Why scheduler do not re-emit REG_DEAD notes?

[Jeff Law]

On 09/07/2015 10:05 AM, Konstantin Vladimirov wrote:
> Hi,
>
> In private backend for GCC 5.2.0, we do have target-specific scheduler
> (running in TARGET_SCHED_FINISH hook), that do some instruction
> packing/pairing on sched2 and relies on REG_DEAD notes, that should be
> correct.
>
> But they aren't because inside haifa-sched.c, that is being run first
> in the sched2 pass, reemit_notes function processes only REG_SAVE_NOTE
> case, and, after this scheduler, some insns with REG_DEAD on register,
> say r1, might be moved before previous r1 usage (input dependency
> case) and things become totally wrong.
>
> Now I appllied some minimal patch locally to fix it (just added
> separate REG_DEAD case).
>
> But may be it is part of design and may be it is generally true, that
> we can't rely on correct REG_DEAD notes in platform-specific
> scheduler?
You can not rely on death notes within the scheduler.  That's been in 
its design as long as I can remember (circa 1992).

jeff