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

Ard Biesheuvel · herbertx · commit 27c539aeffe2 · 2017-03-09T18:34:17.000+08:00
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 &lt;crypto_inc+0x38&gt;
     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 &lt;crypto_inc+0x28&gt;
     9cc:   ret

where the two remaining branch conditions (one for size &lt; 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 &lt;Jason@zx2c4.com&gt;
Signed-off-by: Ard Biesheuvel &lt;ard.biesheuvel@linaro.org&gt;
Signed-off-by: Herbert Xu &lt;herbert@gondor.apana.org.au&gt;
diff --git a/crypto/algapi.c b/crypto/algapi.c
@@ -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;
 		}
 
