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Re: GCC 5 snapshots produce broken kernel for powerpc-e500v2-linux-gnuspe?


On 2014.09.09 at 17:35 +0800, Arseny Solokha wrote:
> Hello,
> 
> I've recently faced an issue I'm afraid I currently unable to debug. When
> building an arbitrary version of Linux kernel for powerpc-e500v2-linux-gnuspe
> target, it seems gcc prior to 5 produces a good image which boots just fine, and
> current gcc 5 snapshots (4.10.0-alpha20140810 for example) produce an image
> which hangs just after U-Boot hands over to the kernel.
> 
> This behavior is well reproducible on real hardware as well as under qemu. I've
> prepared a minimal kernel config which is dysfunctional as is but still enough
> to demonstrate the problem in qemu. I believe the exact Linux version number
> doesn't actually matter here, but see the attachment for details.
> 
> Compare the output produced by u-boot and this minified kernel build using
> gcc 4.9.1 and 4.10.0-alpha20140810 snapshot.
> 
> I now have completely no idea what to do next to find a cause of (1) gcc 5
> snapshots producing unbootable kernel, 

gcc trunk also miscompiles x86_64 kernels currently, but I haven't
looked deeper yet. 
The best way to narrow down the issue is to use git (or svn) bisect to
find out which gcc revision causes the miscompile.  Then you can md5sum
the kernel object files for the bad revision and for the first good
revision and compare the results. After that you can look at the
disassembly of the object files, for which md5sum differs, and try to
figure out the reason why.

-- 
Markus


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