alloc_system: don’t assume MIN_ALIGN for small sizes, fix #45955
The GNU C library (glibc) is documented to always allocate with an alignment
of at least 8 or 16 bytes, on 32-bit or 64-bit platforms:
https://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html
This matches our use of `MIN_ALIGN` before this commit.
However, even when libc is glibc, the program might be linked
with another allocator that redefines the `malloc` symbol and friends.
(The `alloc_jemalloc` crate does, in some cases.)
So `alloc_system` doesn’t know which allocator it calls,
and needs to be conservative in assumptions it makes.
The C standard says:
https://port70.net/%7Ensz/c/c11/n1570.html#7.22.3
> The pointer returned if the allocation succeeds is suitably aligned
> so that it may be assigned to a pointer to any type of object
> with a fundamental alignment requirement
https://port70.net/~nsz/c/c11/n1570.html#6.2.8p2
> A fundamental alignment is represented by an alignment less than
> or equal to the greatest alignment supported by the implementation
> in all contexts, which is equal to `_Alignof (max_align_t)`.
`_Alignof (max_align_t)` depends on the ABI and doesn’t seem to have
a clear definition, but it seems to match our `MIN_ALIGN` in practice.
However, the size of objects is rounded up to the next multiple
of their alignment (since that size is also the stride used in arrays).
Conversely, the alignment of a non-zero-size object is at most its size.
So for example it seems ot be legal for `malloc(8)` to return a pointer
that’s only 8-bytes-aligned, even if `_Alignof (max_align_t)` is 16.
2017-11-20 08:30:04 -06:00
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#![feature(allocator_api)]
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2015-03-10 23:58:16 -05:00
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#![feature(box_syntax)]
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2020-09-03 19:35:27 -05:00
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#![feature(cow_is_borrowed)]
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2020-09-17 13:02:56 -05:00
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#![feature(const_cow_is_borrowed)]
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2017-07-14 20:54:17 -05:00
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#![feature(drain_filter)]
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#![feature(exact_size_is_empty)]
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2019-10-16 13:33:55 -05:00
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#![feature(new_uninit)]
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2015-06-10 15:33:52 -05:00
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#![feature(pattern)]
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2020-07-24 02:39:09 -05:00
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#![feature(str_split_once)]
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2019-06-20 20:52:38 -05:00
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#![feature(trusted_len)]
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2018-03-08 08:36:43 -06:00
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#![feature(try_reserve)]
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2015-03-10 23:58:16 -05:00
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#![feature(unboxed_closures)]
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2019-08-08 17:33:57 -05:00
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#![feature(associated_type_bounds)]
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2019-10-01 05:16:51 -05:00
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#![feature(binary_heap_into_iter_sorted)]
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#![feature(binary_heap_drain_sorted)]
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2020-08-04 11:03:34 -05:00
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#![feature(slice_ptr_get)]
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#![feature(split_inclusive)]
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2020-04-23 14:07:50 -05:00
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#![feature(binary_heap_retain)]
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#![feature(deque_range)]
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#![feature(inplace_iteration)]
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#![feature(iter_map_while)]
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2020-09-08 14:39:13 -05:00
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#![feature(int_bits_const)]
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2020-10-09 06:06:04 -05:00
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#![feature(vecdeque_binary_search)]
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#![feature(slice_group_by)]
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2015-07-11 06:34:57 -05:00
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2016-09-28 19:23:36 -05:00
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use std::collections::hash_map::DefaultHasher;
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2019-12-22 16:42:04 -06:00
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use std::hash::{Hash, Hasher};
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2015-08-11 19:27:05 -05:00
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2018-07-06 12:30:09 -05:00
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mod arc;
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2015-03-10 23:58:16 -05:00
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mod binary_heap;
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2020-04-22 16:16:21 -05:00
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mod borrow;
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2019-10-16 13:33:55 -05:00
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mod boxed;
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mod btree_set_hash;
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mod cow_str;
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2015-03-10 23:58:16 -05:00
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mod fmt;
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alloc_system: don’t assume MIN_ALIGN for small sizes, fix #45955
The GNU C library (glibc) is documented to always allocate with an alignment
of at least 8 or 16 bytes, on 32-bit or 64-bit platforms:
https://www.gnu.org/software/libc/manual/html_node/Aligned-Memory-Blocks.html
This matches our use of `MIN_ALIGN` before this commit.
However, even when libc is glibc, the program might be linked
with another allocator that redefines the `malloc` symbol and friends.
(The `alloc_jemalloc` crate does, in some cases.)
So `alloc_system` doesn’t know which allocator it calls,
and needs to be conservative in assumptions it makes.
The C standard says:
https://port70.net/%7Ensz/c/c11/n1570.html#7.22.3
> The pointer returned if the allocation succeeds is suitably aligned
> so that it may be assigned to a pointer to any type of object
> with a fundamental alignment requirement
https://port70.net/~nsz/c/c11/n1570.html#6.2.8p2
> A fundamental alignment is represented by an alignment less than
> or equal to the greatest alignment supported by the implementation
> in all contexts, which is equal to `_Alignof (max_align_t)`.
`_Alignof (max_align_t)` depends on the ABI and doesn’t seem to have
a clear definition, but it seems to match our `MIN_ALIGN` in practice.
However, the size of objects is rounded up to the next multiple
of their alignment (since that size is also the stride used in arrays).
Conversely, the alignment of a non-zero-size object is at most its size.
So for example it seems ot be legal for `malloc(8)` to return a pointer
that’s only 8-bytes-aligned, even if `_Alignof (max_align_t)` is 16.
2017-11-20 08:30:04 -06:00
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mod heap;
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2015-03-10 23:58:16 -05:00
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mod linked_list;
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2018-07-06 12:30:09 -05:00
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mod rc;
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2015-03-10 23:58:16 -05:00
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mod slice;
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mod str;
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mod string;
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mod vec;
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2019-12-22 16:42:04 -06:00
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mod vec_deque;
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2015-08-11 19:27:05 -05:00
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fn hash<T: Hash>(t: &T) -> u64 {
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2016-09-28 19:23:36 -05:00
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let mut s = DefaultHasher::new();
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2015-08-11 19:27:05 -05:00
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t.hash(&mut s);
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s.finish()
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}
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2017-08-21 09:15:02 -05:00
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2017-09-10 11:13:19 -05:00
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// FIXME: Instantiated functions with i128 in the signature is not supported in Emscripten.
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// See https://github.com/kripken/emscripten-fastcomp/issues/169
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#[cfg(not(target_os = "emscripten"))]
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2017-08-21 09:15:02 -05:00
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#[test]
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fn test_boxed_hasher() {
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let ordinary_hash = hash(&5u32);
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let mut hasher_1 = Box::new(DefaultHasher::new());
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5u32.hash(&mut hasher_1);
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assert_eq!(ordinary_hash, hasher_1.finish());
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2018-07-13 00:25:22 -05:00
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let mut hasher_2 = Box::new(DefaultHasher::new()) as Box<dyn Hasher>;
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2017-08-21 09:15:02 -05:00
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5u32.hash(&mut hasher_2);
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assert_eq!(ordinary_hash, hasher_2.finish());
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}
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