[cfe-dev] Disable certain llvm optimizations at clang frontend

[Y Song via cfe-dev]

Hi,

This is to seek some advice on how to disable certain llvm
optimizations when compiled with clang for BPF backend. For example,
we want to disable two optimizations, instcombine and simplifyCFG. I
would like to get some advice about what is the best way to do it.

Some Background on Why We want to do this
===================================

Any BPF code needs to go through a kernel verifier
   https://github.com/torvalds/linux/blob/master/kernel/bpf/verifier.c
before it can be executed in the kernel. verifier tries to ensure the safety
of the program, no illegal memory access, etc. In certain cases, clang
generated optimized codes make verification very hard, see pending
commits with another approach focused on BPF backend and some core
optimizations.
  https://reviews.llvm.org/D72787
  https://reviews.llvm.org/D82112
  https://reviews.llvm.org/D81859
To workaround the issue, people have to write inline asm or tweak
their program in a very user-unfriendly way to disable certain
transformations, in particular, instcombine and simplifyCFG.

Alternative Method in clang Frontend
============================

The above approach tried to disable/undo/influence specific
optimizations. Another approach is to disable instcombine and
simplifyCFG during compilation process. This is a little bit heavy
weight. But it may still be fine as this will ease the pain for
development. For most customers, a little performance loss vs.
development gain probably will be fine.

How to disable instcombine/simplifyCFG in clang frontends? It looks
like function attributes and insn metadatas are used to influence
optimizations.
The following approach might work, I think:
   BPF backend may add the following two attributes to all functions:
   "disable-instcombine" = "true"
   "disable-simplifyCFG" = "true"
And during instcombine and simplifyCFG, the above function attribute will
be checked, it is true, the pass will be skipped.

Do you think the above method could work? Any alternative suggestions
are also appreciated.

Thanks!

Yonghong