Bug 1369458

Description of problem:

`disassemble /m <function>` seems to silently remove instructions.

I understand that it might be a hard job due to optimizations, however, silently missing out on some instructions should probably be a avoided because it might be misleading. 

If this is a known weakness, then there should be at the very minimum a warning when the command has some chance of giving wrong results.

Version-Release number of selected component (if applicable):
I tried on the following versions with similar results :
gdb-7.6.1-80.el7.x86_64
gdb-7.10.1-30.fc23.x86_64

How reproducible: 100%


Steps to Reproduce:
1. debuginfo-install rpm
2. gdb /usr/bin/rpm
3. break main
4. run
5. disassemble /m headerVerifyInfo
6. disassemble headerVerifyInfo

Actual results:
The '/m' output is incorrect. In particular, the bswap instructions (corresponding to the ntohl() calls) instructions totally disappeared :

200             info->tag = ntohl(pe[i].tag);
   0x00007ffff7b8024b <+91>:    mov    %r9d,(%rcx)

201             info->type = ntohl(pe[i].type);
   0x00007ffff7b80255 <+101>:   mov    %r9d,0x4(%rcx)

Expected results:

Either of the following :
1- a complete output if possible

The corresponding, but more correct version of above would probably be like :
200             info->tag = ntohl(pe[i].tag);
   0x00007ffff7b80242 <+82>:    mov    (%rdx),%r9d
   0x00007ffff7b80248 <+88>:    bswap  %r9d
   0x00007ffff7b8024b <+91>:    mov    %r9d,(%rcx)
201             info->type = ntohl(pe[i].type);
   0x00007ffff7b8024e <+94>:    mov    0x4(%rdx),%r9d
   0x00007ffff7b80252 <+98>:    bswap  %r9d
   0x00007ffff7b80255 <+101>:   mov    %r9d,0x4(%rcx)


Note: The instructions at +82, +88, +94 and +98 are shown nowhere in the /m output.

2- if gdb is unsure of the exactitude of the output, then a big warning should be printed, telling the user not to trust the data.

Additional info:
I ll attach the output of the disassembled function.