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Fix inaccurate round while being fast #7308

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Fix inaccurate round while being fast #7308

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77dc09b
Fix inaccurate round while being fast
raphlinus Oct 18, 2018
78f88f8
Improve algorithm, fix copy-paste bug
raphlinus Oct 18, 2018
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12 changes: 8 additions & 4 deletions src/library.js
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Original file line number Diff line number Diff line change
Expand Up @@ -1566,30 +1566,34 @@ LibraryManager.library = {

round__asm: true,
round__sig: 'dd',
round__deps: ['llvm_copysign_f64'],
round: function(d) {
d = +d;
return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
return +Math_trunc(d + +_llvm_copysign_f64(+0.49999999999999994, d));
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Note that this is handwritten JS code, and in particular, llvm_copysign_* is present here as a fallback, implementing it in JS when it can't be done otherwise. It's a slow code path, see https://github.com/kripken/emscripten/blob/incoming/src/library.js#L1559

To actually use copysign, this would need to be written in C, perhaps in ./system/lib/libc/extras.c, and it could use a copysign intrinsic there.

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I think the CI failures are because this function is marked as asm.js, and I don't think trunc is a math intrinsic there.

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Yeah, between these two things and f32, it sounds like writing it in C is the better choice.

I could do it here with a ternary and floor, but it would be slightly inferior wasm, with a branch. I'll have a try at the C version.

},

roundf__asm: true,
roundf__sig: 'ff',
roundf__deps: ['llvm_copysign_f64'],
roundf: function(d) {
d = +d;
return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
return +Math_trunc(d + +_llvm_copysign_f64(+0.49999999999999994, d));
},

llvm_round_f64__asm: true,
llvm_round_f64__sig: 'dd',
llvm_round_f64__deps: ['llvm_copysign_f64'],
llvm_round_f64: function(d) {
d = +d;
return d >= +0 ? +Math_floor(d + +0.5) : +Math_ceil(d - +0.5);
return +Math_trunc(d + +_llvm_copysign_f64(+0.49999999999999994, d));
},

llvm_round_f32__asm: true,
llvm_round_f32__sig: 'ff',
llvm_round_f32__deps: ['llvm_copysign_f64'],
llvm_round_f32: function(f) {
f = +f;
return f >= +0 ? +Math_floor(f + +0.5) : +Math_ceil(f - +0.5); // TODO: use fround?
return +Math_trunc(f + +_llvm_copysign_f64(+0.49999999999999994, f));
},

rintf__asm: true,
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