[LLVMdev] Would like to force one minor, mechanical change on out-of-tree users of the old pass manager

[Chandler Carruth]

Greetings folks.

I had really wanted out-of-tree folks to be able to make only a single
change to their code due to the new pass manager; essentially, by the time
they had to touch the code at all I wanted them to be able to port
completely to the new pass manager.

However, Richard has raised the issue that this is nearly impossible to
make work with C++ modules, and we've lost the modules build bot that was
checking our self host in this mode because of this.

So, now that 3.6 is branched, I wondered if folks would be OK with me
switching code to explicitly refer to some of the legacy constructs with
the "legacy" namespace. I'm absolutely committed to the new pass manager
being done prior to 3.7 (actually, a lot sooner). So this would require
(entirely mechanical) source changes to out-of-tree users who are tracking
trunk in the interim.

However, the changes would only be required for Pass*Manager* and related
classes. Neither Pass, FunctionPass, or PassManagerBuilder would change.

Any objections to this? While it clearly has cost and would not be my
preferred approach, the benefits seem to outweigh the costs here.

-Chandler
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