Open
Bug 531915
Opened 11 years ago
Updated 1 year ago
Floating point differences between platforms
Categories
(Core :: JavaScript Engine, defect)
Tracking
()
NEW
People
(Reporter: philip, Unassigned)
Details
(Whiteboard: [fingerprinting])
With a 32bit Linux build of SM tip: js> Math.exp(10) 22026.465794806725 js> Math.sin(Math.PI) 1.2246063538223773e16 js> Math.cos(Math.PI/2) 6.123031769111886e17 js> Math.tan(Math.PI) 1.2246063538223773e16 js> Math.atan2(1e310, 2) 5e311 js> Math.pow(Math.PI, 100) 1.9275814160560332e50 With a 64bit Linux build, all the answers are slightly different: js> Math.exp(10) 22026.465794806718 js> Math.sin(Math.PI) 1.2246467991473532e16 js> Math.cos(Math.PI/2) 6.123233995736766e17 js> Math.tan(Math.PI) 1.2246467991473532e16 js> Math.atan2(1e310, 2) 4.9999999999997e311 js> Math.pow(Math.PI, 100) 1.9275814160560206e50 Also I see some bigger differences between Linux and Windows (testing in Firefox 3.5 on both, since I don't have a convenient newer SpiderMonkey on Windows): Math.tan(1e300) = 4.802497308567303 (Linux), 4.987183803371025 (Windows) (plus similar things with other trig functions) 1/((0) % 1.5) = Infinity (Linux), Infinity (Windows) I'd personally like (for embedding) to have identical output for all conceivable inputs across 32 and 64bit Linux, Windows and OS X (but I don't really care what output, and I could just delete some of the functions if necessary).
Comment 1•11 years ago


Adding our resident IEEE double gurus. The differences look like permitted rounding differences to me (we use the platform's FPU hardware, and its allowed to produce a 1bit difference as per spec). Waldo, do you concur?
Comment 2•11 years ago


(In reply to comment #1) > 1/((0) % 1.5) = Infinity (Linux), Infinity (Windows) That looks like a bug in js_fmod from jslibmath.h
Comment 3•11 years ago


With respect to the function offbyone differences: ECMAScript never required precise behavior for any of these functions except for when given certain mathematically exact inputs or by specifying some quality of the result when given inputs comparing in a precise way to some mathematically exact boundary (mathematically exact inputs/boundaries: +0, 0, 1, 1, infinities, NaN, etc. but not Math.PI or similar approximate values). None of these inputs here are exact in the necessary manner, so our behavior is permitted by ES3/ES5. It would be nice to have exactly consistent results across architectures, but unless we revert back to a fdlibmstyle implementation we use everywhere, I don't see it happening. We're not going to munge FP mode so as to avoid or ensure extra precision for a leastsignificant digit difference.
Comment 4•11 years ago


I agree with comment 2. (zero % finite) is clearly specified as zero for either zero value.
Comment 5•11 years ago


Yeah looking at #2 right now.
Comment 6•11 years ago


1/((0) % 1.5) is Infinity on my windows build with msvc8. Philip what version of ff did you use for testing this?
Reporter  
Comment 7•11 years ago


I was using Firefox 3.5.3. But I just built a new version of the shell from Hg on Windows and can't reproduce the problem  I guess it was fixed in bug 503595, so that's okay. The sin/cos/tan cases are not just leastsignificantbit differences in the output, they differ around the 6th decimal digit. I accept they're not violating specs and are not behaving unreasonably, though.
Comment 8•11 years ago


We use libm these days. We used to use fdlibm for crossplatform consistency but it was slow compared to OS library implementations, and the code was crufty. Are the sin/cos/tan differences legit? I crave a numericslawyer's opinion ;). /be
Comment 9•11 years ago


Hm, nonMSB differences are more sadmaking. I don't think we should actually do anything about it for 1e6order differences, tho, at least not unless some more satisfactory solution than shipping our own unoptimized libm functions is possible.
Assignee  
Updated•6 years ago

Assignee: general → nobody
Whiteboard: [fingerprinting]
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Description
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