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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``````
The files here use the LLVM FileCheck framework, documented at https://llvm.org/docs/CommandGuide/FileCheck.html.
One extension worth noting is the use of revisions as custom prefixes for FileCheck. If your codegen test has different behavior based on the chosen target or different compiler flags that you want to exercise, you can use a revisions annotation, like so:
// revisions: aaa bbb
// [bbb] compile-flags: --flags-for-bbb
After specifying those variations, you can write different expected, or
explicitly unexpected output by using <prefix>-SAME: and <prefix>-NOT:,
like so:
// CHECK: expected code
// aaa-SAME: emitted-only-for-aaa
// aaa-NOT: emitted-only-for-bbb
// bbb-NOT: emitted-only-for-aaa
// bbb-SAME: emitted-only-for-bbb