[Bug rtl-optimization/57968] New: MODE_EXIT switches inserted too late

[amylaar at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57968

            Bug ID: 57968
           Summary: MODE_EXIT switches inserted too late
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amylaar at gcc dot gnu.org

I see a miscompilation of ___muldi3 for epiphany-elf; the witch to MODE_EXIT
is done at the start of the exit block, even though it contains instructions
that require a different mode.

The return register on the epiphany is not likely spilled, and a large register
file gives lots of freedom to do optimizations; when optimizig ___muldi3, at
end end the return value is not so much copied as calculated piecemeal.

There is some existing code in compute_pre_exit that is supposed to preserve
computations that need a non-exit mode, but the way it is placed means it is
only active when the immediate result is part of the return register.