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SubjectRe: Surprising code generated for vdso_read_begin()
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Le 09/01/2020 à 21:07, Segher Boessenkool a écrit :
> On Thu, Jan 09, 2020 at 05:52:34PM +0000, Christophe Leroy wrote:
>> Wondering why we get something so complicated/redundant for
>> vdso_read_begin() <include/vdso/helpers.h>
>>
>> static __always_inline u32 vdso_read_begin(const struct vdso_data *vd)
>> {
>> u32 seq;
>>
>> while ((seq = READ_ONCE(vd->seq)) & 1)
>> cpu_relax();
>>
>> smp_rmb();
>> return seq;
>> }
>>
>>
>> 6e0: 81 05 00 f0 lwz r8,240(r5)
>> 6e4: 71 09 00 01 andi. r9,r8,1
>> 6e8: 41 82 00 10 beq 6f8 <__c_kernel_clock_gettime+0x158>
>> 6ec: 81 05 00 f0 lwz r8,240(r5)
>> 6f0: 71 0a 00 01 andi. r10,r8,1
>> 6f4: 40 82 ff f8 bne 6ec <__c_kernel_clock_gettime+0x14c>
>> 6f8:
>>
>> r5 being vd pointer
>>
>> Why the first triplet, not only the second triplet ? Something wrong
>> with using READ_ONCE() for that ?
>
> It looks like the compiler did loop peeling. What GCC version is this?
> Please try current trunk (to become GCC 10), or at least GCC 9?
>

It is with GCC 5.5

https://mirrors.edge.kernel.org/pub/tools/crosstool/ doesn't have more
recent than 8.1

With 8.1, the problem doesn't show up.

Thanks
Christophe

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