core: disable ptr::swap_nonoverlapping_one's block optimization on …

…SPIR-V.

library/core/src/ptr/mod.rs

@@ -473,12 +473,23 @@ pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
 #[inline]
 #[rustc_const_unstable(feature = "const_swap", issue = "83163")]
 pub(crate) const unsafe fn swap_nonoverlapping_one<T>(x: *mut T, y: *mut T) {
- // Only apply the block optimization in `swap_nonoverlapping_bytes` for types
- // at least as large as the block size, to avoid pessimizing codegen.
- if mem::size_of::<T>() >= 32 {
- // SAFETY: the caller must uphold the safety contract for `swap_nonoverlapping`.
- unsafe { swap_nonoverlapping(x, y, 1) };
- return;
+ // NOTE(eddyb) SPIR-V's Logical addressing model doesn't allow for arbitrary
+ // reinterpretation of values as (chunkable) byte arrays, and the loop in the
+ // block optimization in `swap_nonoverlapping_bytes` is hard to rewrite back
+ // into the (unoptimized) direct swapping implementation, so we disable it.
+ // FIXME(eddyb) the block optimization also prevents MIR optimizations from
+ // understanding `mem::replace`, `Option::take`, etc. - a better overall
+ // solution might be to make `swap_nonoverlapping` into an intrinsic, which
+ // a backend can choose to implement using the block optimization, or not.
+ #[cfg(not(target_arch = "spirv"))]
+ {
+ // Only apply the block optimization in `swap_nonoverlapping_bytes` for types
+ // at least as large as the block size, to avoid pessimizing codegen.
+ if mem::size_of::<T>() >= 32 {
+ // SAFETY: the caller must uphold the safety contract for `swap_nonoverlapping`.
+ unsafe { swap_nonoverlapping(x, y, 1) };
+ return;
+ }
  }
 
  // Direct swapping, for the cases not going through the block optimization.