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

Am 10.05.2018 um 12:51 schrieb Junio C Hamano:
> René Scharfe <l.s.r@xxxxxx> 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.

Yes, fast-export seems to be the only place that stores an integer as
a decoration.

>  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 ;-)

Right, it looks deceptively good and works fine if memory is flat and
valid values for pointers are in a contiguous range starting at zero.
The standard allows for other models as well, though.

> 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.

That's right, but I don't see what's naive about it, or how a 32-bit
architecture could avoid wasting those two bits.

Using struct decorate in fast-export has the benefit of not
requiring separate allocations for individual entries.  Switching to
struct hashmap would require individual allocations.  Adding a
custom clone of decorate with a uint32_t payload would be an option.