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Date: Thu, 6 Aug 2020 05:46:18 +0000 From: Christophe Leroy <christophe.leroy@...roup.eu> To: Segher Boessenkool <segher@...nel.crashing.org> 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 08/05/2020 06:40 PM, Segher Boessenkool wrote: > 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?) GCC 10.1 > >>> 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. As a standalone function: With your suggestion: 000006ac <vdso_shift_ns>: 6ac: 54 a5 06 fe clrlwi r5,r5,27 6b0: 35 25 ff e0 addic. r9,r5,-32 6b4: 41 80 00 10 blt 6c4 <vdso_shift_ns+0x18> 6b8: 7c 64 4c 30 srw r4,r3,r9 6bc: 38 60 00 00 li r3,0 6c0: 4e 80 00 20 blr 6c4: 54 69 08 3c rlwinm r9,r3,1,0,30 6c8: 21 45 00 1f subfic r10,r5,31 6cc: 7c 84 2c 30 srw r4,r4,r5 6d0: 7d 29 50 30 slw r9,r9,r10 6d4: 7c 63 2c 30 srw r3,r3,r5 6d8: 7d 24 23 78 or r4,r9,r4 6dc: 4e 80 00 20 blr With the version as is in my series: 000006ac <vdso_shift_ns>: 6ac: 21 25 00 20 subfic r9,r5,32 6b0: 7c 69 48 30 slw r9,r3,r9 6b4: 7c 84 2c 30 srw r4,r4,r5 6b8: 7d 24 23 78 or r4,r9,r4 6bc: 7c 63 2c 30 srw r3,r3,r5 6c0: 4e 80 00 20 blr Christophe
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