> On 1 Dec 2014, at 17:15, Andrew Cooper <andrew.coop...@citrix.com> wrote:
>
> On 01/12/14 14:37, John Haxby wrote:
>> With gcc 4.8.3, compiling xen-detect gives a compilation warning if
>> you're optimising:
>>
>> $ cc -Wall -Os xen-detect.c
>> xen-detect.c: In function ‘check_for_xen’:
>> xen-detect.c:65:9: warning: dereferencing type-punned pointer will break
>> strict-aliasing rules [-Wstrict-aliasing]
>> *(uint32_t *)(signature + 0) = regs[1];
>> ^
>>
>> Signed-off-by: John Haxby <john.ha...@oracle.com
>> <mailto:john.ha...@oracle.com>>
>
> Why are you compiling without the CFLAGS from the Xen build system?
>
> We explicitly disable strict alias optimisations, because optimisations
> based upon the aliasing rules in C is mad. Even when you eliminate all
> the warnings, there are still subtle bugs because the compiler is free
> to assume a lot more than a programmer would typically deem reasonable.
I wasn’t building the whole system, I just wanted xen-detect so I pulled it out
and compiled it; I usually use "-Wall -Os” because the combination finds
problems I might otherwise overlook. The patch also removes three lines of
code :) but you can take it or leave it as you choose. The other patch —
reversing the code of pv and hvm checking — was the real problem.
jch
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