Auto merge of #95317 - Jules-Bertholet:round_ties_to_even, r=pnkfelix,m-ou-se,scottmcm

Add `round_ties_even` to `f32` and `f64`

Tracking issue: #96710

Redux of #82273. See also #55107

Adds a new method, `round_ties_even`, to `f32` and `f64`, that rounds the float to the nearest integer , rounding halfway cases to the number with an even least significant bit. Uses the `roundeven` LLVM intrinsic to do this.

Of the five IEEE 754 rounding modes, this is the only one that doesn't already have a round-to-integer function exposed by Rust (others are `round`, `floor`, `ceil`, and `trunc`).  Ties-to-even is also the rounding mode used for int-to-float and float-to-float `as` casts, as well as float arithmentic operations. So not having an explicit rounding method for it seems like an oversight.

Bikeshed: this PR currently uses `round_ties_even` for the name of the method. But maybe `round_ties_to_even` is better, or `round_even`, or `round_to_even`?

1 files changed, 4 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/context.rs b/compiler/rustc_codegen_llvm/src/context.rs
index 3d29968d5d6..f0d729d4779 100644
--- a/compiler/rustc_codegen_llvm/src/context.rs
+++ b/compiler/rustc_codegen_llvm/src/context.rs
@@ -735,9 +735,13 @@ impl<'ll> CodegenCx<'ll, '_> {
 
         ifn!("llvm.copysign.f32", fn(t_f32, t_f32) -> t_f32);
         ifn!("llvm.copysign.f64", fn(t_f64, t_f64) -> t_f64);
+
         ifn!("llvm.round.f32", fn(t_f32) -> t_f32);
         ifn!("llvm.round.f64", fn(t_f64) -> t_f64);
 
+        ifn!("llvm.roundeven.f32", fn(t_f32) -> t_f32);
+        ifn!("llvm.roundeven.f64", fn(t_f64) -> t_f64);
+
         ifn!("llvm.rint.f32", fn(t_f32) -> t_f32);
         ifn!("llvm.rint.f64", fn(t_f64) -> t_f64);
         ifn!("llvm.nearbyint.f32", fn(t_f32) -> t_f32);