Figure out rules for minimal alignment of system allocator #812
Description
libstd assumes that the system allocator (which does not have an explicitly alignment parameter) still provides some minimal alignment, given by MIN_ALIGN:
// The minimum alignment guaranteed by the architecture. This value is used to
// add fast paths for low alignment values.
#[cfg(all(any(target_arch = "x86",
target_arch = "arm",
target_arch = "mips",
target_arch = "powerpc",
target_arch = "powerpc64",
target_arch = "asmjs",
target_arch = "wasm32")))]
pub const MIN_ALIGN: usize = 8;
#[cfg(all(any(target_arch = "x86_64",
target_arch = "aarch64",
target_arch = "mips64",
target_arch = "s390x",
target_arch = "sparc64")))]
pub const MIN_ALIGN: usize = 16;
However, at least on Unix, MIN_ALIGN is only exploited for allocations where the size is at least as big as the alignment. On Windows, OTOH, MIN_ALIGN is exploited regardless of the size of the allocation.
Does that mean that we can reduce the alignment we provide for small allocations?