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Groups > linux.kernel > #1326820
| From | Christophe Leroy <christophe.leroy@c-s.fr> |
|---|---|
| Newsgroups | linux.kernel |
| Subject | Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync() |
| Date | 2016-02-04 14:50 +0100 |
| Message-ID | <qYssj-RV-39@gated-at.bofh.it> (permalink) |
| References | <qYez0-8kz-11@gated-at.bofh.it> <qYez3-8kz-87@gated-at.bofh.it> <qYqJQ-88K-9@gated-at.bofh.it> |
| Organization | linux.* mail to news gateway |
Le 04/02/2016 12:37, Denis Kirjanov a écrit : > On 2/4/16, Christophe Leroy <christophe.leroy@c-s.fr> wrote: >> This simplification helps the compiler. We now have only one test >> instead of two, so it reduces the number of branches. >> >> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr> >> --- >> v2: new >> v3: no change >> v4: no change >> v5: no change >> >> arch/powerpc/mm/dma-noncoherent.c | 2 +- >> 1 file changed, 1 insertion(+), 1 deletion(-) >> >> diff --git a/arch/powerpc/mm/dma-noncoherent.c >> b/arch/powerpc/mm/dma-noncoherent.c >> index 169aba4..2dc74e5 100644 >> --- a/arch/powerpc/mm/dma-noncoherent.c >> +++ b/arch/powerpc/mm/dma-noncoherent.c >> @@ -327,7 +327,7 @@ void __dma_sync(void *vaddr, size_t size, int direction) >> * invalidate only when cache-line aligned otherwise there is >> * the potential for discarding uncommitted data from the cache >> */ >> - if ((start & (L1_CACHE_BYTES - 1)) || (size & (L1_CACHE_BYTES - 1))) >> + if ((start | end) & (L1_CACHE_BYTES - 1)) >> flush_dcache_range(start, end); >> else >> invalidate_dcache_range(start, end); > The previous version of address cache-line aligned check reads perfectly fine. > What's the benefit of this micro optimization? With this optimisation we avoid one unneccessary test and two associated jumps. Taking into account that __dma_sync() is one of the top ten CPU consummers, I believe it is worth it: Without the patch: c000d894: 70 6a 00 0f andi. r10,r3,15 c000d898: 39 29 00 0f addi r9,r9,15 c000d89c: 54 63 00 36 rlwinm r3,r3,0,0,27 c000d8a0: 7d 23 48 50 subf r9,r3,r9 c000d8a4: 41 82 00 84 beq c000d928 <__dma_sync+0xb8> [...] c000d8c0: 7c 00 04 ac sync c000d8c4: 4e 80 00 20 blr [...] c000d928: 70 8a 00 0f andi. r10,r4,15 c000d92c: 40 a2 ff 7c bne c000d8a8 <__dma_sync+0x38> c000d930: 55 2a e1 3f rlwinm. r10,r9,28,4,31 c000d934: 41 a2 ff 8c beq c000d8c0 <__dma_sync+0x50> With the patch: c000d894: 7c 89 1b 78 or r9,r4,r3 c000d898: 71 2a 00 0f andi. r10,r9,15 c000d89c: 54 63 00 36 rlwinm r3,r3,0,0,27 c000d8a0: 38 84 00 0f addi r4,r4,15 c000d8a4: 7c 83 20 50 subf r4,r3,r4 c000d8a8: 41 82 00 84 beq c000d92c <__dma_sync+0xbc> [...] c000d8c4: 7c 00 04 ac sync c000d8c8: 4e 80 00 20 blr [...] c000d92c: 54 89 e1 3f rlwinm. r9,r4,28,4,31 c000d930: 41 a2 ff 94 beq c000d8c4 <__dma_sync+0x54> Christophe >> -- >> 2.1.0 >> >> _______________________________________________ >> Linuxppc-dev mailing list >> Linuxppc-dev@lists.ozlabs.org >> https://lists.ozlabs.org/listinfo/linuxppc-dev
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[PATCH v5 21/23] powerpc: Simplify test in __dma_sync() Christophe Leroy <christophe.leroy@c-s.fr> - 2016-02-04 00:00 +0100
Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync() Denis Kirjanov <kda@linux-powerpc.org> - 2016-02-04 13:00 +0100
Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync() Christophe Leroy <christophe.leroy@c-s.fr> - 2016-02-04 14:50 +0100
Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync() Christophe Leroy <christophe.leroy@c-s.fr> - 2016-02-05 09:00 +0100
Re: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync() Denis Kirjanov <kda@linux-powerpc.org> - 2016-02-05 09:00 +0100
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