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From: Segher Boessenkool <segher@kernel.crashing.org>
To: Michael Ellerman <mpe@ellerman.id.au>
Cc: parkerderek86@gmail.com, pmenzel@molgen.mpg.de,
	laboger@linux.vnet.ibm.com, xaionaro@gmail.com,
	Paul Murphy <murp@ibm.com>,
	paulus@samba.org, murphyp@linux.ibm.com,
	linuxppc-dev@lists.ozlabs.org
Subject: Re: Possible regression by ab037dd87a2f (powerpc/vdso: Switch VDSO to generic C implementation.)
Date: Mon, 2 Aug 2021 13:07:04 -0500	[thread overview]
Message-ID: <20210802180704.GO1583@gate.crashing.org> (raw)
In-Reply-To: <87czqwl67h.fsf@mpe.ellerman.id.au>

On Mon, Aug 02, 2021 at 04:18:58PM +1000, Michael Ellerman wrote:
> > Go up to this point has only used the vdso function __kernel_clock_gettime; it
> > is the only entry point which would need to explicitly avoid R30 for
> > Go's sake.
> 
> I thought about limiting the workaround to just that code, but it seemed
> silly and likely to come back to bite us. Once the compilers decides to
> spill a non-volatile there are plenty of other ones to choose from.

It can be cheaper to spill N..31 consecutively, using stmw for example.
For 64-bit Power implementations it doesn't currently make any
difference.  Since none of this will be inlined it doesn't have any real
impact.

(This also happens with -m32 -fpic, which always sets GPR30 as fixed, it
is the offset table register.  With those flags -ffixed-r30 doesn't do
anything btw (r30 already *is* a fixed function register), and this will
not work with a Go that clobbers r30.  But this is academic :-) )


Segher

  parent reply	other threads:[~2021-08-02 18:10 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-27  8:45 Paul Menzel
2021-07-27 23:14 ` Benjamin Herrenschmidt
2021-07-28  8:26   ` Paul Menzel
2021-07-28 12:43     ` Michael Ellerman
2021-07-28 12:53       ` Paul Menzel
2021-07-29  7:41         ` Michael Ellerman
2021-07-29  8:33           ` Paul Menzel
2021-07-29 12:58             ` Michael Ellerman
     [not found]         ` <875ywt1s9r.fsf__45665.8238823124$1627544516$gmane$org@mpe.ellerman.id.au>
2021-07-29  8:48           ` Andreas Schwab
2021-08-02  6:04             ` Michael Ellerman
     [not found]       ` <OF44F7146F.67A4C1C2-ON00258720.004DBF64-00258720.004FCFCC@ibm.com>
2021-08-02  6:18         ` Michael Ellerman
2021-08-02 12:48           ` Benjamin Herrenschmidt
2021-08-02 18:07           ` Segher Boessenkool [this message]
2021-08-02 18:14           ` Lynn Boger

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