Re: 0001-Don-t-skip-prologue-instructions-as-it-could-affect-.patch

[Jeff Law <jeffreyalaw@gmail.com>]

On 7/26/2021 4:55 PM, Segher Boessenkool wrote:
> On Fri, Jul 23, 2021 at 10:27:37AM -0600, Jeff Law wrote:
>> On 7/22/2021 7:04 AM, Richard Biener via Gcc-patches wrote:
>>> On Thu, Jul 22, 2021 at 9:02 AM Bin.Cheng via Gcc-patches
>>> <gcc-patches@gcc.gnu.org> wrote:
>>>> Gentle ping.  Any suggestions would be appreciated.
>>> So just to say something - does the existing code mean that any use of
>>> the alias info on prologue/epilogue insns is wrong?  We have
>>>
>>>    /* The prologue/epilogue insns are not threaded onto the
>>>       insn chain until after reload has completed.  Thus,
>>>       there is no sense wasting time checking if INSN is in
>>>       the prologue/epilogue until after reload has completed.  */
>>>    bool could_be_prologue_epilogue = ((targetm.have_prologue ()
>>>                                        || targetm.have_epilogue ())
>>>                                       && reload_completed);
>>>
>>> so when !could_be_prologue_epilogue then passes shouldn't run into
>>> them if the comment is correct.  But else even epilogue stmts could appear
>>> anywhere (like scheduled around)?  So why's skipping those OK?
>> These insns don't exist until after reload has completed.  I think this
>> code is just trying to be more compile-time efficient and not look for
>> them when they're known to not exist.
> Yeah.  Unfortunately it isn't trivial to see if this is a premature
> optimisation, or if this is needed for correctness instead.  But it is
> stage 1 still :-)


https://gcc.gnu.org/pipermail/gcc-patches/2000-May/031560.html

Jeff