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Optimize large array creation in const-eval

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This changes repeated memcpy's to a memset for the case that we're
propagating a single byte into a region of memory.
This commit is contained in:
Mark Rousskov 2024-01-17 13:53:25 -05:00
parent c58a5da7d4
commit e68f3039d4
1 changed files with 19 additions and 12 deletions
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31
compiler/rustc_const_eval/src/interpret/memory.rs
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@ -1209,21 +1209,28 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
throw_ub_custom!(fluent::const_eval_copy_nonoverlapping_overlapping); throw_ub_custom!(fluent::const_eval_copy_nonoverlapping_overlapping);
} }
} }
}
for i in 0..num_copies { let size_in_bytes = size.bytes_usize();
ptr::copy( // For particularly large arrays (where this is perf-sensitive) it's common that
src_bytes, // we're writing a single byte repeatedly. So, optimize that case to a memset.
dest_bytes.add((size * i).bytes_usize()), // `Size` multiplication if size_in_bytes == 1 && num_copies >= 1 {
size.bytes_usize(), // 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.
let value = *src_bytes;
dest_bytes.write_bytes(value, (size * num_copies).bytes_usize());
} else if src_alloc_id == dest_alloc_id {
let mut dest_ptr = dest_bytes;
for _ in 0..num_copies {
ptr::copy(src_bytes, dest_ptr, size_in_bytes);
dest_ptr = dest_ptr.add(size_in_bytes);
} }
} else { } else {
for i in 0..num_copies { let mut dest_ptr = dest_bytes;
ptr::copy_nonoverlapping( for _ in 0..num_copies {
src_bytes, ptr::copy_nonoverlapping(src_bytes, dest_ptr, size_in_bytes);
dest_bytes.add((size * i).bytes_usize()), // `Size` multiplication dest_ptr = dest_ptr.add(size_in_bytes);
size.bytes_usize(),
);
} }
} }
} }