Re: [Valgrind-developers] Idea for a MMX vs. FPU checker tool

[Nicholas N. <nj...@cs...>]

On Fri, 23 Nov 2007, Gregor Jasny wrote:

> I've got an idea for a MMX checker tool. First some background:
> When you use MMX/SSE instructions on the x86 platform you have to reset
> the FPU stack with the 'emms' instruction after calculations finished.
> Otherwise the following floating point instructions will fail.
>
> I have a large C++ project here and somewhere in an static initializer
> such an reset is omitted. Spotting the missing place is quite hard.
> I've tried to use GDB to watch the $ftag register, but GDBs single
> instruction stepping takes ages.
>
> So the valgrind tool should do the following: If an FPU instruction is
> executed with a corrupt FPU stack, print the position of the last
> MMX/SSE instruction.
>
> I've skimmed the lackey source code, but have still no idea where to
> start. It would be nice if you could provide me some assistance or even
> some source code.
>
> What do you think?

It might be possible.  If it is possible, it probably wouldn't be all that 
hard, i.e. the source code needn't be that long, but understanding Valgrind 
enough to write it requires some effort.

The key factor is that Valgrind translates x86 (or PPC) code into an 
intermediate representation (IR).  The question is whether the IR preserves 
the 'emms' instruction in such a way that you can recognise it.  Judging 
from VEX/priv/guest-x86/toIR.c:do_EMMS_preamble() (around line 5090), it's 
not represented explicitly, but rather all the FP regs get zeroed, as does 
the FP stack top.  You might be able to detect that manually;  it's 
conceivable that an optimisation pass might remove some of the zeroings if 
it knew they weren't necessary, but I don't know how likely that is.

To understand the IR, look at VEX/pub/libvex_ir.h.  Make sure you use the 
SVN version, it has much better comments in it than the 3.2.3 version. 
Then, Memcheck might be a better example tool than Lackey, to see how a 
wider range of instructions are handled.

Hope this helps, feel free to ask more questions if you have trouble.

Nick