From: Segher Boessenkool <segher@kernel.crashing.org> To: Christophe Leroy <christophe.leroy@csgroup.eu> Cc: nathanl@linux.ibm.com, linux-arch@vger.kernel.org, vincenzo.frascino@arm.com, arnd@arndb.de, linux-kernel@vger.kernel.org, Paul Mackerras <paulus@samba.org>, luto@kernel.org, tglx@linutronix.de, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation. Date: Wed, 5 Aug 2020 13:40:54 -0500 [thread overview] Message-ID: <20200805184054.GQ6753@gate.crashing.org> (raw) In-Reply-To: <3db2a590-b842-83db-ed2b-f3ee62595f18@csgroup.eu> 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
WARNING: multiple messages have this Message-ID (diff)
From: Segher Boessenkool <segher@kernel.crashing.org> To: Christophe Leroy <christophe.leroy@csgroup.eu> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>, Paul Mackerras <paulus@samba.org>, Michael Ellerman <mpe@ellerman.id.au>, nathanl@linux.ibm.com, anton@ozlabs.org, linux-arch@vger.kernel.org, arnd@arndb.de, linux-kernel@vger.kernel.org, luto@kernel.org, tglx@linutronix.de, vincenzo.frascino@arm.com, linuxppc-dev@lists.ozlabs.org Subject: Re: [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation. Date: Wed, 5 Aug 2020 13:40:54 -0500 [thread overview] Message-ID: <20200805184054.GQ6753@gate.crashing.org> (raw) Message-ID: <20200805184054.xOnIXdPE6ot-_QFyA13htJ6b4oUiNYXUAi4YTAjG07w@z> (raw) In-Reply-To: <3db2a590-b842-83db-ed2b-f3ee62595f18@csgroup.eu> 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
next prev parent reply other threads:[~2020-08-05 18:40 UTC|newest] Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-08-05 7:09 [PATCH v10 0/5] powerpc: switch VDSO to C implementation Christophe Leroy 2020-08-05 7:09 ` [PATCH v10 1/5] powerpc/processor: Move cpu_relax() into asm/vdso/processor.h Christophe Leroy 2020-08-05 7:09 ` [PATCH v10 2/5] powerpc/vdso: Prepare for switching VDSO to generic C implementation Christophe Leroy 2020-08-05 14:03 ` Segher Boessenkool 2020-08-05 14:03 ` Segher Boessenkool 2020-08-05 16:40 ` Christophe Leroy 2020-08-05 16:40 ` Christophe Leroy 2020-08-05 18:40 ` Segher Boessenkool [this message] 2020-08-05 18:40 ` Segher Boessenkool 2020-08-06 5:46 ` Christophe Leroy 2020-08-05 16:51 ` Christophe Leroy 2020-08-05 16:51 ` Christophe Leroy 2020-08-05 20:55 ` Segher Boessenkool 2020-08-05 7:09 ` [PATCH v10 3/5] powerpc/vdso: Save and restore TOC pointer on PPC64 Christophe Leroy 2020-08-05 7:09 ` [PATCH v10 4/5] powerpc/vdso: Switch VDSO to generic C implementation Christophe Leroy 2020-08-05 7:09 ` Christophe Leroy 2020-08-05 7:09 ` [PATCH v10 5/5] powerpc/vdso: Provide __kernel_clock_gettime64() on vdso32 Christophe Leroy 2020-08-05 7:09 ` Christophe Leroy
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