Uros Bizjak <ubiz...@gmail.com> writes:
> On Fri, Jan 13, 2012 at 10:00 AM, Eric Botcazou <ebotca...@adacore.com> wrote:
>>> The problem my patch solves is the answer to the question "Is the
>>> choosen non-live temporary register untouched over the insn
>>> sequence?". The answer: "Yes, if it was not set or clobbered by any
>>> insn in the sequence".
>>
>> Are you sure that you don't need to do this in addition to the existing test,
>> instead of in lieu of the existing test?
>
> Yes, because new test ALWAYS includes the registers that were wrongly
> marked as dead by previous test due to REG_UNUSED and noclobber
> processing.
Yeah, the patch looks like the right fix to me FWIW. Being able to use
DF_INSN_DEFS in this way is part of the point of having df: when going
from one insn to the next within the same bb, there are no "hidden"
influences that would cause a register to become live outside of
DF_INSN_DEFS. The only requirement is that we call df_insn_rescan
on new instructions, and we do (in peep2_update_life, where we need
it for df_simulate_one_insn_backwards).
Or to put it another way: the insn-to-insn changes in the current liveness
sets are all produced by df_simulate_one_insn_{backwards,forwards}, which
uses the same information. So I don't think there's any reason why we
need to keep the current code as well.
Richard
