Avoid stack -> Box copy when data can be put in Box directly #101829
Description
In https://bugzilla.mozilla.org/show_bug.cgi?id=1787715 (currently not public for security reasons) we ran into a stack overflow when flate2 calls Box::<CompressorOxide>>::default() in DeflateBackend::make() here:
https://github.com/rust-lang/flate2-rs/blob/cc5ed7f817cc5e5712b2bb924ed19cab4f389a47/src/ffi/rust.rs#L131
DeflateBackend::make() is getting compiled with a call to chkstk(65664)
A reduced version of this is here:
https://rust.godbolt.org/z/hTE3785xY
pub const LZ_CODE_BUF_SIZE: usize = 64 * 1024; struct LZOxide { pub codes: [u8; LZ_CODE_BUF_SIZE], } impl LZOxide { #[inline(never)] const fn new() -> Self { LZOxide { codes: [0; LZ_CODE_BUF_SIZE], } } } pub struct CompressorOxide { lz: LZOxide, huff: Box<u32>, pub codes: [u8; LZ_CODE_BUF_SIZE], } impl Default for CompressorOxide { #[inline(always)] fn default() -> Self { CompressorOxide { lz: LZOxide::new(), huff: Box::default(), codes: [0; LZ_CODE_BUF_SIZE], } } } pub fn f() -> Box<CompressorOxide> { Box::default() }A further reduced version that doesn't require any of the special box syntax in Default() is here:
https://rust.godbolt.org/z/8e5EPafzb
pub const LZ_CODE_BUF_SIZE: usize = 64 * 1024; struct LZOxide { pub codes: [u8; LZ_CODE_BUF_SIZE], } impl LZOxide { #[inline(never)] const fn new() -> Self { LZOxide { codes: [0; LZ_CODE_BUF_SIZE], } } } pub struct CompressorOxide { lz: LZOxide, huff: Box<u32>, //pub codes: [u8; LZ_CODE_BUF_SIZE], } impl Default for CompressorOxide { #[inline(always)] fn default() -> Self { CompressorOxide { lz: LZOxide::new(), huff: Box::default(), //codes: [0; LZ_CODE_BUF_SIZE], } } } use std::ptr; use std::alloc::{Layout, alloc}; pub fn g() -> Box<CompressorOxide> { let layout = Layout::new::<CompressorOxide>(); let ptr = unsafe { alloc(layout) as *mut CompressorOxide }; assert!(!ptr.is_null()); unsafe { ptr::write(ptr, CompressorOxide { lz: LZOxide::new(), huff: Box::default(), }); Box::from_raw(ptr)} }This seems to have to do with LZOxide::new() not being inlined. If it's inlined the problem goes away. If the field initialization order of CompressorOxide is swapped
so that we call Box::<u32>::default() before LZOxide::new() the problem also goes away.