavior is different. All this instruction does is perform a standard fpu operation, but instead of passing the current mode flags to the fpu, it passes a fixed set irrespective of the current processor mode. As far as I can tell, any performance win comes from removing the branches after the ToInt conversion that are normally used to match x86 behavior.
why_only_15 6y ago
The JSC JIT people seemed kind of surprised by this, which was weird. Maybe the V8 JIT people asked for it?
- monocasa 6y ago
  Is there a source for that? I was under the impression this initially shipped on iOS devices so it'd be weird for JSC to be surprised by it.
  - why_only_15 6y ago
    According to this bug it was shipped six months after the bug was created and they didn't seem super familiar with it.
    - monocasa 6y ago
      It was public knowledge that Apple had a shipped implementation using this instruction before that patch was merged. https://news.ycombinator.com/item?id=18163433 https://news.ycombinator.com/item?id=18163433 I also don't really see any indication that the project maintainers don't know about the instruction. Looks like an example of JSC not being developed in the open more than anything else.
      - why_only_15 6y ago
        are you referring to the tweet linked there? That tweet is wrong in a lot of ways, because that instruction gives you a 1-2% boost, not a 30% boost like they claimed.
        monocasa 6y ago
        They're talking about a 30% boost overall on a new release which is an amalgamation of many pieces, including use of this new instruction. Can you give an example of something the maintainers said that made them seem "not super familiar with it"?

My guess is that this is what JIT people asked for.

by kanox 6y ago

My guess is that this is what JIT people asked for.

amelius 6y ago

Don't sufficiently advanced compilers infer what the real type of a variable is, in the most important cases?

chrisseaton 6y ago
Many important cases are polymorphic so have to be able to handle both.
robocat 6y ago
First comment on question: “The JavaScript engine has to do this operation (which is called ToInt32 in the spec) whenver you apply a bitwise operator to a number and at various other times (unless the engine has been able to maintain the number as an integer as an optimization, but in many cases it cannot). – T.J. Crowder” Edit: From https://www.ecma-international.org/ecma-262/5.1/#sec-9.5 https://www.ecma-international.org/ecma-262/5.1/#sec-9.5 9.5 ToInt32: (Signed 32 Bit Integer) The abstract operation ToInt32 converts its argument to one of 2 integer values in the range −2³¹ through 2³¹−1, inclusive. This abstract operation functions as follows: Let number be the result of calling ToNumber on the input argument. If number is NaN, +0, −0, +∞, or −∞, return +0. Let posInt be sign(number) * floor(abs(number)). Let int32bit be posInt modulo 2³²; that is, a finite integer value k of Number type with positive sign and less than 2³² in magnitude such that the mathematical difference of posInt and k is mathematically an integer multiple of 2³². If int32bit is greater than or equal to 2³¹, return int32bit − 2³², otherwise return int32bit. NOTE Given the above definition of ToInt32: The ToInt32 abstract operation is idempotent: if applied to a result that it produced, the second application leaves that value unchanged. ToInt32(ToUint32(x)) is equal to ToInt32(x) for all values of x. (It is to preserve this latter property that +∞ and −∞ are mapped to +0.) ToInt32 maps −0 to +0.
- domenicd 6y ago
  Remember not to refer to outdated specs; the modern version is at https://tc39.es/ecma262/#sec-toint32 https://tc39.es/ecma262/#sec-toint32 . The changes look editorial modernizations (i.e., I don't think there have been any bugfixes to this low-level operation in the 9 years since ES 5.1 was published), but it's better to be safe than sorry, and build the right habits.
olliej 6y ago
We do, but there are still times when a double -> int conversion is necessary, this is true in every other language as well. The real problem is that JS inherited the x86 behavior, so everyone has to match that. The default ARM behavior is different. All this instruction does is perform a standard fpu operation, but instead of passing the current mode flags to the fpu, it passes a fixed set irrespective of the current processor mode. As far as I can tell, any performance win comes from removing the branches after the ToInt conversion that are normally used to match x86 behavior.

why_only_15 6y ago

The JSC JIT people seemed kind of surprised by this, which was weird. Maybe the V8 JIT people asked for it?

monocasa 6y ago
Is there a source for that? I was under the impression this initially shipped on iOS devices so it'd be weird for JSC to be surprised by it.
- why_only_15 6y ago
  According to this bug it was shipped six months after the bug was created and they didn't seem super familiar with it.
  - monocasa 6y ago
    It was public knowledge that Apple had a shipped implementation using this instruction before that patch was merged. https://news.ycombinator.com/item?id=18163433 https://news.ycombinator.com/item?id=18163433 I also don't really see any indication that the project maintainers don't know about the instruction. Looks like an example of JSC not being developed in the open more than anything else.
    - why_only_15 6y ago
      are you referring to the tweet linked there? That tweet is wrong in a lot of ways, because that instruction gives you a 1-2% boost, not a 30% boost like they claimed.
      - monocasa 6y ago
        They're talking about a 30% boost overall on a new release which is an amalgamation of many pieces, including use of this new instruction. Can you give an example of something the maintainers said that made them seem "not super familiar with it"?