[llvm-commits] ideas for 10096

[Rafael Ávila de Espíndola]

I am trying to fix 10203 and currently I have three ideas of how it 
might be fixed:

*) Fix bit rot on the strong phi elimination. IT currently crashes when 
building firefox.


*) A simple preprocessioning pass that would try to convert

     %vreg40<def> = COPY %vreg45
     %vreg42<def> = COPY %vreg45

to

     %vreg40<def> = COPY %vreg45
     %vreg42<def> = COPY %vreg40

That is, try to use in the RHS values that have been defined in the same bb.

* The third idea (which I like a bit more right now) is to change the 
coalescer itself

  Right now it works in three stages if I understand it correctly.
  1) it collects which value numbers of A are copies of B and B of A.
  2) It then optimistically merges the intervals, by first copying the 
LHS and then the RHS with exceptions for the A=B and B=A cases.
  3) It now walks the ranges of both registers looking for overlapping 
ones. If it finds one, it checks if both have the same value number in 
the new merged interval.

  I think this can be done in one pass. As we walk the ranges, we can 
find the value numbers and check if they are compatible. Doing this in 
one pass then has the advantage that it is easy to add support for the 
case we have in the bug.

In this case, the dominance check can be as simple an "defined earlier 
in the same bb".

So, do you agree with the third option being the best or would a 
preprocess pass (or fixing the strong phi elimination) be better?

Cheers,
Rafael