Date: Sat, 02 Mar 2013 19:35:58 +0200 From: Andriy Gapon <avg@FreeBSD.org> To: freebsd-hackers@FreeBSD.org Subject: clang generated code sometimes confuses fbt Message-ID: <5132387E.8010808@FreeBSD.org>
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I observe the following problem.
There are two tiny wrapper functions around a larger implementation function:
int
bpobj_iterate(bpobj_t *bpo, bpobj_itor_t func, void *arg, dmu_tx_t *tx)
{
return (bpobj_iterate_impl(bpo, func, arg, tx, B_TRUE));
}
int
bpobj_iterate_nofree(bpobj_t *bpo, bpobj_itor_t func, void *arg, dmu_tx_t *tx)
{
return (bpobj_iterate_impl(bpo, func, arg, tx, B_FALSE));
}
On a clang compiled system:
$ dtrace -l | fgrep bpobj_iterate
1483 fbt kernel bpobj_iterate_impl entry
1484 fbt kernel bpobj_iterate_impl return
On a gcc compiled system:
dtrace -l | fgrep bpobj_iterate
647 fbt kernel bpobj_iterate_impl entry
648 fbt kernel bpobj_iterate_impl return
20656 fbt kernel bpobj_iterate entry
20657 fbt kernel bpobj_iterate return
28426 fbt kernel bpobj_iterate_nofree entry
28427 fbt kernel bpobj_iterate_nofree return
Examination reveals why that is so.
clang:
Dump of assembler code for function bpobj_iterate:
0xffffffff802d5a90 <bpobj_iterate+0>: mov $0x1,%r8d
0xffffffff802d5a96 <bpobj_iterate+6>: jmp 0xffffffff802d5aa0
<bpobj_iterate_impl>
gcc:
Dump of assembler code for function bpobj_iterate:
0xffffffff802d3f43 <bpobj_iterate+0>: push %rbp
0xffffffff802d3f44 <bpobj_iterate+1>: mov %rsp,%rbp
0xffffffff802d3f47 <bpobj_iterate+4>: mov $0x1,%r8d
0xffffffff802d3f4d <bpobj_iterate+10>: callq 0xffffffff802d3787
<bpobj_iterate_impl>
0xffffffff802d3f52 <bpobj_iterate+15>: pop %rbp
0xffffffff802d3f53 <bpobj_iterate+16>: retq
So quite obviously fbt can not really entry/return points for the clang function.
This is not a big problem on its own, of course, but here is a bad twist.
On the clang system:
$ ctfdump -f /boot/kernel/kernel | fgrep bpobj_iterate
[8975] FUNC (bpobj_iterate) returns: 24 args: (2601, 4824, 34, 2221)
[13093] FUNC (bpobj_iterate_nofree) returns: 24 args: (2601, 4824, 34, 2221)
Now that's the problem: fbt sees only bpobj_iterate_impl, but CTF data is
generated/present only for bpobj_iterate and bpobj_iterate_nofree.
On the gcc system:
ctfdump -f /boot/kernel/kernel | fgrep bpobj_iterate
[323] FUNC (bpobj_iterate_impl) returns: 1 args: (5153, 5661, 6, 5078, 1350)
[10439] FUNC (bpobj_iterate) returns: 1 args: (5153, 5661, 6, 5078)
[14377] FUNC (bpobj_iterate_nofree) returns: 1 args: (5153, 5661, 6, 5078)
To summarize: I would be glad of either clang generated code was "fbt-friendly"
or if ctf information was generated for bpobj_iterate_impl. Either is perfect
for me.
Now, I am not quite sure why ctfconvert skips bpobj_iterate_impl in the
clang-generated code. Seems like some sort of a bug in ctfconvert.
--
Andriy Gapon
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