I reviewed the current SIMD implementation (as given by vector.h and in
library.js). Unfortunately, the documentation on Javascript's
implementation of SIMD is somewhat sparse, so I am using the documentation
given in the polyfill implementation
(https://github.com/johnmccutchan/ecmascript_simd/blob/master/src/ecmascript_simd.js)
as a reference.
Some of the implementations in library.js do a lot of copying/casting, e.g.
emscripten_float32x4_and__inline: function(a, b) {
return
'SIMD.int32x4.bitsToFloat32x4(SIMD.int32x4.and(SIMD.float32x4.bitsToInt32x4('
+ a + '), SIMD.float32x4.bitsToInt32x4(' + b + ')))';
},
Given the fact that float32x4 and int32x4 objects are immutable, this has
noticable performance penalties. Also, looking at the polyfill
implementation of SIMD.float32x4.and(...), I don't think I understand the
rationale behind these copies:
/**
* @param {float32x4} a An instance of float32x4.
* @param {float32x4} b An instance of float32x4.
* @return {float32x4} New instance of float32x4 with values of a & b.
*/
SIMD.float32x4.and = function(a, b) {
var aInt = SIMD.float32x4.bitsToInt32x4(a);
var bInt = SIMD.float32x4.bitsToInt32x4(b);
return SIMD.int32x4.bitsToFloat32x4(SIMD.int32x4.and(aInt, bInt));
}
In other cases, the emscripten function signature is different from the
Javascript SIMD implementation. Like in case of
emscripten_float32x4_lessThan__inline: function(a, b) {
return 'SIMD.int32x4.bitsToFloat32x4(SIMD.float32x4.lessThan(' + a + ', ' +
b + '))';
},
which returns an float32x4 object, whereas the polyfill implementation returns
an int32x4:
/**
* @param {float32x4} t An instance of float32x4.
* @param {float32x4} other An instance of float32x4.
* @return {int32x4} 0xFFFFFFFF or 0x0 in each lane depending on
* the result of t < other.
*/
SIMD.float32x4.lessThan = function(t, other) {
var cx = t.x < other.x;
var cy = t.y < other.y;
var cz = t.z < other.z;
var cw = t.w < other.w;
return SIMD.int32x4.bool(cx, cy, cz, cw);
}
Can someone educate me why that is, please?
Soeren
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