Date: Sun, 4 Dec 2016 17:38:42 +0100 From: Dimitry Andric <dim@FreeBSD.org> To: Hans Petter Selasky <hps@selasky.org> Cc: Colin Percival <cperciva@tarsnap.com>, freebsd-current@freebsd.org Subject: Re: clang/llvm 3.9.0 mysteriously zeroing variables? Message-ID: <78FB227F-3542-452F-9A16-4FB0E0E698AC@FreeBSD.org> In-Reply-To: <e0646eb8-d793-1ffb-bd12-febbce86a4f8@selasky.org> References: <01000158c7252f0c-6c3198b0-fbef-4a60-ade9-e3b91d9e83bd-000000@email.amazonses.com> <e0646eb8-d793-1ffb-bd12-febbce86a4f8@selasky.org>
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On 04 Dec 2016, at 10:52, Hans Petter Selasky <hps@selasky.org> wrote:
>=20
> On 12/04/16 01:04, Colin Percival wrote:
>> Starting with r309124 (when clang/llvm 3.9.0 was imported) I'm seeing =
EC2
>> instances panic on boot with a division-by-zero error; the code in =
question
>> is in blkfront.c, printing out the size of disks:
>>=20
>>> device_printf(dev, "%juMB <%s> at %s",
>>> (uintmax_t) sectors / (1048576 / sector_size),
>>> device_get_desc(dev),
>>> xenbus_get_node(dev));
>>=20
>> My first thought was that 'sector_size' must be either zero or very =
large...
>> but no, when I add printf("sector_size =3D %ju\n", =
(uintmax_t)sector_size), it's
>> entirely normal. What's more, adding that printf makes the =
division-by-zero
>> panic go away.
>>=20
>> I'd think I was just hallucinating, but earlier today I heard that a =
similarly
>> "impossible" panic had been observed in the NFS client code when =
compiled with
>> clang/llvm 3.9.0.
>>=20
>> So... is anyone else seeing unexpected panics or other odd behaviour =
starting
>> after clang/llvm 3.9.0 was imported?
>>=20
>=20
> Hi,
>=20
> Can you look at the code with "objdump -Dx --source" and see what is =
going on there? Might it be the "sector" variable is shadowed?
I don't see anything in the generated code for the call that can cause
this, except for sector_size really being zero, or the result of
1048576/sector_size being zero.
On i386, you get this:
.loc 1 1349 19 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:19
movl -56(%ebp), %ecx # -56(%rbp) =3D sectors
.Ltmp1148:
#DEBUG_VALUE: xbd_connect:sectors <- %ECX
.loc 1 1349 38 is_stmt 0 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:38
movl $1048576, %eax # imm =3D 0x100000
xorl %edx, %edx
divl -52(%ebp) # -52(%ebp) =3D sector_size
movl %eax, %edi
.loc 1 1349 27 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:27
xorl %edx, %edx
movl %ecx, %eax
divl %edi
movl %eax, -32(%ebp) # 4-byte Spill
On amd64, it looks pretty similar:
.loc 1 1349 19 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:19
movq -112(%rbp), %rcx # -112(%rbp) =3D sectors
.Ltmp1128:
#DEBUG_VALUE: xbd_connect:sectors <- %RCX
.loc 1 1349 38 is_stmt 0 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:38
movl $1048576, %eax # imm =3D 0x100000
xorl %edx, %edx
divq -88(%rbp) # -88(%rbp) =3D sector_size
movq %rax, %rsi
.loc 1 1349 27 # =
/usr/src/sys/dev/xen/blkfront/blkfront.c:1349:27
xorl %edx, %edx
movq %rcx, %rax
divq %rsi
movq %rax, %r15
Colin, does it panic for you in the first or the second div?
-Dimitry
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