Fwd: [RFC][gomp4] Offloading patches (2/3): Add tables generation

[Jakub Jelinek]

On Fri, Feb 14, 2014 at 04:50:34PM +0100, Bernd Schmidt wrote:
> How many offloaded functions do we really expect to have in an
> executable? I don't think that's likely to be a bottleneck.

First of all, this isn't just about offloaded functions, but also any
global variables that need to be mapped (for OpenMP #pragma omp declare
target surrounded vars).  Like functions, also the vars can be global
(but, with multiple DSOs, even global can be interposed and thus not unique
name), or static, at which point you can have the same name in between
different TUs.

> The use of a random-seed is really just a fallback, preferrably it
> uses the name of first symbol defined in the current translation
> unit which I think ought to be reliable enough.

Many TUs don't have any non-weak global symbols at all, if the symbols
they provide are all comdat etc., then you hit the random seed all the time.
Encoding the random seed into data sections of the binary is a problem for
build reproduceability, unless you always supply -frandom-seed=XXXX, but
then it isn't really that much random (e.g. the often used
-frandom-seed=$(@) or similar).  If the weirdo names are only used to name
symbols in .symtab, that randomness at least can be stripped off, but not
if it is in data sections.  So, to me this is far less reliable and against
the spirit of static symbols.

> >I still don't see what you find wrong on the approach with host/target
> >address arrays, if you are afraid something will reorder the arrays
> >(but, what would do that), one can insert indexes into both arrays as well,
> >which the linker can fill in and you can then verify.
> 
> It strikes me as really unnecessarily brittle. On the host side we'd
> have multiple objects linked together in some order to produce such
> a table, on the ptx side we'd have to produce the table all in one

Sure.  So, the linker/linker plugin orders the objects in some order
and thus by concatenation of the smaller per-TU tables creates the
host table, then the same linker/linker plugin just creates the to be target
table with the same order, and feeds that to the offloading target
compiler.

> go. Factor in possibilities like function cloning and I just think
> there are too many ways in which this can utterly fail. I'd rather
> have something that is more robust from the start even if it's
> slightly less efficient.

I don't see how function cloning or anything similar can make a difference
here, you have a function which is address taken and it's address is tracked
in some array, such function can't be cloned (well, can be cloned for
unrelated callers, but the table still keeps the original function, with the
same public ABI etc.).

	Jakub