[klibc] fwd: fix up ARM assembly to use 'bx lr' in place of 'mov pc, lr'.

On Wed, Feb 16, 2011 at 04:01:44PM -0800, Steve Langasek wrote:

Thanks for forwarding this on, maks.

On Thu, Feb 17, 2011 at 01:50:19AM +0200, Kirill A. Shutemov wrote:

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Date: Wed, 16 Feb 2011 22:05:42 -0000
From: Steve Langasek <redacted>
Subject: [Bug 527720] Re: thumb2 porting issues identified: klibc uses mov.*pc

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I've also touched it up to be mergeable with Debian (support v4t builds
with #ifdef).

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Confirmed that installing the resulting klibc packages on my beagleboard
gives me a successfully-booting initramfs with klibc; and the
vfork/setjmptest test cases all pass in the klibc package tree.

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Fixes: https://bugs.launchpad.net/ubuntu/+source/klibc/+bug/527720

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--- klibc-1.5.20.orig/usr/klibc/arch/arm/vfork.S
+++ klibc-1.5.20/usr/klibc/arch/arm/vfork.S

@@ -25,7 +25,11 @@ vfork:
         ldrcs	r3, 1f
         mvncs	r0, #0
         strcs	r2, [r3]
+#if defined (___ARM_ARCH_4T__) || defined (__ARM_ARCH_4__)

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NAK. ARMv4T supports bx intruction. ARMv4 doesn't.

I see from
https://wiki.ubuntu.com/ARM/Thumb2PortingHowto#Identifying%20the%20Target%20Architecture
that you are correct - but then in
https://wiki.ubuntu.com/ARM/Thumb2PortingHowto#Detailed%20Instruction%20Behaviour
the recommendation given is to use exactly this guard for backwards
compatibility.

glibc-ports:

sysdeps/arm/sysdep.h:
#if (!defined (__ARM_ARCH_2__) && !defined (__ARM_ARCH_3__) \
     && !defined (__ARM_ARCH_3M__) && !defined (__ARM_ARCH_4__))
# define __USE_BX__
#endif

sysdeps/arm/dl-machine.h:
#if defined(__USE_BX__)
#define BX(x) "bx\t" #x
#else
#define BX(x) "mov\tpc, " #x
#endif

I'm cc:ing Dave Martin from ARM who drafted this porting guide.  Dave, was
there some other reason besides instruction compatibility to prefer the
original "mov pc,lr" on armv4t, or is this simply a miscopy?

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 	mov	pc, lr
+#else
+	bx	lr
+#endif

Thanks,
-- 
Steve Langasek                   Give me a lever long enough and a Free OS
Debian Developer                   to set it on, and I can move the world.
Ubuntu Developer                                    http://www.debian.org/
slangasek at ubuntu.com                                     vorlon at debian.org

-- 
 Kirill A. Shutemov