The stack should be aligned at a 8 mod 16 boundary
at function entry, under both Apple's ABI and under
"System V Application Binary Interface AMD64
Architecture Processor Supplement, draft 0.99".
GCC aligns the stack at a 0 mod 8 boundary.
To quote from the above mentioned document:
The end of the input argument area shall be aligned
on a 16 byte boundary. In other words, the value
(%rsp - 8) is always a multiple of 16 when control
is transferred to the function entry point.
How come this hasn't been discovered before (at least I
cannot find any bug reports about it)? It is because the
x86 is very lax about alignment. But a few instructions
are not that lax, MOVDQA will trigger a SIGSEGV on *nix
systems. It is a performance issue for other 16-byte
loads and stores.
I have no test case for this, although it is possible to
trigger in a shared library under darwin, since the runtime
loader used MOVDQA.
Note that this bug has been verified to exist also for
x86_64-*-freebsd, and from reading the compiler sources.
it also affects gnu/linux.
--
Summary: Stack not aligned at mod 16 byte boundary in x86_64 code
Product: gcc
Version: 4.2.2
Status: UNCONFIRMED
Severity: major
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: tege-gcc at swox dot com
GCC build triplet: i386-apple-darwin8.11.1
GCC host triplet: i386-apple-darwin8.11.1
GCC target triplet: i386-apple-darwin8.11.1
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35271
