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SubjectRe: Surprising code generated for vdso_read_begin()
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?


Segher

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