portable-simd: use simd_arith_offset to avoid ptr-int transmutation

Author: RalfJung

library/portable-simd/crates/core_simd/src/intrinsics.rs

@@ -61,6 +61,10 @@ extern "platform-intrinsic" {
     /// xor
     pub(crate) fn simd_xor<T>(x: T, y: T) -> T;
 
+    /// getelementptr (without inbounds)
+    #[cfg(not(bootstrap))]
+    pub(crate) fn simd_arith_offset<T, U>(ptrs: T, offsets: U) -> T;
+
     /// fptoui/fptosi/uitofp/sitofp
     /// casting floats to integers is truncating, so it is safe to convert values like e.g. 1.5
     /// but the truncated value must fit in the target type or the result is poison.

library/portable-simd/crates/core_simd/src/vector/ptr.rs

@@ -1,5 +1,8 @@
 //! Private implementation details of public gather/scatter APIs.
+#[cfg(not(bootstrap))]
+use crate::simd::intrinsics;
 use crate::simd::{LaneCount, Simd, SupportedLaneCount};
+#[cfg(bootstrap)]
 use core::mem;
 
 /// A vector of *const T.
@@ -21,12 +24,16 @@ where
     #[inline]
     #[must_use]
     pub fn wrapping_add(self, addend: Simd<usize, LANES>) -> Self {
+        #[cfg(bootstrap)]
         // Safety: converting pointers to usize and vice-versa is safe
         // (even if using that pointer is not)
         unsafe {
             let x: Simd<usize, LANES> = mem::transmute_copy(&self);
             mem::transmute_copy(&{ x + (addend * Simd::splat(mem::size_of::<T>())) })
         }
+        #[cfg(not(bootstrap))]
+        // Safety: this intrinsic doesn't have a precondition
+        unsafe { intrinsics::simd_arith_offset(self, addend) }
     }
 }
 
@@ -49,11 +56,15 @@ where
     #[inline]
     #[must_use]
     pub fn wrapping_add(self, addend: Simd<usize, LANES>) -> Self {
+        #[cfg(bootstrap)]
         // Safety: converting pointers to usize and vice-versa is safe
         // (even if using that pointer is not)
         unsafe {
             let x: Simd<usize, LANES> = mem::transmute_copy(&self);
             mem::transmute_copy(&{ x + (addend * Simd::splat(mem::size_of::<T>())) })
         }
+        #[cfg(not(bootstrap))]
+        // Safety: this intrinsic doesn't have a precondition
+        unsafe { intrinsics::simd_arith_offset(self, addend) }
     }
 }