Why does the Mac ABI require 16-byte stack alignment for x86-32?

Question

I can understand this requirement for the old PPC RISC systems and even for x86-64, but for the old tried-and-true x86? In this case, the stack needs to be aligned on 4 byte

Answer 1

Hmm, didn't OS X ABI also do funny RISC like things like passing small structs in registers?

So that points to the consistency with other platforms theory.

Come to think of it, the FreeBSD syscall api also aligns 64-bit values. (like e.g. lseek and mmap)

Answer 2

This is an efficiency issue.

Making sure the stack is 16-byte aligned in every function that uses the new SSE instructions adds a lot of overhead for using those instructions, effectively reducing performance.

On the other hand, keeping the stack 16-byte aligned at all times ensures that you can use SSE instructions freely with no performance penalty. There is no cost to this (cost measured in instructions at least). It only involves changing a constant in the prologue of the function.

Wasting stack space is cheap, it is probably the hottest part of the cache.

Answer 3

While I cannot really answer your question of WHY, you may find the manuals at the following site useful:

http://www.agner.org/optimize/

Regarding the ABI, have a look especially at:

http://www.agner.org/optimize/calling_conventions.pdf

Hope that's useful.

Answer 4

In order to maintain consistency in kernel. This allows the same kernel to be booted on multiple architectures without modicfication.

Answer 5

From "Intel®64 and IA-32 Architectures Optimization Reference Manual", section 4.4.2:

"For best performance, the Streaming SIMD Extensions and Streaming SIMD Extensions 2 require their memory operands to be aligned to 16-byte boundaries. Unaligned data can cause significant performance penalties compared to aligned data."

From Appendix D:

"It is important to ensure that the stack frame is aligned to a 16-byte boundary upon function entry to keep local __m128 data, parameters, and XMM register spill locations aligned throughout a function invocation."

http://www.intel.com/Assets/PDF/manual/248966.pdf

Answer 6

First, note that the 16 bytes alignment is an exception introduced by Apple to the System V IA-32 ABI.

The stack alignment is only needed when calling system functions, because many system libraries are using SSE or Altivec extensions which require the 16 bytes alignment. I found an explicit reference in the libgmalloc MAN page.

You can perfectly handle your stack frame the way you want, but if you try to call a system function with a misaligned stack, you will end up with a misaligned_stack_error message.

Edit: For the record, you can get rid of alignment problems when compiling with GCC by using the mstack-realign option.