> I've got two versions of the same piece of code, the only difference
> is that lines are reordered:
>
> #if 1
> u32Tmp = ntohl(*reinterpret_cast<u32*>(pDat+4));
> sPhasor.angle = interpretU32AsFloat(u32Tmp);
> u32Tmp = ntohl(*reinterpret_cast<u32*>(pDat));
> sPhasor.mag = interpretU32AsFloat(u32Tmp);
> #else
> u32Tmp = ntohl(*reinterpret_cast<u32*>(pDat));
> sPhasor.mag = interpretU32AsFloat(u32Tmp);
> u32Tmp = ntohl(*reinterpret_cast<u32*>(pDat+4));
> sPhasor.angle = interpretU32AsFloat(u32Tmp);
> #endif
>
> In case '#if 1' sPhasor.angle is assigned before sPhasor.mag, and
> everything works fine.
>
> In case '#if 0' it's the other way round, and I get mode switches
> caused by the first (but not second) ntohl().
I forgot to mention the mode switch is actually a unaligned access
exception as shown here:
$> cat /proc/xenomai/faults
TRAP CPU0 CPU1
0: 0 50 (Data or instruction access)
1: 0 0 (Section fault)
2: 0 0 (Generic data abort)
3: 0 0 (Unknown exception)
4: 0 0 (Instruction breakpoint)
5: 0 0 (Floating point exception)
6: 0 0 (VFP Floating point exception)
7: 0 0 (Undefined instruction)
8: 0 14629654 (Unaligned access exception)
>
> Looking at the assembly, there is a clear difference:
>
> #if 1
> 0x2ac5bcb8 <+304>: ldr r3, [r5]
> 0x2ac5bcbc <+308>: ldr r2, [r5, #4]
> 0x2ac5bcc0 <+312>: rev r3, r3
> 0x2ac5bcc4 <+316>: rev r2, r2
> [...]
> #else
> 0x2abc3cb8 <+304>: ldm r5, {r2, r3}
> => 0x2abc3cbc <+308>: rev r2, r2
> 0x2abc3cc0 <+312>: rev r3, r3
> [...]
> #endif
>
> Here the arrow indicates where the mode switch occurs, which is at the
> 'rev' (for reverse byte order instruction), hence ntohl(). It seems
> that the assembler optimization that collates the two ' ldr r3, [r5]'
> and 'ldr r2, [r5, #4]' into ' ldm r5, {r2, r3}' could have something
> to do with the mode switches...
>
> Any clues what causes the mode switches and how to avoid them?
>
> Cheers,
>
> Andreas
>
>
> Setup:
> i.MX6 dual (ARM Cortex A9)
> $> cat /proc/version
> Linux version 3.0.43-tpcom_run2-PD13.2.4 (andreas@gorse) (gcc version
> 4.6.2 (OSELAS.Toolchain-2011.11.3 linaro-4.6-2011.11) ) #1 SMP PREEMPT
> Tue Sep 26 16:17:30 BST 2017
> $> cat /proc/xenomai/version
> 2.6.4
> $> cat /proc/ipipe/version
> 1.18-13
> $> arm-cortexa9-linux-gnueabi-gcc --version
> arm-cortexa9-linux-gnueabi-gcc (OSELAS.Toolchain-2011.11.3
> linaro-4.6-2011.11) 4.6.2
> $> arm-cortexa9-linux-gnueabi-ld --version
> GNU ld (GNU Binutils) 2.21.1
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