Why GDB jumps unpredictably between lines and prints variables as value optimized out

Question

Can anyone explain this behavior of gdb   900         memset  amp new ckpt info   0  sizeof CKPT INFO     gdb    903         prev offset     cp node- gt offset     gdb    905         m CPND CKPTINFO READ ckpt info  char   cb- gt shm addr ckpt addr sizeof CKPT    HDR  i offset    gdb    903         prev offset     cp node- gt offset     gdb    905         m CPND CKPTINFO READ ckpt info  char   cb- gt shm addr ckpt addr sizeof CKPT  HDR  i offset      gdb    908         bitmap offset    client hdl 32     gdb    910         bitmap value   cpnd client bitmap set client hdl 32      gdb    908         bitmap offset    client hdl 32     gdb    910         bitmap value   cpnd client bitmap set client hdl 32      gdb    908         bitmap offset    client hdl 32     gdb    910         bitmap value   cpnd client bitmap set client hdl 32      gdb  913         found   cpnd find exact ckptinfo cb    amp ckpt info   bitmap offset    amp offset    amp prev offset    gdb  916         if  found   gdb  p found  1    lt value optimized out gt   gdb  set found 0 Left operand of assignment is not an lvalue    Why after executing line 903 it again executes the same for 905 908 910   Another things is found is a bool-type variable  so why it is showing value optimized out  I am not able to set the value of found as well   This seems to be a compiler optimization  in this case its -O2   how can I still set the value of found

User · Accepted Answer

To debug optimized code  learn assembly machine language   Use the GDB TUI mode  My copy of GDB enables it when I type the minus and Enter  Then type C-x 2  that is hold down Control and press X  release both and then press 2   That will put it into split source and disassembly display  Then use stepi and nexti to move one machine instruction at a time  Use C-x o to switch between the TUI windows   Download a PDF about your CPU s machine language and the function calling conventions  You will quickly learn to recognize what is being done with function arguments and return values   You can display the value of a register by using a GDB command like p  eax

User · Answer

Recompile without optimizations  -O0 on gcc

User · Answer

Declare found as  volatile    This should tell the compiler to NOT optimize it out   volatile int found   0

User · Answer

Im using QtCreator with gdb   Adding  QMAKE CXXFLAGS    -O0 QMAKE CXXFLAGS -  -O1 QMAKE CXXFLAGS -  -O2 QMAKE CXXFLAGS -  -O3   Works well for me

User · Answer

Typically  boolean values that are used in branches immediately after they re calculated like this are never actually stored in variables  Instead  the compiler just branches directly off the condition codes that were set from the preceding comparison  For example   int a   SomeFunction    bool result   --a  gt   0     use subtraction as example computation if   result         foo       else      bar      return    Usually compiles to something like   call  SomeFunction    calls to SomeFunction    which stores its return value in eax sub eax  1   subtract 1 from eax and store in eax  set S  sign  flag if result is negative jl ELSEBLOCK   GOTO label  ELSEBLOCK  if S flag is set call  foo   this is the  if  black  call foo   j FINISH   GOTO FINISH  skip over the  else  block ELSEBLOCK    label this location to the assembler call  bar FINISH    both paths end up here ret   return   Notice how the  bool  is never actually stored anywhere

User · Answer

You pretty much can t set the value of found  Debugging optimized programs is rarely worth the trouble  the compiler can rearrange the code in ways that it ll in no way correspond to the source code  other than producing the same result   thus confusing debuggers to no end

User · Answer

The compiler will start doing very clever things with optimisations turned on  The debugger will show the code jumping forward and backwards alot due to the optimized way variables are stored in registers  This is probably the reason why you can t set your variable  or in some cases see its value  as it has been cleverly distributed between registers for speed  rather than having a direct memory location that the debugger can access   Compile without optimisations

User · Answer

When debugging optimized programs  which may be necessary if the bug doesn t show up in debug builds   you often have to understand assembly compiler generated   In your particular case  return value of cpnd find exact ckptinfo will be stored in the register which is used on your platform for return values  On ix86  that would be  eax  On x86 64   rax  etc  You may need to google for   your processor  procedure calling convention  if it s none of the above   You can examine that register in GDB and you can set it  E g  on ix86    gdb  p  eax  gdb  set  eax   0

[c] Why GDB jumps unpredictably between lines and prints variables as "<value optimized out>"?

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