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4f44014c5c
fix bitcast of single-element SIMD vectors in effect this reverts https://github.com/rust-lang/rust/pull/142768 and adds additional tests. That PR relaxed the conditions on an early return in an incorrect way that would create broken LLVM IR. https://godbolt.org/z/PaaGWTv5a ```rust #![feature(repr_simd)] #[repr(simd)] #[derive(Clone, Copy)] struct S([i64; 1]); #[no_mangle] pub extern "C" fn single_element_simd(b: S) -> i64 { unsafe { std::mem::transmute(b) } } ``` at the time of writing generates this LLVM IR, where the type of the return is different from the function's return type. ```llvm define noundef i64 ``````@single_element_simd(<1`````` x i64> %b) unnamed_addr { start: ret <1 x i64> %b } ``` The test output is actually the same for the existing tests, showing that the change didn't actually matter for any tested behavior. It is probably a bit faster to do the early return, but, well, it's incorrect in general. zullip thread: [#t-compiler > Is transmuting a `T` to `Tx1` (one-element SIMD vector) UB?](https://rust-lang.zulipchat.com/#narrow/channel/131828-t-compiler/topic/Is.20transmuting.20a.20.60T.60.20to.20.60Tx1.60.20.28one-element.20SIMD.20vector.29.20UB.3F/with/526262799) cc ``````@sayantn`````` r? ``````@scottmcm``````
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