Seems like a small surface-area with a large impact on compilers. At first glance, looks good to me.
Curiosity: Does this overlap with Brendan's work on value objects (i.e. will it become moot in the face of them) -- Yehuda Katz (ph) 718.877.1325 On Nov 2, 2012 12:49 PM, "David Herman" <dher...@mozilla.com> wrote: > I'd like to add a Math.imul function for doing 32-bit integer > multiplication. The use case is for compilers like Emscripten and Mandreel. > They generate code to emulate machine operations that can be efficiently > JIT compiled, but they don't currently have a good solution for doing > integer multiplication. For other operations, like addition, they can use > the composition of JS's floating-point addition and coercion to int32 via > bitwise or: > > t = (a+b)|0; > > But you can't do the same trick with multiplication, because multiplying > large numbers may lose low bits of precision when it overflows to double. > So Emscripten has a compiler switch where you can do the fast-but-incorrect > (a*b)|0 or you can do a slow-but-correct pure-JS multiply, which has to > split the operation into two multiplications with manual carry. As it turns > out, multiplication is the only major operation that doesn't have a clear > implementation pattern with existing JS operators. > > The semantics is straightforward: > > Math.imul(a, b) === ToInt32((ToUint32(a) x ToUint32(b)) mod 2^32) > > In other words, convert the two arguments to 32-bit integers and "do what > C does." The result is signed, simply as a convention -- signed and > unsigned integers are isomorphic (ToUint32 and ToInt32 form the bijection > between them). We could also consider adding Math.umul: > > Math.umul(a, b) === ToUint32((ToUint32(a) x ToUint32(b)) mod 2^32) > > but it's not strictly necessary; it's equivalent to: > > Math.imul(a, b)>>>0 > > It might be nice to have Math.umul available out of the box, but for > Emscripten and Mandreel, only one of the two is strictly necessary. > > At any rate, this function would be easy for JS engines to implement and > optimize, and code generators could benefit from it immediately on engines > that support it. > > I believe we could add this to the ES6 extended Math operations, because > it's extremely simple and well-understood; there's no design necessary. > > Dave > > _______________________________________________ > es-discuss mailing list > es-discuss@mozilla.org > https://mail.mozilla.org/listinfo/es-discuss >
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