[llvm-branch-commits] [llvm] [LivePhysRegs] Add callee-saved regs from MFI in addLiveOutsNoPristines. (PR #73553)

[Florian Hahn via llvm-branch-commits]

fhahn wrote:

@kparzysz please take a loo at https://gist.github.com/fhahn/67937125b64440a8a414909c4a1b7973, which has much more limited impact. 

> I haven't looked at the updated testcases in detail, but I see that most of the changes are in treating LR as live (whereas it was dead before). At the first glance that doesn't look right.

We might now overestimate liveness, which only results in missed perf correct? Although I think this is mostly theoretical at this point, as there's no test cases that show that. The issue is that underestimating as we currently do leads to incorrect results, in particular with tail calls that use LR implicitly. If LR isn't marked as live in that case, other passes are free to clobber LR (e.g. the machine-outliner by introducing calls using BL, as in https://github.com/llvm/llvm-project/blob/20f634f275b431ff256ba45cbcbb6dc5bd945fb3/llvm/test/CodeGen/Thumb2/outlined-fn-may-clobber-lr-in-caller.ll

https://github.com/llvm/llvm-project/pull/73553