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crypto: algapi - annotate expected branch behavior in crypto_inc() 

To prevent unnecessary branching, mark the exit condition of the
primary loop as likely(), given that a carry in a 32-bit counter
occurs very rarely.

On arm64, the resulting code is emitted by GCC as

     9a8:   cmp     w1, #0x3
     9ac:   add     x3, x0, w1, uxtw
     9b0:   b.ls    9e0 <crypto_inc+0x38>
     9b4:   ldr     w2, [x3,#-4]!
     9b8:   rev     w2, w2
     9bc:   add     w2, w2, #0x1
     9c0:   rev     w4, w2
     9c4:   str     w4, [x3]
     9c8:   cbz     w2, 9d0 <crypto_inc+0x28>
     9cc:   ret

where the two remaining branch conditions (one for size < 4 and one for
the carry) are statically predicted as non-taken, resulting in optimal
execution in the vast majority of cases.

Also, replace the open coded alignment test with IS_ALIGNED().

Cc: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
This commit is contained in:
Ard Biesheuvel 2017-02-14 21:51:02 +00:00 committed by Herbert Xu
parent b56f5cbc7e
commit 27c539aeff
1 changed files with 2 additions and 2 deletions
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4
crypto/algapi.c
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@ -963,11 +963,11 @@ void crypto_inc(u8 *a, unsigned int size)
u32 c;
if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
!((unsigned long)b & (__alignof__(*b) - 1)))
IS_ALIGNED((unsigned long)b, __alignof__(*b)))
for (; size >= 4; size -= 4) {
c = be32_to_cpu(*--b) + 1;
*b = cpu_to_be32(c);
if (c)
if (likely(c))
return;
}