Today is the last day of mediocre library call optimizations

To this day, we still do too little when it comes to optimizing well
known library calls.

Given that various libraries, especially the C/C++ standard library, are
relatively clear defined, and LLVM has all the infrastructure already in
place, e.g., SimplifyLibCalls.cpp, it seems odd that we still miss so
many optimization opportunities.

To show how easy it is, and how much impact it can have, I crafted a
very simple, yet very effective patch which will hopefully only be the
first of its kind.

The attached twelve line addition to SimplifyLibCalls.cpp provides
subsequent passes with a better understanding of the "getenv" function.

This function was chosen to showcase the enormous impact well placed
information can have on real world software, including LLVM itself.

While you could take my word for it, I would encourage you to try it out
yourself. Apply the patch and create *a new build* of LLVM to compare
against your existing one. I firmly believe the result will be rather
surprising for most.

With a bit of luck we may even see more patches like this one today.
Even if you are all too busy today, there is always next year I guess.

Thanks in advance,
  Johannes

0001-LibCalls-Improve-optimizations-for-getenv-library-ca.patch (1.64 KB)

Hi Johannes,

It would help if the patch had some explanation (ideally in the comments) of what it is trying to accomplish. If I’m reading the integer promotion correctly, it is indicating that the pointer value for a getenv return is in the low 4GB of the address space. I suspect that this is unsound on platforms with non-integer pointer representations and I don’t believe it’s correct on any platform with ASLR.

I may be completely misunderstanding what the patch does, but without comments it’s difficult to tell.

David

I suspect that your understanding of the patch will improve if you pay careful attention to the date of the submission.

Hi David,

thanks for your input!

I was actually trying to eliminate only one value from the set of
possible results, namely -1, though it was pretty late and I might have
overdone it.

Sorry for any confusion this might have caused.

Cheers,
  Johannes