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Rollup merge of #120387 - RalfJung:large-array-followup, r=oli-obk

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interpret/memory: fix safety comment for large array memset optimization

Also fix the doc comment for `check_and_deref_ptr`.
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Guillaume Gomez 2024-01-30 11:19:14 +01:00 committed by GitHub
commit e0e96a1829
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1 changed files with 4 additions and 5 deletions
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9
compiler/rustc_const_eval/src/interpret/memory.rs
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@ -396,7 +396,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
/// to the allocation it points to. Supports both shared and mutable references, as the actual
/// checking is offloaded to a helper closure.
///
/// If this returns `None`, the size is 0; it can however return `Some` even for size 0.
/// Returns `None` if and only if the size is 0.
fn check_and_deref_ptr<T>(
&self,
ptr: Pointer<Option<M::Provenance>>,
@ -1214,10 +1214,9 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
let size_in_bytes = size.bytes_usize();
// For particularly large arrays (where this is perf-sensitive) it's common that
// we're writing a single byte repeatedly. So, optimize that case to a memset.
if size_in_bytes == 1 && num_copies >= 1 {
// SAFETY: `src_bytes` would be read from anyway by copies below (num_copies >= 1).
// Since size_in_bytes = 1, then the `init.no_bytes_init()` check above guarantees
// that this read at type `u8` is OK -- it must be an initialized byte.
if size_in_bytes == 1 {
debug_assert!(num_copies >= 1); // we already handled the zero-sized cases above.
// SAFETY: `src_bytes` would be read from anyway by `copy` below (num_copies >= 1).
let value = *src_bytes;
dest_bytes.write_bytes(value, (size * num_copies).bytes_usize());
} else if src_alloc_id == dest_alloc_id {