Re: [PATCH] fast-export: avoid NULL pointer arithmetic

> René Scharfe <l.s.r@web.de> writes:
> 
>> The standard says about uintptr_t that "any valid pointer to void can
>> be converted to this type, then converted back to pointer to void, and
>> the result will compare equal to the original pointer".  So void * ->
>> uintptr_t -> void * is a proper roundtrip, but that doesn't imply that
>> casting arbitrary uintptr_t values to void * would be lossless.
>>
>> I don't know an architecture where this would bite us, but I wonder if
>> there is a cleaner way.  Perhaps changing the type of the decoration
>> member of struct decoration_entry in decorate.h to uintptr_t?
> 
> In order to ensure "void * -> uintptr_t -> void *" roundtrip holds,
> the implementation would guarantee that uintptr_t is wider than
> void*, so what you suggest technically makes sense.  We should be
> able to store any pointer in the field.  And we should be able to
> store any value of an unsigned integral type that is narrower than
> uintptr_t.
> 
> But it somehow feels backwards in spirit to me, as the reason why we
> use "void *" there in the decoration field is because we expect that
> we'd have a pointer to some struture most of the time, and we have
> to occasionally store a small integer there.

>  So I'd naively expect
> that
> 
> 	uint32_t mark = 23;
> 	de->decoration = (void *)mark;
> 
> would be a good way to store mark #23 in the field and
> 
> 	uint32_t mark;
> 	mark = (typeof(mark))de->decoration;
> 
> would be a good way to read it off of the "void *" field.  Of
> course, this assume that (void *) is at least as wide as 32-bit and
> it also ignores the standard ;-)

> This is an unrelated tangent but the mark-to-ptr() and ptr-to-mark()
> implementations feel wasteful, especially when we worry about 32-bit
> archs.  A naive platform implementation of
> 
> 	(uint32_t *)mark - (uint32_t *)NULL;
> 
> would be ((uintptr_t)mark) / 4, i.e. the de->decoration field will
> always have two LSB clear and only utilize top 30-bit to represent
> the value of mark.