C23 specifies a new set of `roundeven` functions that round to the
nearest integral, with ties to even. It does not raise any floating
point exceptions.
This behavior is similar to two other functions:
1. `rint`, which rounds to the nearest integer respecting rounding mode
and possibly raising exceptions.
2. `nearbyint`, which is identical to `rint` except it may not raise
exceptions.
Technically `rint`, `nearbyint`, and `roundeven` all behave the same in
Rust because we assume default floating point environment. The backends
are allowed to lower to `roundeven`, however, so we should provide it in
case the fallback is needed.
Add the `roundeven` family here and convert `rint` to a function that
takes a rounding mode. This currently has no effect.
These don't have much content since they now use the generic
implementation. There will be more similar functions in the near future
(fminimum, fmaximum, fminimum_num, fmaximum_num); start the pattern of
combining similar functions now so we don't have to eventually maintain
similar docs across 24 different files.
Many routines have some form of handling for rounding mode and floating
point exceptions, which are implemented via a combination of stubs and
`force_eval!` use. This is suboptimal, however, because:
1. Rust does not interact with the floating point environment, so most
of this code does nothing.
2. The parts of the code that are not dead are not testable.
3. `force_eval!` blocks optimizations, which is unnecessary because we
do not rely on its side effects.
We cannot ensure correct rounding and exception handling in all cases
without some form of arithmetic operations that are aware of this
behavior. However, the cases where rounding mode is explicitly handled
or exceptions are explicitly raised are testable. Make this possible
here for functions that depend on `math::fenv` by moving the
implementation to a nonpublic function that takes a `Round` and returns
a `Status`.
Link: https://github.com/rust-lang/libm/issues/480
This produces better assembly, e.g. on aarch64:
.globl libm::u128_wmul
.p2align 2
libm::u128_wmul:
Lfunc_begin124:
.cfi_startproc
mul x9, x2, x0
umulh x10, x2, x0
umulh x11, x3, x0
mul x12, x3, x0
umulh x13, x2, x1
mul x14, x2, x1
umulh x15, x3, x1
mul x16, x3, x1
adds x10, x10, x14
cinc x13, x13, hs
adds x13, x13, x16
cinc x14, x15, hs
adds x10, x10, x12
cinc x11, x11, hs
adds x11, x13, x11
stp x9, x10, [x8]
cinc x9, x14, hs
stp x11, x9, [x8, rust-lang/libm#16]
ret
The original was ~70 instructions so the improvement is significant.
With these changes, the result is reasonably close to what LLVM
generates using `u256` operands [1].
[1]: https://llvm.godbolt.org/z/re1aGdaqY
For some reason, the upcoming limb changes in [1] seem to ignore the
black boxing when applied to the operator function. Changing to instead
black box the inputs appears to fix this.
[1]: https://github.com/rust-lang/libm/pull/503
With the correctly rounded implementation, we can reduce the ULP
requirement for `cbrt` to zero. There is still an override required for
`i586` because of the imprecise FMA.
We only round using nearest, but some incoming code has more handling of
rounding modes that would be nice to `match` on. Rather than checking
integer values, add an enum representation.
Usually `cargo binstall iai-callgrind-runner` handles apt dependencies.
However, the following has been happening:
Err:11 mirror+file:/etc/apt/apt-mirrors.txt noble-updates/main amd64 libc6-dbg amd64 2.39-0ubuntu8.3
404 Not Found [IP: 40.81.13.82 80]
E: Failed to fetch mirror+file:/etc/apt/apt-mirrors.txt/pool/main/g/glibc/libc6-dbg_2.39-0ubuntu8.3_amd64.deb 404 Not Found [IP: 40.81.13.82 80]
Fetched 19.8 MB in 6s (3138 kB/s)
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?
Installing the dependencies manually seems to resolve the issue.
Introduce a version of generic `fma` that works when there is a larger
hardware-backed float type available to compute the result with more
precision. This is currently used only for `f32`, but with some minor
adjustments it should work for `f16` as well.
When `fmaf128` was introduced in [1], it included a bug where `self`
gets returned rather than the expected minimum positive value. Resolve
this and add a regression test.
[1]: https://github.com/rust-lang/libm/pull/494
Add checks at the max subnormal value and a couple values scatted
throughout the subnormal range. This helped identifiy a bug in
`fmaf128`.
As part of this, slightly reduce the amount of edge cases checked
without optimizations because the change makes it become noticible.
To ensure we don't waste time running extensive tests when there is an
easily identifiable failure, run the normal test suite for relevant
functions before starting extensive tests.
Adjust paths such that these macros don't go through the private `math`
module. `feature = "private-test-deps"` is still needed.
Additionally, ensure that `cargo check` for this crate gets run in CI
because `cargo test` does not seem to identify this problem.
`compiler_builtins` will need to reexport the `support` module.
We need someplace to collect known failures, previous regressions, edge
cases that are difficult to construct from generics, and similar.
Introduce this here.
A few bugs have been fixed, including the sign of `fma(tiny, -tiny,
0.0)`. Switch to tracking `master` rather than the latest tag so we
don't need to xfail these tests.
Currently, `fma(tiny, -tiny, 0.0)` returns 0.0 while the answer should
be -0.0. This is because `-0.0 + 0.0 = +0.0` in the default rounding
mode; however, the result should be negative. Musl has the same pattern
but that version worked because the C compiler was contracting `x*y + z`
to (ironically) `fmadd`.
Musl was fixed in 9683bd6241 ("math: fix fma(x,y,0) when x*y rounds to
-0"). Add the same fix here, which allows dropping the xfails.
Now that we have a hex float formatter, make use of it for test output.
This produces values that are easier to read than the bitwise hex
representation.
Example:
thread 'mp_quickspace_fmaf128' panicked at crates/libm-test/tests/multiprecision.rs:17:48:
called `Result::unwrap()` on an `Err` value:
input: (0xe38d71c71c71c71c71c71c71c71c71c8, 0xe38d71c71c71c71c71c71c71c71c71c8, 0xffff0000000000000000000000000000)
as hex: (-0x1.71c71c71c71c71c71c71c71c71c8p+9102, -0x1.71c71c71c71c71c71c71c71c71c8p+9102, -inf)
as bits: (0xe38d71c71c71c71c71c71c71c71c71c8, 0xe38d71c71c71c71c71c71c71c71c71c8, 0xffff0000000000000000000000000000)
expected: 0xffff0000000000000000000000000000 -inf 0xffff0000000000000000000000000000
actual: 0x7fff8000000000000000000000000000 NaN 0x7fff8000000000000000000000000000
Caused by:
real value != NaN
In order to make these more interchangeable in more places, always
return `(impl Iterator, u64)`. This will facilitate using other
generators for extensive tests.
This is the first step toward making `fma` usable for `f128`, and
possibly `f32` on platforms where growing to `f64` is not fast. This
does not yet work for anything other than `f64`.
This crate has a handful of lists that need to list all API and can't
easily be verified. Additionally, some longer lists should be kept
sorted so they are easier to look through. Resolve both of these by
adding a check in `update-api-list.py` that looks for annotations and
verifies the contents are as expected.
Annotations are `verify-apilist-start`, `verify-apilist-end`,
`verify-sorted-start`, and `verify-sorted-end`.
This includes fixes for anything that did not meet the criteria.
Use the generic `scalbn` to provide `f16` and `f128` versions, which
also work for `ldexp`.
This involves a new algorithm for `f16` because the default does not
converge fast enough with a limited number of rounds.
`EXP_MAX` sounds like it would be the maximum value representable by
that float type's exponent, rather than the maximum unsigned value of
its bits. Clarify this by renaming to `EXP_SAT`, the "saturated"
exponent representation.
Simplify the SPDX string to the user-facing version to make it easier for
users and tooling to understand. Contributions must still be `MIT OR Apache-2.0`.
[ add commit body with context - Trevor ]
