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RFC: cgraph/lowering vs. finish_file for GCC/UPC rewrites?

Recently, we have been working on upgrading GCC/UPC (see to the GCC trunk.  Previously,
we've sync'ed with the latest stable release, but
now we want to stay more current.

When built with GCC versions 4.0 through 4.3, we used
the gimplify language hook, LANG_HOOKS_GIMPLIFY_EXPR,
to rewrite trees that refer to UPC constructs and UPC
shared variable references - converting them into
non-UPC, gimplified, tree structures.  This worked
well, though we did need to extend the language hook
to include a gimplify test predicate and fallback
so that we can rewrite modify_expr's involving UPC
shared variables as the target:

upc_gimplify_expr (tree *expr_p,
   gimple_seq *pre_p, gimple_seq *post_p,
   bool (* gimple_test_f) (tree),
   int fallback)

Working with the latest GCC 4.5 snapshot, we have run
into a problem that leads me to believe that the current
approach will no longer work with the 4.5/trunk
version of GCC.

In prior GCC versions, the gimplify pass was called
before the call graph pass.  This meant that we could
safely employ the gimplify language hook to perform
the rewrites, which may emit inlined runtime calls.

An example UPC-related rewrite is to transform
UPC shared variable references into runtime calls.
This program:

shared int x;
shared int y;

int main()
  x = y;

might be translated into something like:

int main()
  int y_tmp = upc_get_int(upc_shared_addr(&y));
  upc_put_int(upc_shared_addr(&x), &y_tmp);

The definitions of the runtime functions upc_put_int()
and upc_get_int() are found in a pre-included header
file (the UPC driver adds a -include switch on the
command line).

Depending upon optimization level and compile time
switches - calls to the UPC runtime functions can
be implemented as either inlined function calls or
conventional calls to pre-compiled library routines.
At optimization levels above -O0, most of the UPC
runtime is inlined, by default.

With the new/current organization of the
compilation/call graph passes, we end up with the
surprising result that the inlined runtime function
definitions "disappear" before UPC's gimplify pass
can refer to them.  That's because the call graph
pass noticed that the inline runtime functions were
declared, but not referenced (yet).  The gimplify pass
is then run against the remaining function bodies,
but the UPC runtime functions are no longer available.

One workaround for this issue might be to mark the
runtime functions, in a fashion similar to ctors/dtors
so that the call graph pass won't eliminate them.
I'm unsure if that will get the inlining aspects of
those routines right, and it might retain unused
function definitions in the form of compiled
non-inlined code.

GOMP appears to use a "lowering" pass that runs after
the call graph and gimplify passes.  It calls runtime
routines via builtin function definitions, ensuring
that the function definitions won't go away.  However,
it looks to me as if GOMP does not inline those
runtime functions?

OBJC implements some post-processing in the
finish_file() hook routine, which in turn calls
objc_finish_file().  That may be a reasonable place
to relocate UPC's tree rewrites, but that leads to
a few questions:

Can gimplify_expr() be safely called on the same tree
more than once?  The question comes up because the
simplest thing is to retain the current infrastructure
where UPC rewrites occur in the gimplify language
hook.  The second gimplify pass will redo some
work, calling out to the UPC language hook again,
but since all UPC constructs have been rewritten and
gimplified, there will be no additional work done,
besides the traversal.

How about an alternative approach that implements a
custom tree-walk inside finish_file() (that is similar
in structure to that implemented in omp-low.c).
Is this rewrite routine allowed to selectively
gimplify parts of the tree and/or to create temp
variables managed by the code in gimplify.c?

Is the description above, of the interactions
between the cgraph, gimplify and lowering passes

What approach would you recommend for the
implementation of UPC tree re-writes that will
support calls to the runtime (that are inlined,
if applicable)?


- Gary

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