Ruslan Ermilov wrote: > On Sun, Nov 12, 2006 at 01:21:05PM -0500, Alexander Kabaev wrote: >> GCC expects 4-byte aligned structured on ARM but does not necessarily >> have to. We can change the default at the expense of possible more >> inefficient code generated and lost binary compatibility with other ARM >> OSes out there. So this is trade off between unclear performance >> penalty and an unspecified but certainly sizable number of other >> landmines like this lurking on the code. >> >> We should decide what evil we regard as lesser. >> > This is the only buildworld problem so far on FreeBSD/ARM, so my > feeling is that we can actually benefit from leaving it "as is", > as it has a potential of making our code more portable. Of course > if binary compatibility for structs across platforms is an issue, > a structure should be "packed", because otherwise the C standard > says that "Each non-bit-field member of a structure or union object > is aligned in an implementation-defined manner appropriate to its > type." > > On the other hand, having GCC align "struct foo { char x[11]; }" > on a four-byte boundary (other than for backward compatibility) > doesn't make too much sense to me. > > I don't know GCC rules for alignment of structure members. For > example, if it's guaranteed (in GCC) that offsetof(struct foo, bar) > is always 1 for "struct foo { char foo; char bar; }" (without the > "packed" attribute) on all platforms and OSes GCC supports? > I'd expect the latter to be "4" for FreeBSD/ARM but fortunately > it stays "1", i.e., only the structure alignment is affected, > and not of structure members (which is POLA but makes the 4 byte > for structure alignment questionable). This issue appears to have broken if_bridge. On my avila board I align rx packets to be aligned s.t. the ip header lands on a 32-bit boundary. This results in the ethernet header being 2-byte aligned which is ok on other platforms. But the compiler takes this code in bridge_forward: /* * If the interface is learning, and the source * address is valid and not multicast, record * the address. */ if ((bif->bif_flags & IFBIF_LEARNING) != 0 && ETHER_IS_MULTICAST(eh->ether_shost) == 0 && (eh->ether_shost[0] == 0 && eh->ether_shost[1] == 0 && eh->ether_shost[2] == 0 && eh->ether_shost[3] == 0 && eh->ether_shost[4] == 0 && eh->ether_shost[5] == 0) == 0) { (void) bridge_rtupdate(sc, eh->ether_shost, src_if, 0, IFBAF_DYNAMIC); } and converts the 6 byte compares to a 32-bit and 16-bit compare. Since the data is only 16-bit aligned the 32-bit load faults. So the point is that just because things compile doesn't necessarily mean they work. And worse code that works on many/most other architectures may not work. SamReceived on Sun Nov 12 2006 - 19:22:05 UTC
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