New DSDL directive for simple compile-time checks

[kjetilkjeka]

@pavel.kirienko

We want to be sure that fields that follow variable-length entities are still byte-aligned (or otherwise aligned) at all times. Here’s an example:

uint7 node_id
void1
@assert offset % 8 == 0
void3
uint8[<=16] unique_id
@assert min_offset / 8 == 2
@assert max_offset / 8 == 18

# Making sure that the next field is always aligned
@assert min_offset % 8 == 0
@assert max_offset % 8 == 0
uint16 foo

# Eventually (in a future release) we could go further and ask the compiler to
# verify the offset against every possible permutation of the preceding
# variable-length fields.
@assert any_offset % 8 == 0 # I have not given much thought to this yet

@assert max_offset / 8 < 400

Yes, it won’t work with variable-length fields where the element sizes are not a multiple of 8 bits, but those are rare (they don’t exist in the standard data type set, for example).

This is a big problem with min/max offset. We can only check the arrays that are incredibly hard to get wrong, to begin with. We could make @assert aligned work equal to your proposed @assert any_offset % 8 == 0 without needing to reason about min/max/any offsets and mod operators.

@pavel.kirienko

I am not sure why do we need @assert aligned data or end since the same goals can be achieved by moving the assertion statement after the field it is intended to protect.

@assert aligned
uint7 data0
uint8[<=1] data1
@assert aligned   # Applies both to the above and to the below
void3
uint5 data2
@assert aligned
void2
uint64 data3

Stuff like @assert aligned +2 doesn’t seem to be really needed, does it? Unconventional checks can always be implemented by checking against the offset variables directly, e.g. @assert (offset - 2) % 8 == 0

Do you take the challange to remove end and data in the following examples?

void7
@assert aligned data
uint47[<=1] data

void5
@assert aligned data # After you've looked up SomeType you will realize that the definition is not a multiple of 8 bytes
SomeType[<=7] data

void5
@assert aligned end
SomeType[13]

@pavel.kirienko

If strongly desired, we could add some sugar that would expand as follows:

@assert aligned
# Is equivalent to:
@assert min_offset % 8 == 0
@assert max_offset % 8 == 0

@assert aligned should check that it’s always aligned. Not just in 2 arbitrarily chosen cases (min/max length).