[llvm-dev] Misleading documentation on FP to integer conversion instructions?

[Neil Nelson via llvm-dev]

Rounding towards zero
https://en.wikipedia.org/wiki/Rounding#Rounding_towards_zero

/Rounding/ appears to be a kind of generic term for a variety of 
operations not always with our more common sense.

Neil Nelson

On 9/4/20 12:02 PM, Stephen Neuendorffer via llvm-dev wrote:
>
> If fptosi takes 0.9 -> 0, then that is not 'rounding' in any sense I'm 
> aware of (IEEE754 or otherwise).
> Rounding (in the IEE754 sense) determines how a number is converted 
> when it is halfway between two
> candidate results.  (see round(), ceil(), floor()).
>
> fptosi seems to model the behavior of a C cast from float to int, 
> which truncates the fractional bits (as in trunc()).
>
> Steve
>
>
> On Fri, Sep 4, 2020 at 10:51 AM Owen Anderson via llvm-dev 
> <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>
>
>
>>     On Sep 4, 2020, at 6:26 AM, David Jones via llvm-dev
>>     <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>
>>     The LLVM IR reference manual states, for fptosi:
>>
>>     "The ‘|fptosi|’ instruction converts its floating-point
>>     <http://llvm.org/docs/LangRef.html#t-floating> operand into the
>>     nearest (rounding towards zero) signed integer value."
>>
>>     I interpreted this to mean that it rounds:
>>
>>     The nearest integer to 0.3 is 0.
>>
>>     The nearest integer to 0.9 is 1.
>>
>>     The nearest integer to 0.5 is either 0 or 1. And this is where
>>     the "rounding towards zero" applies - the result is prescribed to
>>     be 0.
>>
>>     In actuality, the instruction truncates, returning 0 for all cases.
>>
>>     Should this be reworded, perhaps to use the word "truncate”?
>
>     “Rounding towards zero” is a term of art from IEEE 754.  See
>     https://en.wikipedia.org/wiki/IEEE_754#Directed_roundings
>
>     I agree that the documentation could be made more accessible by
>     *also* using the colloquial term “truncation”, but
>     round-towards-zero is the correct name for this behavior and
>     should be retained.
>
>     —Owen
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