linux-kernel - Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation.

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Date:   Wed, 5 Aug 2020 13:40:54 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, nathanl@...ux.ibm.com,
        anton@...abs.org, linux-arch@...r.kernel.org, arnd@...db.de,
        linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de,
        vincenzo.frascino@....com, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation.

Hi!

On Wed, Aug 05, 2020 at 04:40:16PM +0000, Christophe Leroy wrote:
> >It cannot optimise it because it does not know shift < 32.  The code
> >below is incorrect for shift equal to 32, fwiw.
> 
> Is there a way to tell it ?

Sure, for example the &31 should work (but it doesn't, with the GCC
version you used -- which version is that?)

> >What does the compiler do for just
> >
> >static __always_inline u64 vdso_shift_ns(u64 ns, unsigned long shift)
> >	return ns >> (shift & 31);
> >}
> >
> 
> Worse:

I cannot make heads or tails of all that branch spaghetti, sorry.

>  73c:	55 8c 06 fe 	clrlwi  r12,r12,27
>  740:	7f c8 f0 14 	addc    r30,r8,r30
>  744:	7c c6 4a 14 	add     r6,r6,r9
>  748:	7c c6 e1 14 	adde    r6,r6,r28
>  74c:	34 6c ff e0 	addic.  r3,r12,-32
>  750:	41 80 00 70 	blt     7c0 <__c_kernel_clock_gettime+0x114>

This branch is always true.  Hrm.


Segher