I use GCC mainly to compile Scheme code that has been translated to
large, heavily macrofied C routines by the Gambit Scheme compiler:
https://github.com/gambit/gambit
Scheme semantics require that all procedure calls in "tail call
position" are, in fact, tail called.
For calls between Scheme procedures defined in the same file, Gambit's
compiler generates a C goto; intermodule calls use a trampoline.
Recently, the Gambit-generated C code was modified to take advantage of
LLVM's musttail attribute, when available.
So I've been investigating whether gcc's -foptimize-sibling-calls option
(for which I've found very little documentation) might produce similar
results.
This commit to GCC's source tree defines must_tail_call_plugin.c, which
seems (a) to be used only in some tests, and (b) to have had no patches
since first committed:
https://gcc.gnu.org/git/?p=gcc.git;a=commit;h=9a385c2d3d74ffed78f2ed9ad47b844d2f294105
I found this similar, more recent plugin:
https://github.com/pietro/gcc-musttail-plugin/
This seems to set up maybe_complain_about_tail_call in calls.c to fail
when a call is not compiled to a tail call.
So, my understanding of what I need to do is:
1. Built and dynamically link a plugin to set CALL_EXPR_MUST_TAIL_CALL
on all calls.
2. Use -foptimize-sibling-calls.
3. gcc fails with an appropriate message on the first call it can't
"tail call" in each file, or it succeeds.
So some questions:
1. Is this how you recommend I proceed?
2. Which plugin do you recommend?
3. I think it would be better to have a flag named something like
-Wcant-tail-call that would generate a warning in
maybe_complain_about_tail_call, so that (a) one could catch all such
cases at once and (b) one can throw an error if needed by specifying
-Werror, and (c) I wouldn't need to fiddle with a plugin.
Do you agree with this idea? I added a few flags to gcc about 20 years
ago, I guess I could look at it again to see how to do such a thing now.
Brad Lucier
