An tiny optimization to the ARMv6 backend

[Ben Shi]

Hello, Golang developers,

I find the following lines (line 95 - 107) https://github.com/golang/go/blob/master/src/cmd/compile/internal/ssa/gen/ARM.rules

// byte swap
// let (a, b, c, d) be the bytes of x from high to low
// t1 = x right rotate 16 bits -- (c,   d,   a,   b  )
// t2 = x ^ t1                 -- (a^c, b^d, a^c, b^d)
// t3 = t2 &^ 0xff0000         -- (a^c, 0,   a^c, b^d)
// t4 = t3 >> 8                -- (0,   a^c, 0,   a^c)
// t5 = x right rotate 8 bits  -- (d,   a,   b,   c  )
// result = t4 ^ t5            -- (d,   c,   b,   a  )
// using shifted ops this can be done in 4 instructions.
(Bswap32 <t> x) ->
    (XOR <t>
        (SRLconst <t> (BICconst <t> (XOR <t> x (SRRconst <t> [16] x)) [0xff0000]) [8])
(SRRconst <t> x [8]))

I thought they can be optimized to a simple REV instruction, which is avaliable in ARMv6. This instruction is described on page 562 of the "ARM Architecture Reference Manual" rev C.c.

If I want to make this change, should I follow the "contributing Guildline" https://golang.org/doc/contribute.html ?
 
Ben Shi

[cherry]

In Go 1.8 we still support ARMv5 (https://tip.golang.org/doc/go1.8#ports).

If I want to make this change, should I follow the "contributing Guildline" https://golang.org/doc/contribute.html ?

Yes, go submit the change when Go 1.9 tree is open in February.

[Ben Shi]

So currently the ARM.rules and ARMOps.go are not optimized due to compatibility of ARMv5?

And no submit about ARM is accepted until Feb 2017?

[minux]

On Mon, Dec 12, 2016 at 10:46 PM, Ben Shi <power...@163.com> wrote:

So currently the ARM.rules and ARMOps.go are not optimized due to compatibility of ARMv5?

Yes. That's at least part of the reason. We plan to drop support for ARMv5 in

Go 1.9, so we can use all ARMv6 instructions then.

And no submit about ARM is accepted until Feb 2017?

We're in code freeze in preparation for the 1.8 release, so only bug fixes

and docs changes are accepted until the tree reopens in Feb 2017. It's

not only about ARM changes.

Please see golang.org/s/releasesched for details about Go's release schedule.

[Keith Randall]

Note that you can get your code all done, tested, code reviewed, and ready to submit right now.  We just can't press the submit button until Feb.

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[Dan Kortschak]

Is this set in stone? I have hardware that cannot be upgraded and is
ARMv5l.

[Brad Fitzpatrick]

You're now the second person asking for continued arm5 support.

Not in stone yet.

[Dan Kortschak]

Yeah, thanks - I just got to that thread. Thank you for the
consideration.

[minux]

On Thu, Dec 15, 2016 at 7:11 PM, Dan Kortschak <dan.ko...@adelaide.edu.au> wrote:

Is this set in stone? I have hardware that cannot be upgraded and is
ARMv5l.

There will always be ARMv5 hardware out there. I think we need to set a

criteria for retaining the support for ARMv5 and ARMv6 (prior to ARMv6K).

One such criteria could be finding a real ARMv5E builder. Without a builder,

there is no point in maintaining ARMv5 support.

Let's continue the discussion in #17082.