Hi, list:
We have some efforts on building and tuning linux kernel on open64, we
have fixed several bugs and now we have only two building issues and
several running issues at x86_64 target, O2 level.
linux 2.6.32 as an example, the two building issues are:
A). bug787, Rao Shivarama has submitted a patch, but due to the regression
filed as bug882, the patch is backed out. This is still a pending issue. My
personally investigation shows Rao's approach is on the right track, the
bug882 shows the fix triggers the SSA version verification issues which are
already known in open64, filed as bug889/891/892. We will continue work on
it.
B). the Variable Length Array in Structs (VLAS), this is a common issue for
non-gnu compilers.
clang documents the issue below: Intentionally unsupported GCC extensions
- clang does not support the gcc extension that allows variable-length
arrays in structures. This is for a few reasons: one, it is tricky to
implement, two, the extension is completely undocumented, and three, the
extension appears to be rarely used. Note that clang *does* support
flexible array members (arrays with a zero or unspecified size at the end
of a structure).
I review the failure case of kernel build, the VLAS are *not the flexible
array members(case like this : struct A { int a; int b; char ctx[]} is
called Flexible array menbers. It is a newly introduced feature c99 or
later, open64 supports it well.)*
VLAS does not appear in linux kernel source code frequently:
lib/libcrc32c.c
u32 crc32c(u32 crc, const void *address, unsigned int length)
43{
44 struct {
45 struct shash_desc shash;
46 char ctx[crypto_shash_descsize(tfm)];
47 } desc;
crypto/testmgr.c
struct {
1432 struct shash_desc shash;
1433 char ctx[crypto_shash_descsize(tfm)];
1434 } sdesc;
crypo/hmac.c
struct {
1432 struct shash_desc shash;
1433 char ctx[crypto_shash_descsize(tfm)
];
1434 } sdesc;
I just speculate that clang will support kernel build by flexible array
members. It is not the case,
http://llvm.org/bugs/show_bug.cgi?id=4071
and people claim that they build up kernel with clang, but it is not fully
functional.
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2010-October/011711.html
as this guy pointed out,
* SELinux, Posix ACLs, IPSec, eCrypt, anything that uses the crypto API -
None
of these will compile, due to either an ICE or variable-length arrays in
structures (don't remember which, it's in my notes somewhere). If it's
variable-length arrays or another intentionally unsupported GNUtension,
I'm
hoping it's just used in some isolated implementation detail (or
details),
and not a fundamental part of the crypto API (honestly just haven't had
a
chance to dive into the crypto source yet). I'm really hoping it's an
issue
in Clang, though, as it's easier for me to hack Clang and I'm trying to
avoid kernel patches as much as possible.
We meet the problem and post it out, we wish we could get input from
community and go on the kernel building/tuning work to broaden the open64's
application and improve the code quality.
Thanks.
Regards
Gang
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