Re: Creating a wrapper around a function at compile time

[Erick Ochoa <eochoa@gcc.gnu.org>]

Awesome! Thanks!

Let's go a little bit into the transformation mechanics itself. I am
somewhat familiar with LTO but I am always confused about the
transformations scheduled during WPA.

Let's assume that:

1. we have the profiling pass, which profiles each argument in each
callsite.
2. we are now running in the -fprofile-use so that means that we have
access to the value profiles for each of the arguments
3. there's an IPA/LTO pass which creates edge summaries storing "likely"
values for each of callsite/argument
4. we have a heuristic that let's us decide to some extent which subset of
likely values will yield good results (heavily used and specialization is
greatly beneficial if found) and run it during WPA.

Then, what happens next? I know that the idea is to create some parameter
tests at the callsite and call the respective parameter. The gap in my
understanding is that at this point I am assuming we are in WPA and
therefore have no function bodies to add these parameter tests. Would it be
sufficient to do the following:

1. Store in optimization (edge) summaries the (argument position x value x
specialized function version)
2. During LTRANS actually perform the transformation for the parameter test

Or is there some other way? I am still working on understanding how
ipa_make_edge_direct_to_target with speculative = true adds the edge and
function test during WPA. Otherwise, if we go with the two steps above, the
edges won't be visible to the rest of the IPA optimizations.

I'll be reading the cgraph_edge::make_speculative and the rest of the
infrastructure a bit more closely...

Any help or direction is greatly appreciated. Thanks!

-Erick