Hello everyone,
I wanted to highlight this ticket on bugzilla [0]. It is a missed
optimization that I worked on. It involves propagating constants across
function calls at link-time. I am relatively new to GCC and this would
be my first significant contribution. I have developed a prototype of
the solution that seems to work well enough to compile and run CPU2017
intrate benchmarks correctly. I am now in the process of running the
full suite. Feedback would be appreciated.
Here's an overview of how it works:
ipa-initcall-cp collects all variables with static lifetime that contain
only a single constant write. Then, for each read of the variable, we
determine (statically) if the write occurs before read. In order to do
this, we use both the dominators graph and the static call graph. If the
write occurs before all reads, then we can safely substitute the read
with the constant being written to the variable. ipa-initcall-cp works
across function calls so the read and write do not need to occur on the
same function.
There are some issues which still need to be addressed, particularly,
ipa-initcall-cp is at the moment just a prototype and as such it will
stream in functions and modify them during WPA. I would like to fix
this, however I am not sure how to properly use clones when modifying
the function body.
I have tested this against applied my patch to GCC version 10.1. If you
have any questions, comments, let me know.
[0] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92538
