Add mul_add_relaxed methods for floating-point types

[landsharkiest]

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Implements mul_add_relaxed for f16, f32, f64, and f128, which computes (self * a) + b with relaxed precision semantics. Unlike mul_add which guarantees a fused operation, this variant allows the compiler to choose between fused or separate operations based on target performance.

This fills the gap between the precision-guaranteed mul_add and the fully-optimizable algebraic operators, providing target-specific optimization while maintaining reasonable floating-point semantics.

Tracking issue: #151770

[rustbot]

r? @Mark-Simulacrum

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[RalfJung]

@landsharkiest friendly reminder that this PR is waiting for you to react to the reviewer's feedback and to fix CI. :)
if you no longer have time to work on this, that's fine -- just let us know so the task can be picked up by someone else.

[folkertdev]

Another thing, please update the doc comment of the intrinsics to point to the public methods, like here

/// The stabilized version of this intrinsic is 
/// [`f128::mul_add`](../../std/primitive.f128.html#method.mul_add) 

rust/library/core/src/intrinsics/mod.rs

Lines 1459 to 1480 in a6050b7

	/// Returns `a * b + c` for `f128` values.
	///
	/// The stabilized version of this intrinsic is
	/// [`f128::mul_add`](../../std/primitive.f128.html#method.mul_add)
	#[rustc_intrinsic_const_stable_indirect]
	#[rustc_intrinsic]
	#[rustc_nounwind]
	pub const fn fmaf128(a: f128, b: f128, c: f128) -> f128;
	/// Returns `a * b + c` for `f16` values, non-deterministically executing
	/// either a fused multiply-add or two operations with rounding of the
	/// intermediate result.
	///
	/// The operation is fused if the code generator determines that target
	/// instruction set has support for a fused operation, and that the fused
	/// operation is more efficient than the equivalent, separate pair of mul
	/// and add instructions. It is unspecified whether or not a fused operation
	/// is selected, and that may depend on optimization level and context, for
	/// example.
	#[rustc_intrinsic]
	#[rustc_nounwind]
	pub const fn fmuladdf16(a: f16, b: f16, c: f16) -> f16;

We use the prasing of "stabilized" here even if the methods are not actually stable.

[rustbot]

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

[landsharkiest]

@landsharkiest friendly reminder that this PR is waiting for you to react to the reviewer's feedback and to fix CI. :) if you no longer have time to work on this, that's fine -- just let us know so the task can be picked up by someone else.

Sorry got caught up in school and job searching, will finally put this away! :)

[JonathanBrouwer]

💔 I suspect this PR failed tests as part of a rollup
@bors r-

After fixing the problem, consider running a try job for the failed job before re-approving.

Link to failure: #159573 (comment)

[rust-bors]

This pull request was unapproved.

This PR was contained in a rollup (#159573), which was unapproved.

View changes since this unapproval

[Mark-Simulacrum]

@bors squash msg="Add mul_add_relaxed methods for floating-point types"

[rust-bors]

🔨 2 commits were squashed into ace8a46.

[Mark-Simulacrum]

@bors r+ rollup

[rust-bors]

📌 Commit ace8a46 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

🌲 The tree is currently closed for pull requests below priority 100. This pull request will be tested once the tree is reopened.

Reason for tree closure: spurious failures

[folkertdev]

We may as well try

@bors try jobs=dist-i586-gnu-i586-i686-musl

[rust-bors]

💔 Test for 2e2b7e3 failed: CI. Failed job:

• try - dist-i586-gnu-i586-i686-musl (web logs, enhanced plaintext logs)

[folkertdev]

Looks like the fix was not quite sufficient

@bors r-

[rust-bors]

This pull request was unapproved.

View changes since this unapproval

[scottmcm]

i586 is known sketchy for floats always, right? Could potentially just skip them for that...

[folkertdev]

Yeah some tests were already skipped since the last CI failure, seems like a bit more ignoring is needed.

https://triagebot.infra.rust-lang.org/gh-changes-since/rust-lang/rust/151793/234c31cd674e11703f15d290cba7ff81dfe8b4b8..3b7025f3a35e74943089a7e2dfe139bf2f821a89

[rust-log-analyzer]

The job dist-i586-gnu-i586-i686-musl failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

stderr:

thread 'main' (464263) panicked at library/core/src/num/f64.rs:16:1:
assertion failed: r == 9.020562075079397e-19 || r == 1.734723475976807e-18
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- library/core/src/num/f64.rs - f64::f64::mul_add_relaxed (line 1749) stdout end ----

failures:

[RalfJung]

Yeah if you check precise results then it's not surprising to see failures on that target.