Re: Question about genassym, locore.s and 0-sizedarrays(showstopper for an icc compiled kernel)

Dan Nelson wrote:
> I guess the correct question to be asking is "does the ELF format allow
> 0-length symbols?"

It does, according to my reading of it.  They may have an issue with
dead code removal or element aliasing.  The way to find out would be
to see what they emit for "[]"... 0 lenth, or 1?


> If not, then gcc is generating invalid objects, and
> genassym will have to be rewritten to not use them (maybe add one to
> the array size, and have genassym.sh subtract it).  If it does, then
> genassym.c (sys/assym.h actually) is legal code.  If Intel doesn't want
> to change icc, we can still work around it, but there may be other code
> that will break on icc just like assym.h.

For example, an actual array of length 1 would mean you were
screwed, since treating 1 as if it were 0 would treat that as
zero, as well, and get the wrong answer.

The real answer is that the code should probably use an array
length of one, and then use the address of the array length 1
element, rather than the address of the object plus the size
of the object, when it's trying to use the zero length array
trick to glue data with the correct object alignment (according
to the array type) onto the end of a structure, which is where
most of this type of code comes from.

Unfortunately, the genassym.sh is a special case; for it to work
out if you changed the nature of the trick, the script would
need to grow special knowledge of the symbols in question.  That
is probably going to be the answer in any case, if Intel is
unwilling/unable to adopt the GCC-ism.

Anecdote on "unable":

	At one point in time SEF had to deal with a compiler
	issue with the Microsoft C compiler used by SCO; it
	took code like this:

		char *hw = "Hello World!\n" + 6;

	and generated a data segment missing the "Hello ".

	When the code later did this:

		printf( "%s", hw - 6);

	It got the wrong answer.  The problem was the MS
	compiler was unable to gerate a pseudo-symbol for
	the start of the data, and so had no way of taking
	the origina string, and allocating storage, with a
	static symbol offset +6 into the string.

If Intel ICC makes an optimization based on pruning of actually
zero-length element, or not aligning them to their type boundary,
etc., it would take deep compiler voodoo to correct this.

Hopefully, they do the right thing for "[]".

-- Terry