intrinsics::simd: document that masks must be signed integer vectors

this is because they may be widened, and that only works when sign extension is used: zero extension would produce invalid results

Author: folkertdev

Diff for: library/core/src/intrinsics/simd.rs

@@ -273,7 +273,7 @@ pub unsafe fn simd_shuffle<T, U, V>(_x: T, _y: T, _idx: U) -> V;
 ///
 /// `U` must be a vector of pointers to the element type of `T`, with the same length as `T`.
 ///
-/// `V` must be a vector of integers with the same length as `T` (but any element size).
+/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
 ///
 /// For each pointer in `ptr`, if the corresponding value in `mask` is `!0`, read the pointer.
 /// Otherwise if the corresponding value in `mask` is `0`, return the corresponding value from
@@ -294,7 +294,7 @@ pub unsafe fn simd_gather<T, U, V>(_val: T, _ptr: U, _mask: V) -> T;
 ///
 /// `U` must be a vector of pointers to the element type of `T`, with the same length as `T`.
 ///
-/// `V` must be a vector of integers with the same length as `T` (but any element size).
+/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
 ///
 /// For each pointer in `ptr`, if the corresponding value in `mask` is `!0`, write the
 /// corresponding value in `val` to the pointer.
@@ -318,7 +318,7 @@ pub unsafe fn simd_scatter<T, U, V>(_val: T, _ptr: U, _mask: V);
 ///
 /// `U` must be a pointer to the element type of `T`
 ///
-/// `V` must be a vector of integers with the same length as `T` (but any element size).
+/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, read the corresponding
 /// pointer offset from `ptr`.
@@ -341,7 +341,7 @@ pub unsafe fn simd_masked_load<V, U, T>(_mask: V, _ptr: U, _val: T) -> T;
 ///
 /// `U` must be a pointer to the element type of `T`
 ///
-/// `V` must be a vector of integers with the same length as `T` (but any element size).
+/// `V` must be a vector of signed integers with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, write the corresponding
 /// value in `val` to the pointer offset from `ptr`.
@@ -523,9 +523,9 @@ pub unsafe fn simd_bitmask<T, U>(_x: T) -> U;
 
 /// Selects elements from a mask.
 ///
-/// `M` must be an integer vector.
+/// `T` must be a vector.
 ///
-/// `T` must be a vector with the same number of elements as `M`.
+/// `M` must be a signed integer vector with the same length as `T` (but any element size).
 ///
 /// For each element, if the corresponding value in `mask` is `!0`, select the element from
 /// `if_true`.  If the corresponding value in `mask` is `0`, select the element from