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Rollup merge of #133844 - RalfJung:simd_relaxed_fma-nondet, r=workingjubilee

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clarify simd_relaxed_fma non-determinism

This is the safer spec in the sense that it is more likely to be satisfied by the backend -- and if people are okay with a non-deterministic result, I assume they don't care whether it's the same choice across all lanes or not?

Cc ``@calebzulawski`` ``@workingjubilee``
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Jacob Pratt 2024-12-05 05:50:51 -05:00 committed by GitHub
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library/core/src/intrinsics/simd.rs
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@ -619,7 +619,8 @@ extern "rust-intrinsic" {
/// set has support for a fused operation, and that the fused operation is more efficient /// 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 /// 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 /// whether or not a fused operation is selected, and that may depend on optimization
/// level and context, for example. /// level and context, for example. It may even be the case that some SIMD lanes get fused
/// and others do not.
/// ///
/// `T` must be a vector of floats. /// `T` must be a vector of floats.
#[cfg(not(bootstrap))] #[cfg(not(bootstrap))]