Add documentation note about signed overflow direction#152179
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nickkuk wants to merge 1 commit intorust-lang:mainfrom
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Add documentation note about signed overflow direction#152179nickkuk wants to merge 1 commit intorust-lang:mainfrom
nickkuk wants to merge 1 commit intorust-lang:mainfrom
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In #151989 (comment) I noticed that signed overflow direction can be determined by returned wrapped value. It is not very obvious (especially, assuming additional
carry: boolsummand), but it is important if we want to add new leading (signed) limb to big integer in this case.Examples for small summands
x, y: i8with result extension:Here is general proof.
iN::carrying_add(x, y, c)returns(result, true)thenFirst case is overflowing below
iN::MINand we haveso we obtain$[0; s-1]$ which is exactly range of non-negative
iN.Second case is overflowing above
iN::MAXandthat is,$[-s,-1]$ which is exactly range of negative
iN.iN::borrowing_sub(x,y,b)returns(result, true)thenFirst case is overflowing below
iN::MINand we haveSecond case is overflowing above
iN::MAXand