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    SubjectRe: [PATCH v5 21/23] powerpc: Simplify test in __dma_sync()
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    On 2/4/16, Christophe Leroy <christophe.leroy@c-s.fr> wrote:
    >
    >
    > 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>

    Yeah, looks better. Did you compile the kernel with default compiler flags?

    Thanks!

    >
    >
    > Christophe
    >>> --
    >>> 2.1.0
    >>>
    >>> _______________________________________________
    >>> Linuxppc-dev mailing list
    >>> Linuxppc-dev@lists.ozlabs.org
    >>> https://lists.ozlabs.org/listinfo/linuxppc-dev
    >
    >

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