On 13 February 2017 at 21:55, Jason A. Donenfeld <Jason@xxxxxxxxx> wrote:
> On Sun, Feb 5, 2017 at 11:06 AM, Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
>> + if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) ||
>> + !((unsigned long)b & (__alignof__(*b) - 1)))
>
> Why not simply use the IS_ALIGNED macro?
>
Good point.
> Also, are you might consider checking to see if this is a constant, so
> that you can avoid an unnecessary branch.
Check if what is a constant? The buffer address?
> Alternatively, if you want
> to use the branch, I'd be interested in you writing back saying, "I
> tested both cases, and branching is faster than always using the slow
> unaligned path."
When using the 4-byte wide path, the loop terminates after 1 iteration
unless there is a carry in the low word. The likelihood of the loop
iterating multiple times in the 1-byte wide path is 1 in 256.
I suppose we could add this if there is concern about the branching
diff --git a/crypto/algapi.c b/crypto/algapi.c
index 6b52e8f0b95f..03670390a2e4 100644
--- a/crypto/algapi.c
+++ b/crypto/algapi.c
@@ -967,7 +967,7 @@ void crypto_inc(u8 *a, unsigned int size)
for (; size >= 4; size -= 4) {
c = be32_to_cpu(*--b) + 1;
*b = cpu_to_be32(c);
- if (c)
+ if (likely(c))
return;
}
but other than that, I see little reason to introduce complicated logic here.
>> + while (((unsigned long)dst & (relalign - 1)) && len > 0) {
>
> IS_ALIGNED
>
>> +static inline void crypto_xor(u8 *dst, const u8 *src, unsigned int size)
>> +{
>> + if (IS_ENABLED(CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS) &&
>> + __builtin_constant_p(size) &&
>> + (size % sizeof(unsigned long)) == 0) {
>
> You can expand this condition to be:
>
> if ( (is_constant(size) && size%sizeof(ulong)==0) &&
> (efficient_unaligned || (is_constant(dst) && is_constant(src) &&
> is_aligned(dst) && is_aligned(src))) )
>
> It might seem complex, but it all gets compiled out.
Care to explain how? Could you point me to any references to
crypto_xor() where either of the buffer addresses are compile time
(not link time) constants?
