Re: bitfields: types vs modes?

[Mark Mitchell <mark at codesourcery dot com>]

DJ Delorie wrote:

> Yes, that's pretty much what they asked for.  However, it is still
> somewhat ambiguous - for example, if you have an 8-bit "int" field,
> which 4 bytes are read to get that field? 

I haven't gone back just now to try to find the right words in the ABI,
so I don't know for sure that they're there, but the intent is the
types' natural alignment.  All "int" bit-fields in that ABI are located
within a 32-bit aligned range of memory; the intent is that you
read/write that entire 4-byte range.  In other words, take the byte
address of the start of the bit-field, mask off the two low-order bits,
that's the 4-byte address to read.

On an ARM, you can never cross a protection boundary in that way; you're
never going to get a SEGV from the bytes you read in that way if you
wouldn't have gotten one from the byte containing the start of the
bit-field.  Protection is done at the page (or cache-line, I forget)
boundary; in either case, the granularity of protection is greater than
the granularity of any basic type.  And, if it's not, that's your
problem; the ABI semantics of a volatile bit-field of type T are that
you get sizeof(T) bytes, and if it blows up (which I don't think it
can), it's not the compiler's fault.

If you have an ABI where an int bit-field can cross a sizeof(int)-byte
boundary, that's a harder case.  That can happen on ARM with GCC's
packed-structure extension, but that's outside the ARM ABI, and
therefore the behavior is not specified.

> We need to figure out how much information we should pass to the
> target to let it make these decisions, if it needs to.  A common
> function to do by-type access could be called from your target and
> mine.

I'd like to know how many of the assumptions above are valid on your
target.  I would imagine that many of them are.  In which case, the
existing hook is good enough; we just need to implement middle-end
behavior depending on the hook.

-- 
Mark Mitchell
CodeSourcery
mark@codesourcery.com
(650) 331-3385 x713