http://gcc.gnu.org/bugzilla/show_bug.cgi?id=46354
Summary: attribute((aligned(...))) can incorrectly decrease
structure field alignment
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: pagee...@freemail.hu
while investigating a clang error when compiling the linux kernel i narrowed
the problem down to what seems to be a gcc bug. what happens is that despite
what the gcc documentation says about the aligned attribute, gcc does decrease
structure field alignment even when the packed attribute is not specified. the
attached example shows that the issue is only with the attribute attached to a
typedef. the relevant part of the generated asm looks like this for gcc:
movq xx+8(%rip), %rax
addq x+4(%rip), %rax <==== bug, should be +8
addq p+2(%rip), %rax <==== should it be +4?
addq pp+4(%rip), %rax
and for clang:
movq xx+8(%rip), %rax
addq x+8(%rip), %rax
addq p+2(%rip), %rax
addq pp+4(%rip), %rax
the tested gcc versions so far:
gcc version 4.4.5 (Gentoo 4.4.5 p1.0, pie-0.4.5)
gcc version 4.5.1 20100924 (Red Hat 4.5.1-4) (GCC)
note that fixing this bug will have a non-trivial effect on linux as this
construct is relied upon throughout the compat layer (that implements
transforming syscall parameters between 32 bit userland and the 64 bit kernel)
so when gcc stops decreasing the natural alignment of these structure fields,
all such code will have to be changed to explicitly use the packed attribute
lest the kernel/userland syscall ABI break... only to run into the next issue
in that the packed attribute on the structure ignores the aligned attribute in
the typedef (see the +2 above), both in gcc and clang (given both compilers are
affected, i'm not sure if this is a bug or feature). more interestingly, if the
aligned attribute is on the structure field itself then it is properly taken
into account, both in gcc and clang. so this looks like a fine mess to clean up
if/when the root bug gets fixed.
