[lldb-dev] Watching reads/writes on optimized variables?

[Greg Clayton via lldb-dev]

> On Aug 26, 2016, at 7:25 AM, Christian Convey via lldb-dev <lldb-dev at lists.llvm.org> wrote:
> 
> Hi guys,
> 
> I'm trying to use watchpoints to detect user-space reads/writes of an
> arbitrary C/C++ program variable.
> 
> For example:
>   void foo() {
>      int x;   // <-- I'm interested in 'x'
>      x = 10;  // <-- I want to detect this
> 
>      for (int i = 0; i < 4; ++i) {
>         x = i;  // <-- And this
>         bar(x);
>      }
> 
>      x = 20;  // <-- and this
>      bar(x);
>      baz( &x ); // <-- and any updates to 'x' during this call
>   }
> 
> My concern is that the clang+LLVM will sometimes model "x" using a
> register or constant, rather than with memory.  And so a watchpoint
> might miss some reads/writes to "x".
> 
> Does anyone know of a way to minimize or eliminate this problem?

Just take the address of your variable at some point in your code and it will force it into memory.

> 
> Ideas I've considered so far:
> 
> * Compiling the whole program with "-O0".  This might be enough, but
> I'm not sure.
> 
> * Add the "volatile" qualifier to "x".  This might solve the problem,
> but could require countless additions of "volatile" elsewhere as well.
> 
> * Adding a statement of the form "my_dummy_func( &x )".  Assuming this
> reliably causes a memory allocation for "x", this might help.  But I
> wouldn't expect it to reliably preclude "x"'s value from being modeled
> with a register or constant at certain locations in the object code.
> 
> I don't mind modifying compiler/linker flags, but I'd prefer to not
> modify the source code.  I should be able to use most versions of GCC
> and/or Clang/LLVM/LLDB.
> 
> Thanks,
> Christian
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