CppCon 2015: John Farrier “Demystifying Floating Point"

[@skilz8098]

The comment about stack overflow is fairly on point.

[@ehsanamini8501]

@4:31 and @8:10 why does -1^0 compute to zero? Shouldn't it be 1? Is it a typo or am I missing something?

[@ehsanamini8501]

Oh I get it now. 1 represents a negative number, 0 a positive number.

[@janasandeep]

At 11:08 (slide 24), the binary representation of 1.0e-37 shown here is different from what I got using visual studio: EA1C0802 (little endian). Why so? The latter does not seem to be a denormalized number.

[@soulstudiosmusic]

+sandeep jana See section where he mentions the three different types of floating point numbers available in VS.

[@dascandy]

There are the same amount of numbers between 0.5 and 0.25, between 0.25 and 0.125, between 0.125 and 0.0625, ... so if you count the number of floats between 0 and 1, that's a lot more than between 1 and 2. In fact, there are 255 "groups" of numbers, each from N to N*2, each having 8388608 numbers.

[@justcurious1940]

u are totally correct but i just wanna say that we only have 254 groups for normal values and 1 group for NANs and 1 group for subnormals

[@alexloktionoff6833]

Can anybody provide a link to those 50 equations /*test-cases???*/ that must fork same on all IEEE754 machines?

[@richardcavell]

Kudos for the Guru Meditation at 6:15

[@soulstudiosmusic]

Perplexing subject for a good talk.

[@enhex]

Why 0.0 to 0.1 have more precision? Is it because every floating point number is unique, and there's a lot of overlaps up to 2^23?

[@hanyouchu4661]

The difference between float and double for the Kahan version of triangle area is due to input conversion from decimal to binary. If you cast the float input to double, the results are almost identical.

[@alexloktionoff6833]

oh yea, don't forget to put f or d suffixes to all constants in the code to avoid double rounding!

[@RajeshKumar28sep]

5:22, for 64 bits its should be 11 bit of exp

[@Silvertestrun]

Thank you!

[@georganatoly6646]

Very good talk. It wouldn't have occurred to me to group like exponentiation.

[@acmdz]

Oh, that's really helpful!

[@victornoagbodji]

great talk : )

[@CharIie83]

great talk

[@Calm_Energy]

Units in last place = ulps. This is a measurement I've never seen before. Some compilers can control how rounding works via options.

[@alexloktionoff6833]

but not all h/w follows bit exact rounding. +/-.5ulp what we can rely...

[@andik70]

16:56 'on the CPU math is done exactly and then rounded to give it back to you' did you really say that? Do you really mean that?

[@richardcavell]

Theoretically, that’s what happens. Think conceptually.

[@MaherBaba]

what u mean?

[@Xeverous]

-ffast-math not mentioned?

[@Courserasrikanthdrk]

ok,,,,,,,,,,,,,the talk has expressed very important maths topics :----------------------------------------------------------------------)

[@dascandy]

Your 64-bit floats occupy 65 bits.

[@pranavjain3905]

the exponent register should have 11 exponent bits

[@MaherBaba]

you see everything don't you

[@UpstreamNL]

Confusing talk. This guy is all over the place. Every slide in no way connects to the previous slide.