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interpret/memory: fix safety comment for large array memset optimization #120387

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Jan 30, 2024
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interpret/memory: fix safety comment for large array memset optimization #120387

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9 changes: 4 additions & 5 deletions compiler/rustc_const_eval/src/interpret/memory.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -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>>,
Expand Down Expand Up @@ -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());
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This line seems more dangerous and we don't have a comment on it at all 😆 I'll try to add one this week

} else if src_alloc_id == dest_alloc_id {
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