linux-kernel - Re: [PATCH] MIPS: UAPI: Fix unrecognized opcode WSBH/DSBH/DSHD when using MIPS16.

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Date:	Mon, 7 Sep 2015 00:23:12 +0100 (BST)
From:	"Maciej W. Rozycki" <macro@...ux-mips.org>
To:	Yousong Zhou <yszhou4tech@...il.com>
cc:	Ralf Baechle <ralf@...ux-mips.org>, Chen Jie <chenj@...ote.com>,
	linux-mips@...ux-mips.org, linux-kernel@...r.kernel.org,
	Florian Fainelli <florian@...nwrt.org>
Subject: Re: [PATCH] MIPS: UAPI: Fix unrecognized opcode WSBH/DSBH/DSHD when
 using MIPS16.

On Sun, 6 Sep 2015, Yousong Zhou wrote:

> Hi, Maciej, first of all, thank you for your time on this,
> appreciate it.

 You're welcome!

> >  The bug certainly was there, it's just your analysis and consequently the
> > fix that are wrong in the general case for some reason, maybe a buggy
> > compiler.
> >
> 
> This is the compiler "--version",
> 
>     mips-openwrt-linux-gcc (OpenWrt/Linaro GCC 4.8-2014.04 r46763) 4.8.3

 A-ha!  I've checked GCC's history and the symptom you're seeing was PR 
target/55777:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=55777

and the bug has only been fixed pretty recently (r199328), already after 
GCC 4.8 has branched.  From your troubles I infer the fix has not been 
backported to 4.8 and therefore is only available from GCC 4.9 up; of 
course either you or OpenWrt can still backport it and apply locally.

 Fortunately as I noted you need not dive into that hole as the 
reasonable choice here is to avoid the asm for MIPS16 code altogether, 
regardless of the GCC bug.

> >  Now if you stick `.set nomips16' just above WSBH, then this code will
> > happily assemble, because this single instruction only (`.set pop' reverts
> > any previous `.set' directives; I'm assuming you wrote above by hand and
> > `.pop' is a typo) will assemble in the regular MIPS instruction mode.  But
> > if this code is ever reached, then the processor will still execute the
> > machine code produced by the assembler from the WSBH instruction in the
> > MIPS16 mode.
> 
> Yes, I hand-copied it from the output of "gcc -S" just to
> show the form/pattern (the original output is too long for
> this conversation).  No, that `.pop' is not a typo (I just
> did a double-check).

 Well, please check again then as there's no such pseudo-op:

$ cat pop.s
	.pop
$ mips-mti-linux-gnu-as -o pop.o pop.s
pop.s: Assembler messages:
pop.s:1: Error: unknown pseudo-op: `.pop'
$ 

> >  No, it's your bug after all.  I think the last paragraph I wrote quoted
> > above combined with the source code in question make it clear what to do.
> 
> Okay, I will try.  Most of the time when textbooks read
> clearly/obviously/apparently, things go astray ;)

 Well, if you really find yourself stuck with it, then come back for 
more hints, however please do try figuring it out yourself first as 
it'll be a good exercise.

  Maciej
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