RE: [Valgrind-users] Use of VALGRIND_DISCARD_TRANSLATIONS with ne sted functions

[Barrie W. <bw...@cs...>]

> -----Original Message-----
> From: Jeremy Fitzhardinge [mailto:je...@go...] 
> Sent: 16 January 2004 18:54
> To: Barrie Walker
> Cc: 'val...@li...'
> Subject: Re: [Valgrind-users] Use of 
> VALGRIND_DISCARD_TRANSLATIONS with nested functions
> 
> 
> On Fri, 2004-01-16 at 09:58, Barrie Walker wrote:
> > [snip] valgrind
> > [snip] nested functions 
> > [snip] VALGRIND_DISCARD_TRANSLATIONS
> > [snip] what the parameters should be 
> > [snip] where in the code it should be placed

> It's tricky - it isn't the proper solution, but more a hack 
> to get you going.  The trouble is that gcc generates a piece 
> of code dynamically on the stack when you call a nested 
> function; for each call, it generates a different piece of 
> code.  Valgrind sees this the first time, and generates its 
> own instrumented code, but if gcc later uses the same stack 
> location to generate code for a new call to a different 
> function, Valgrind won't notice, and use the old cached code.
> 
> Therefore, to use VALGRIND_DISCARD_TRANSLATIONS you need to 
> guess the location and size of the code which gcc is 
> generating.  Something like this (before each call to a 
> nested function) might work:
> 
> int myfunc()
> {
> 	int mynestedfunc() {
> 	}
> 	char local;
> 
> 	VALGRIND_DISCARD_TRANSLATIONS(&local-32, 128);	// guessing
> 	mynestedfunc();
> }
> 
> It gets more complex if you're passing a pointer to 
> mynestedfunc elsewhere, and/or you use dynamically sized 
> arrays or alloca().  &local is basically a guess at the value 
> of %esp - you could also use:
> 	unsigned int esp;
> 
> 	asm volatile("mov %%esp,%0" : "=r" (esp));
> 	VALGRIND_DISCARD_TRANSLATIONS(esp-32, 128);	// guessing
> 
> The real fix is for Valgrind to cope with this automatically 
> (bug http://bugs.kde.org/show_bug.cgi?id=69511).
> 
> 	J

Hi Jeremy,

Thank you, that was helpful.

I found that the address given for the function was a stack one in amongst
the local data (at least when passed to printf) and it seems to make a
difference when using that one.

Far from ideal, thought.

Barrie