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

[Nemanja Ivanovic via llvm-dev]

One might argue that 0 is "the value closest and no greater in magnitude
than the infinitely precise result" of rounding 0.9 to an integer. But this
seems like nitpicking.
I agree that Owen's suggestion of adding wording similar to the Wikipedia
entry is a reasonable way to disambiguate.

Conversely, we can simply use the wording from the C standard as that is
what the operation implements:

When a finite value of real floating type is converted to an integer type
other than _Bool,
the fractional part is discarded (i.e., the value is truncated toward
zero). If the value of
the integral part cannot be represented by the integer type, the behavior
is undefined.

On Fri, Sep 4, 2020 at 2:02 PM Stephen Neuendorffer via llvm-dev <
llvm-dev at lists.llvm.org> 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> wrote:
>
>>
>>
>> On Sep 4, 2020, at 6:26 AM, David Jones via llvm-dev <
>> 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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