Re: [Qemu-devel] [RFC PATCH qemu] dtc: Allow compiling with old gcc

[Alexey Kardashevskiy <aik@ozlabs.ru>]

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On 09/03/17 12:31, David Gibson wrote:
> On Thu, Mar 09, 2017 at 11:52:17AM +1100, Alexey Kardashevskiy wrote:
>> On 09/03/17 01:21, Peter Maydell wrote:
>>> On 8 March 2017 at 13:49, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>> After 6e85fce0225f "dtc: Update requirement to v1.4.2" QEMU stopped
>>>> compiling in CentOS7:
>>>>
>>>> In file included from /home/aik/p/qemu/dtc/libfdt/libfdt.h:54:0,
>>>>                  from /home/aik/p/qemu/device_tree.c:30:
>>>> /home/aik/p/qemu/dtc/libfdt/libfdt_env.h:64:0: error: "__bitwise" redefined [-Werror]
>>>>  #define __bitwise
>>>>  ^
>>>> In file included from /usr/include/asm/ptrace.h:27:0,
>>>>                  from /usr/include/asm/sigcontext.h:11,
>>>>                  from /usr/include/bits/sigcontext.h:27,
>>>>                  from /usr/include/signal.h:340,
>>>>                  from /home/aik/p/qemu/include/qemu/osdep.h:86,
>>>>                  from /home/aik/p/qemu/device_tree.c:14:
>>>> /usr/include/linux/types.h:21:0: note: this is the location of the previous definition
>>>>  #define __bitwise __bitwise__
>>>>  ^
>>>> cc1: all warnings being treated as errors
>>>> make: *** [device_tree.o] Error 1
>>>> make: *** Waiting for unfinished jobs....
>>>>
>>>> The reason is that CentOS7 comes with libfdt 1.4.0 so QEMU tries using
>>>> the internal one which does not compile as CentOS7 comes with gcc v4.8.5
>>>> which reports warnings which it would not if the OS's libfdt was used
>>>> (libfdt_env.h has not changed between 1.4.0 and 1.4.2).
>>>>
>>>> gcc 6.2.0 from Ubuntu v16.10 handles this fine.
>>>>
>>>> This replaces -I with -isystem to suppress the warning (which turns
>>>> to an error because of -Werror).
>>>
>>> Thanks for the bug report. I think it would be cleaner to fix this
>>
>> Agree.
>>
>>> by fixing the problem upstream in libfdt and then moving our
>>> submodule forward to the fixed version. (libfdt should not be
>>> defining __ prefixed symbols as these are reserved for the
>>> system.)
> 
> Yes.  I've now fixed this in upstream libfdt, at Paolo's suggestion.
> Unfortunately, it came just after the 1.4.3 release.  I was just leery
> about making a new release and qemu pull so soon after the last one.
> Should I do that anyway?
>
>> Also agree, regardless my proposal, libfdt should be fixed. However v2.9
>> won't compile on CentOS7 (and I suppose on RHEL7) which is a bit annoying.
>> And having "-isystem" makes some sense as these headers are normally system
>> ones and I'd think they should be treated the same way (i.e. path should be
>> included via -isystem vs. -I).
> 
> I'm not really clear on why -isystem fixes this problem anyway.

I believe it is a workaround as if all warnings were enabled, than many
(many) system headers would have triggered warnings because many have not
changed for quite a while and do not keep up with the actual gcc version.
"-isystem" tells gcc where it can relax warnings.

Pretty much like moving from older Microsoft Visual Studio to a newer one
would produce thousands warnings in my files (but not in system ones),
mostly about "deprecated" stuff but they could afford this as they ship
both the compiler and the standard library in one package :)


-- 
Alexey


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