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

[Philipp Marek <philipp.marek@linbit.com>]

Hi Matthias, 

thanks for the quick answer!

> >During debugging for some strange behaviour in DRBD I got upon this.
> 
> please could you recheck with gcc-4.9 (unstable) and gcc-6 (experimental)?
well, wouldn't that make the module incompatible with the running kernel?


I switched the GCC version by modifying 
/usr/src/linux-headers-4.3.0-1-amd64/.kernelvariables; "make V=1" correctly 
showed the expected version.


== GCC 4.9

1758            err = __send_command(peer_device->connection, device->vnr, sock, P_TRIM, sizeof(*t), NULL, 0);
   0x00000000000283f1 <+337>:   mov    $0x1c,%r9d
   0x00000000000283f7 <+343>:   mov    $0x31,%r8d
   0x0000000000028402 <+354>:   mov    0x10(%rsp),%rax
   0x0000000000028407 <+359>:   mov    0x18(%r14),%rdi
   0x000000000002840b <+363>:   mov    0x68(%rax),%esi
   0x000000000002840e <+366>:   movl   $0x0,0x8(%rsp)
   0x0000000000028416 <+374>:   movq   $0x0,(%rsp)
   0x000000000002841e <+382>:   callq  0x26750 <__send_command>
   0x0000000000028423 <+387>:   mov    %eax,0x20(%rsp)

1759            goto out;
1760        }
1761        if (dp_flags & DP_WSAME) {
   0x00000000000284d6 <+566>:   test   $0x2,%ah
   0x00000000000284d9 <+569>:   je     0x2869f <drbd_send_dblock+1023>



== GCC 6

1758            err = __send_command(peer_device->connection, device->vnr, sock, P_TRIM, sizeof(*t), NULL, 0);
   0x0000000000029dd7 <+327>:   lea    0x200(%rbx),%rcx
   0x0000000000029dde <+334>:   lea    0x1f8(%rbx),%rdx
   0x0000000000029de5 <+341>:   mov    $0x1c,%r9d
   0x0000000000029deb <+347>:   mov    $0x31,%r8d
   0x0000000000029df6 <+358>:   mov    0x10(%rsp),%rax
   0x0000000000029dfb <+363>:   mov    0x18(%r15),%rdi
   0x0000000000029dff <+367>:   mov    0x68(%rax),%esi
   0x0000000000029e02 <+370>:   movl   $0x0,0x8(%rsp)
   0x0000000000029e0a <+378>:   movq   $0x0,(%rsp)
   0x0000000000029e12 <+386>:   callq  0x27fd0 <__send_command>
   0x0000000000029e17 <+391>:   mov    %eax,0x20(%rsp)

1759            goto out;
1760        }
1761        if (dp_flags & DP_WSAME) {
   0x0000000000029eab <+539>:   test   $0x2,%ah
   0x0000000000029eae <+542>:   je     0x2a07f <drbd_send_dblock+1007>


Hrmpf.

Is there some way that GCC can decide that the "goto" is invalid and/or not 
needed because the later code must not have any side-effects?
Hard to believe that all the rest of the function can be skipped.

I came across that because *this* call to __send_command() returned 
0 (as reported via systemtap), but upon leaving the function it returns an 
error code (-5, EIO) - so something must change the "err" inbetween.
And seeing that there's no "jmp" when optimizations are enabled made me 
believe in a GCC bug...
Do you see an error in that conclusion?



Here's another output; GCC 5, with
    #pragma GCC push_options
    #pragma GCC optimize ("O1")
before the function:

1757            err = __send_command(peer_device->connection, device->vnr, sock, P_TRIM, sizeof(*t), NULL, 0);
   0x00000000000288cd <+285>:   mov    0x68(%r13),%esi
   0x00000000000288d1 <+289>:   mov    0x18(%rbx),%rdi
   0x00000000000288d5 <+293>:   movl   $0x0,0x8(%rsp)
   0x00000000000288dd <+301>:   movq   $0x0,(%rsp)
   0x00000000000288e5 <+309>:   mov    $0x1c,%r9d
   0x00000000000288eb <+315>:   mov    $0x31,%r8d
   0x00000000000288f1 <+321>:   callq  0x26860 <__send_command>
   0x00000000000288f6 <+326>:   mov    %eax,%r14d

1758            goto out;
   0x00000000000288f9 <+329>:   jmpq   0x28a95 <drbd_send_dblock+741>

1759        }
1760    #pragma GCC pop_options
1761        if (dp_flags & DP_WSAME) {
   0x00000000000288fe <+334>:   test   $0x2,%ah
   0x0000000000028901 <+337>:   je     0x28934 <drbd_send_dblock+388>

Here the jmp is still left in...