From: Henri Cloetens <henri.cloetens@blueice.be>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Question on gcc RTL and scheduling step.
Date: Wed, 26 Feb 2020 17:02:00 -0000 [thread overview]
Message-ID: <86e3b82f-61a0-dc59-1511-5b0de7ae0866@blueice.be> (raw)
In-Reply-To: <20200226125927.GM22482@gate.crashing.org>
Hello Segher, all,
- On the issue of the restore of R18 in mode DI, it seems from
 analysing the log file that the compiler does not understand this
 also affects R19. This looks a bug to me ??. Anyway, I have resolved
 this, by adding a 'clobber' statement to R19 when R18 is written
 in mode 'DI'.
- The other issue, I have changed ((set) (pc) (return)) to (return),
 but, as far as I understand the logfiles, and the documentation,
 both forms are equivalent. Anyway, after fixing the first issue, I do
 not see the second issue any more.
Best Regards and thanks,
Henri.
On 02/26/2020 01:59 PM, Segher Boessenkool wrote:
> Hi!
>
> On Wed, Feb 26, 2020 at 01:02:46PM +0100, Henri Cloetens wrote:
>> 1. The epilogue generator emits an instruction "popret_internal" that
>> Â Â Â a. Restores R18 in mode DI. (which corresponds to R18 and R19 in
>> mode SI)
>> Â Â Â b. Does the function return.
>> 2. Then, during scheduling, the compiler shifts an add instruction PAST
>> this return
>> Â Â Â instruction, which is incorrect, because:
>> Â Â (a.) The instruction writes R19. (it restores R18 and R19). The
>> scheduler should see this.
> Yes. So why doesn't it? Look at the various dump files to see what the
> df ("dataflow") framework thinks about this.
>
>> Â Â (b.) The instruction changes the PC (program counter).
> The documentation says you should write just
> (return)
> instead of
> (set (pc) (return))
> (unlikely to be causing any of this, but a good cleanup to make).
>
>> Q. Why is this happening ?. How can I prevent this ?.
> You can have the scheduler print much more debugging info, but I'd
> start with the usual stuff: -dap.
>
>
> Segher
>
next prev parent reply other threads:[~2020-02-26 15:10 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-26 15:10 Henri Cloetens
2020-02-26 16:39 ` Segher Boessenkool
2020-02-26 17:02 ` Henri Cloetens [this message]
2020-02-26 22:00 ` Segher Boessenkool
2020-02-27 7:43 ` Henri Cloetens
2020-02-27 9:23 ` Segher Boessenkool
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