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logic is written up in https://github.com/rust-lang/rust/issues/49048
Also, update the documentation slightly
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`max` on floats returns the other argument if one of them is NaN, which
would be `0.0` in this case. This is unlike the C functions `fdim` and
`fdimf` which return NaN if either of their arguments is NaN.
https://doc.rust-lang.org/1.31.0/std/primitive.f32.html#method.max
https://en.cppreference.com/w/c/numeric/math/fdim
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Thanks to @LukasKalbertodt for catching this. Addresses a comment
raised in #55169 after it was merged.
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Added a #[must_use] annotation on copysign, per review feedback.
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I improved the f32 version and made a copy-paste error for f64.
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This patch adds a `copysign` function to the float primitive types.
It is an exceptionally useful function for writing efficient numeric
code, as it often avoids branches, is auto-vectorizable, and there
are efficient intrinsics for most platforms.
I think this might work as-is, as the relevant `copysign` intrinsic
is already used internally for the implementation of `signum`. It's
possible that an implementation might be needed in japaric/libm for
portability across all platforms, in which case I'll do that also.
Part of the work towards #55107
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This is recommend by [RFC 0505] and as far as I know, the only primitive
types without this heading.
[RFC 0505]: https://github.com/rust-lang/rfcs/blob/c892139be692586e0846fbf934be6fceec17f329/text/0505-api-comment-conventions.md#using-markdown
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Move "See also" disambiguation links for primitive types to top
Closes #50384.
<details>
<summary>Images</summary>
</details>
r? @steveklabnik
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Fixes https://github.com/rust-lang/rust/issues/49842.
Other resources:
- https://users.rust-lang.org/t/why-does-the-mul-add-method-produce-a-more-accurate-result-with-better-performance/1626
- https://en.wikipedia.org/wiki/Multiply%E2%80%93accumulate_operation
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Affected methods are `abs`, `signum`, and `powi`.
CC https://github.com/rust-lang/rust/issues/32110#issuecomment-379503183
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… previously in the unstable core::num::Float trait.
Per https://github.com/rust-lang/rust/issues/32110#issuecomment-379503183,
the `abs`, `signum`, and `powi` methods are *not* included for now
since they rely on LLVM intrinsics and we haven’t determined yet whether
those instrinsics lower to calls to libm functions on any platform.
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Tracking issue: #49048
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and included that it returns radians
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Expose float from_bits and to_bits in libcore.
These methods have no dependencies on libm and thus should be offered in libcore.
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Make float::from_bits transmute
See commit message for details.
See also this discussion here: https://github.com/rust-lang/rust/issues/40470#issuecomment-343803381
(may require libs team discussion before merging)
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The current implementation/documentation was made to avoid sNaN because of
potential safety issues implied by old/bad LLVM documentation. These issues
aren't real, so we can just make the implementation transmute (as permitted
by the existing documentation of this method).
Also the documentation didn't actually match the behaviour: it said we may
change sNaNs, but in fact we canonicalized *all* NaNs.
Also an example in the documentation was wrong: it said we *always* change
sNaNs, when the documentation was explicitly written to indicate it was
implementation-defined.
This makes to_bits and from_bits perfectly roundtrip cross-platform, except
for one caveat: although the 2008 edition of IEEE-754 specifies how to
interpet the signaling bit, earlier editions didn't. This lead to some platforms
picking the opposite interpretation, so all signaling NaNs on x86/ARM are quiet
on MIPS, and vice-versa.
NaN-boxing is a fairly important optimization, while we don't even guarantee
that float operations properly preserve signalingness. As such, this seems like
the more natural strategy to take (as opposed to trying to mangle the signaling
bit on a per-platform basis).
This implementation is also, of course, faster.
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This commit moves the `f32::cmath` and `f64::cmath` modules into the
`sys` module. Note that these are not publicly exported modules, simply
implementation details. These modules are already platform-specific with
shims on MSVC and this is mostly just a reflection of that reality. This
should also help cut down on `#[cfg]` traffic if platforms are brought on
which don't directly support these functions.
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This reverts commit c589f867f89d4e6e48c6602aed8e878208d4822f.
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This reverts commit f74c5d2e18e50c24de2cc1192bf2088cdaa61916.
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This reverts commit 576426a05a1a6cb33eece7082d7341b7c6bb5277.
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This reverts commit 2e34ff767113c6a15c5862b0646ca9ad7ffd81b1.
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This reverts commit b762283e57ff71f6763effb9cfc7fc0c7967b6b0.
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IEEE 754-1985 specifies the encoding of NaN floating point numbers,
but while it mentions that NaNs can be subdivided into signaling
and quiet ones, it doesn't fix the encoding of signaling NaNs in binary
formats. This led to different implementations (CPUs) having different
encodings. IEEE 754-2008 finally specified the encoding of signaling NaNs
but some architectures are compatible with it, while others aren't.
Certain MIPS and PA-RISC CPUs have different encodings for signaling
NaNs.
In order to have the float <-> binary cast feature of the std library be
portable to them, we don't mask any quiet NaNs like we did before (only
being compliant to IEEE 754-2008 and nothing else), but instead we
simply pass a known good NaN instead.
Note that in the code removed there was a bug; the 64 bit mask for quiet
NaNs should have been `0x0008000000000000` instead of the specified
`0x0001000000000000`.
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Fix NaN handling in is_sign_negative/positive
This would be my proposed fix for the #42425 provided we decide it is indeed a problem.
Note this would technically be a breaking change to a stable API. We might want to consider deprecating these methods and adding new ones.
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See #42425
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Re-implement float min/max in rust
This also adds the relevant implementations into libcore.
See #42423
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See #42423
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See #42629 and #42630.
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[unstable, deprecated since 1.11.0]
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