Re: [Valgrind-developers] PATCH: Make Calltree work without --pointercheck=no (2)

[Jeremy F. <je...@go...>]

On Mon, 2004-03-15 at 13:05, Julian Seward wrote:
> First arg is Bool, conceptually 1-bit and in fact unsigned char
> (see include/vg_skin.h).  But then there are calls
> 
>    emit_movb_reg_regmem ( size & DO_BOUNDSCHECK, reg1, reg2 ); 
> 
> This just can't be right, and I'm not even sure what the caller
> intends here. 

Yeah, it's a bit of a nasty hack.  I snarfed a bit in the "size"
parameter to mean "this should be a checked memory operation".  This
test masks out the size part, and just grabs the flag.  The code should
probably be something like ((size & DO_BOUNDSCHECK) != 0) to generate a
proper boolean result.

I guess the cleaner approach is to add a separate flag argument rather
than encoding the bounds-checking flag in the size.

>  Not to mention DO_BOUNDSCHECK is 1 << 8 so is 
> zero when interpreted as Bool.  If the bounds check always happens
> for instructions emitted by this call, it's because size just 
> happens to always be 1, 2 or 4 but never zero -- and completely
> nothing to do with DO_BOUNDSCHECK.

Well, the expression is evaluated as an int, so you'll get 0x100 or 0x0
as the result.  It will also probably be passed as an int; I'm not sure
when the bits are masked off (if ever).

> I guess the C "type system" (so-called) gets to wear the pointy 
> hat and stand in the corner :-)  Anybody know of a way to define
> a Bool type in C in such a way the compiler can't silently confuse
> it with any flavour of int?

Not really.  C has a boolean type, and it's called "int".  The
interpretation is 0 = False, non-0 = True.  The cleanest way of making a
"proper" boolean type would be to use an enum, but C still lets you
easily cast between int and enum types.

	J