Commits on Nov 20, 2017

1. alloc_system: don’t assume MIN_ALIGN for small sizes, fix rust-lang#4… …

   …5955
   
   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.

   

   SimonSapin committed Nov 20, 2017
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2. alloc_jemalloc: don’t assume MIN_ALIGN for small sizes …

   See previous commit’s message for what is expected of allocators
   in general, and jemalloc/jemalloc#1072
   for discussion of what jemalloc does specifically.

   

   SimonSapin committed Nov 20, 2017
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3. Remove comment about a branch being optimized out, fix rust-lang#45831 …

   Most often, this code is used through the `std::heap::Heap`
   and `#[gloabal_allocator]` indirection, so this branch is not
   optimized out anymore.

   

   SimonSapin committed Nov 20, 2017
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