[LLVMdev] Thumb-2 code generation error in Apple LLVM at all optimization levels

[Jean-Daniel Dupas]

Le 13 nov. 2011 à 11:19, Don Quixote de la Mancha a écrit :

> I am thinking now that the Thumb-2  machine code generated by Apple's
> LLVM 3.0+svn compiler is correct, but that when a Release build is
> generated, the use of conditional machine instructions confuses GDB.
> 
> I boiled my apparently erroneous source down to:
> 
> - (void) mySleep: (int)sleepTime
> {
>    if ( sleepTime >= 4000 ){
>        usleep( sleepTime );
>    }
> 
>    return;
> }
> 
> If I set a breakpoint on the usleep call, the breakpoint will be hit
> but usleep() will not actually be called.
> 

Debugging info in optimized build are not reliable.
The compiler reorder instructions, change the program flow, and do a lot of other transformation that alter them.

> I verified this by passing in 5000.  When I do, I can step down into
> the shared libraries that lead to the actual system call.
> 
> I have also tried calling a regular subroutine of my own instead of a
> system call.  When sleepTime is zero, my subroutine is not called even
> though GDB shows my program stepping over the subroutine call.
> 
> I would say that this is a bug in GDB's source code debugger, in that
> it ought to consider the basic block of the if only to be entered if
> the test succeeds.  GDB's assembly code debugger does the right thing.
> 
> In any case this is not a bug in LLVM.  

Why not ? Generating valid debug info is the role of the compiler, not the debugger.

> If it is to be considered a bug in GDB it would be really hard to fix.

-- Jean-Daniel