Partially revert #868 #878
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This commit partially reverts rust-lang#868 to restore the intrinsics to their original implementation to avoid breaking changes. This is done while rust-lang/rust#73166 is running through crater, and should unblock rust-lang/rust#74482.
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jethrogb
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Sep 8, 2020
This reverts commit 311d56c.
bors
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Sep 15, 2020
Update stdarch
This PR **changes the public signature** of the following functions in `core::arch::{x86, x86_64}`:
```patch
-pub unsafe fn _mm256_extract_epi8(a: __m256i, imm8: i32) -> i8
+pub unsafe fn _mm256_extract_epi8(a: __m256i, imm8: i32) -> i32
-pub unsafe fn _mm256_extract_epi16(a: __m256i, imm8: i32) -> i16
+pub unsafe fn _mm256_extract_epi16(a: __m256i, imm8: i32) -> i32
```
This change is desired so that these signatures
* are similar to those of the 128-bit versions `_mm_extract_epi8` and `_mm_extract_epi16`
* match the Intel definitions for the intrinsics
* [RFC 2325](https://github.com/rust-lang/rfcs/blob/master/text/2325-stable-simd.md) specifies that the exact vendor function signatures should be used
A [crater run](rust-lang#73166 (comment)) revealed only a single breakage. The [vektor crate](https://github.com/AdamNiederer/vektor/blob/master/src/x86/avx2.rs#L2436-L2472) copied the incorrect signatures in `core` exactly to their own crate. The functions don't seem to be used by anyone anywhere.
Actual breakage is not expected, since due to the nature of the functions, users would generally write `_mm256_extract_epi8(...) as u8` or `_mm256_extract_epi16(...) as u16`.
See rust-lang/stdarch#868. Note that the changes from that stdarch PR have already partially landed in core after rust-lang/stdarch#878. This PR is now only about the remaining changes.
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This commit partially reverts #868 to restore the intrinsics to their
original implementation to avoid breaking changes. This is done while
rust-lang/rust#73166 is running through crater, and should unblock
rust-lang/rust#74482.