Re-land mm_extract_epi fix (#898)

This reverts commit 311d56cd91609c1c1c0370cbd2ece8e3048653a5.

Co-authored-by: Jethro Beekman <jethro@fortanix.com>

library/stdarch/crates/core_arch/src/x86/avx2.rs

@@ -3743,9 +3743,14 @@ pub unsafe fn _mm256_xor_si256(a: __m256i, b: __m256i) -> __m256i {
 // This intrinsic has no corresponding instruction.
 #[rustc_args_required_const(1)]
 #[stable(feature = "simd_x86", since = "1.27.0")]
-pub unsafe fn _mm256_extract_epi8(a: __m256i, imm8: i32) -> i8 {
-    let imm8 = (imm8 & 31) as u32;
-    simd_extract(a.as_i8x32(), imm8 as u32)
+pub unsafe fn _mm256_extract_epi8(a: __m256i, imm8: i32) -> i32 {
+    let a = a.as_u8x32();
+    macro_rules! call {
+        ($imm5:expr) => {
+            simd_extract::<_, u8>(a, $imm5) as i32
+        };
+    }
+    constify_imm5!(imm8, call)
 }
 
 /// Extracts a 16-bit integer from `a`, selected with `imm8`. Returns a 32-bit
@@ -3759,9 +3764,14 @@ pub unsafe fn _mm256_extract_epi8(a: __m256i, imm8: i32) -> i8 {
 // This intrinsic has no corresponding instruction.
 #[rustc_args_required_const(1)]
 #[stable(feature = "simd_x86", since = "1.27.0")]
-pub unsafe fn _mm256_extract_epi16(a: __m256i, imm8: i32) -> i16 {
-    let imm8 = (imm8 & 15) as u32;
-    simd_extract(a.as_i16x16(), imm8)
+pub unsafe fn _mm256_extract_epi16(a: __m256i, imm8: i32) -> i32 {
+    let a = a.as_u16x16();
+    macro_rules! call {
+        ($imm4:expr) => {
+            simd_extract::<_, u16>(a, $imm4) as i32
+        };
+    }
+    constify_imm4!((imm8 & 15), call)
 }
 
 /// Extracts a 32-bit integer from `a`, selected with `imm8`.
@@ -6120,7 +6130,7 @@ mod tests {
         );
         let r1 = _mm256_extract_epi8(a, 0);
         let r2 = _mm256_extract_epi8(a, 35);
-        assert_eq!(r1, -1);
+        assert_eq!(r1, 0xFF);
         assert_eq!(r2, 3);
     }
 
@@ -6133,7 +6143,7 @@ mod tests {
         );
         let r1 = _mm256_extract_epi16(a, 0);
         let r2 = _mm256_extract_epi16(a, 19);
-        assert_eq!(r1, -1);
+        assert_eq!(r1, 0xFFFF);
         assert_eq!(r2, 3);
     }
 

library/stdarch/crates/stdarch-verify/tests/x86-intel.rs

@@ -323,8 +323,6 @@ fn verify_all_signatures() {
             // take a signed-integer. This breaks `_MM_SHUFFLE` for
             // `_mm_shuffle_ps`:
             "_mm_shuffle_ps" => continue,
-            // FIXME(#867)
-            "_mm256_extract_epi8" | "_mm256_extract_epi16" => continue,
             _ => {}
         }