Re: LLVM--Low Level Virtual Machine--and Ada

[Duncan Sands <baldrick@free.fr>]

Hi Bob, thanks for your informative reply.

> Another problem is that some modern machines use DEP (which I think
> stands for "data execution prevention" or something like that).  DEP
> means the operating system prevents writeable data from being executed
> as code.  The purpose is to prevent certain kinds of security holes
> that are common in languages that don't do array-bounds checking.
> But DEP prevents trampolines from working, so users have to turn
> it off in order to run some Ada programs (such as the compiler).
> It's a pain because users get some mysterious error message
> when trampolines are used.

I'm not sure that this is a problem anymore: gcc uses a bunch of tricks
(eg: setting a flag on the program that notes it runs code on the stack)
to inform the operating system that the trampoline is kosher IIRC.  That
said, I haven't tried to implement any of this in LLVM yet, which is also
why I'm vague on the details.
 
> Another problem (I've heard) is that there's at least one machine
> supported by gcc where trampolines don't work.  I don't remember which
> machine, and I don't even know for sure that this is true.

All architectures currently supported by LLVM support trampolines in gcc.

Ciao,

Duncan.