[LLVMdev] metadata to inform the optimizers that some code should stay unchanged

[Xinfinity]

Hello LLVM,

I am working on some passes that perform code transformations. Since I am
interested in performance, I apply the O3 passes, right after my pass.
However, the optimization passes modify the code inserted by my pass in an
undesirable way. As far I know, there is no way to prevent the optimizers
from optimizing some regions of code. So what I intend to do is to attach
metadata to the instructions contained in basicblocks that I want to remain
unchanged, and to modify some of the optimization passes to be aware of the
metadata. For now, I am interested only in passes that would affect the
control flow graph, so for a start I modify the Simplify CFG pass and the
jump threading pass, but I will check which other passes might duplicate the
code, merge blocks etc.

Do you think the performance will drop significantly if some regions of code
are not optimized ? And do you consider this modification would be of any
use to the community?

Thank you.
Alexandra

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