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Message-ID: <c5c08e3b-1e41-36-a0f5-324f4976b8df@esi.com.au>
Date: Wed, 19 Jun 2024 17:09:25 +1000 (AEST)
From: Damian McGuckin <damianm@....com.au>
To: MUSL <musl@...ts.openwall.com>
Subject: Re: roundf() (and round(), and ...)
Commenting on only part of that:
On Tue, 18 Jun 2024, Szabolcs Nagy wrote:
> it is unspecified if (uint32_t)a raises inexact exception,
> so this will be target and compiler dependent in practice.
Yes. Scary.
> ieee-754-2008:
>
> 5.8 Details of conversions from floating-point to integer formats
> ...
> A language standard that permits implicit conversions or expressions
> involving mixed types should require that these be implemented with
> the inexact-signaling conversion operations below.
IEEE-754 2019 now specifies:
The operations for conversion from floating-point to a specific
signed or unsigned integer format without signaling the inexact
exception are:
One of which is:
intFormatOf-convertToIntegerTowardZero(source)
convertToIntegerTowardZero(x) rounds x to an integral
value toward zero.
AND
The operations for conversion from floating-point to a specific
signed or unsigned integer format, signaling if inexact, are:
intFormatOf-convertToIntegerExactTowardZero(source)
convertToIntegerExactTowardZero(x) rounds x to an integral
value toward zero.
So, it depends as you say, which of those two operations the compiler
uses when it translates:
(integral-type) floating-point-expression
If indeed the target supports both.
> c23:
>
> F.4 Floating to integer conversion
> ...
> ... whether conversion of a non-integral floating value raises the
> "inexact" floating-point exception is unspecified.[note]
> ...
> note: In those cases where it matters, library functions can be used
> to effect such conversions with or without raising the "inexact"
> floating-point exception.
Ah, but then such functions are lower down those that we currently name.
Thanks - Damian
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