Bug#810819: gcc-5: misoptimization, "goto out" removed

["Ph. Marek" <philipp.marek@linbit.com>]

Package: gcc-5
Version: 5.3.1-5
Severity: normal

During debugging for some strange behaviour in DRBD I got upon this.

 * DRBD 8.4 from GIT head (http://git.drbd.org/drbd-8.4.git/)
 * compiling for linux-headers-4.3.0-1-amd64=4.3.3-5
 * drbd_send_dblock(), in drbd/drbd_main.c

For reference: the codeblock is around line 1759, and looks like this:

    if (dp_flags & DP_DISCARD) {
        struct p_trim *t = (struct p_trim*)p;
        t->size = cpu_to_be32(req->i.size);
        err = __send_command(peer_device->connection, device->vnr, sock, P_TRIM, sizeof(*t), NULL, 0);
        goto out;
    }
    if (dp_flags & DP_WSAME) {

Now, when I look at the generated disassembly, the "goto out" instruction 
is simply missing:

   0x0000000000028626 <+358>:   mov    %eax,0x18(%rdi)
   0x0000000000028629 <+361>:   mov    0x10(%rsp),%rax
   0x000000000002862e <+366>:   mov    0x18(%r10),%rdi
   0x0000000000028632 <+370>:   mov    0x68(%rax),%esi
   0x0000000000028635 <+373>:   movl   $0x0,0x8(%rsp)
   0x000000000002863d <+381>:   movq   $0x0,(%rsp)
   0x0000000000028645 <+389>:   callq  0x267d0 <__send_command>
   0x000000000002864a <+394>:   mov    %eax,0x20(%rsp)  # store result
   0x000000000002864e <+398>:   mov    0x40(%rsp),%rdi  # load for DP_WSAME
   0x0000000000028653 <+403>:   callq  0x28658 <drbd_send_dblock+408>

When I insert

    #pragma GCC push_options
    #pragma GCC optimize ("O0")

above the function definition, the result is this:

    0x0000000000027aad <+605>:   movl   $0x0,0x8(%rsp)
    0x0000000000027ab5 <+613>:   movq   $0x0,(%rsp)
    0x0000000000027abd <+621>:   mov    $0x1c,%r9d
    0x0000000000027ac3 <+627>:   mov    $0x31,%r8d
    0x0000000000027ac9 <+633>:   mov    %rax,%rdi
    0x0000000000027acc <+636>:   callq  0x2cc00 <__send_command>
    0x0000000000027ad1 <+641>:   mov    %eax,0x30(%rsp)
    0x0000000000027ad5 <+645>:   jmpq   0x27de0 <drbd_send_dblock+1424>
    0x0000000000027ada <+650>:   mov    0x2c(%rsp),%eax

Hooray, the jump is back!


-- System Information:
Debian Release: stretch/sid
  APT prefers testing
  APT policy: (990, 'testing'), (500, 'unstable'), (500, 'stable'), (1, 'experimental')
Architecture: amd64 (x86_64)

Kernel: Linux 4.3.0-1-amd64 (SMP w/4 CPU cores)
Locale: LANG=de_AT.UTF-8, LC_CTYPE=de_AT.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages gcc-5 depends on:
ii  binutils      2.25.90.20160101-1
ii  cpp-5         5.3.1-5
ii  gcc-5-base    5.3.1-5
ii  libc6         2.21-6
ii  libcc1-0      5.3.1-5
ii  libgcc-5-dev  5.3.1-5
ii  libgcc1       1:5.3.1-5
ii  libgmp10      2:6.1.0+dfsg-2
ii  libisl15      0.15-3
ii  libmpc3       1.0.3-1
ii  libmpfr4      3.1.3-2
ii  libstdc++6    5.3.1-5
ii  zlib1g        1:1.2.8.dfsg-2+b1

Versions of packages gcc-5 recommends:
ii  libc6-dev  2.21-6

Versions of packages gcc-5 suggests:
pn  gcc-5-doc         <none>
pn  gcc-5-locales     <none>
pn  gcc-5-multilib    <none>
pn  libasan2-dbg      <none>
pn  libatomic1-dbg    <none>
pn  libcilkrts5-dbg   <none>
pn  libgcc1-dbg       <none>
pn  libgomp1-dbg      <none>
pn  libitm1-dbg       <none>
pn  liblsan0-dbg      <none>
pn  libmpx0-dbg       <none>
pn  libquadmath0-dbg  <none>
pn  libtsan0-dbg      <none>
pn  libubsan0-dbg     <none>

-- no debconf information