The way the KECCAK transform is currently coded involves many references
into the state array using indexes that are calculated at runtime using
simple but non-trivial arithmetic. This forces the compiler to treat the
state matrix as an array in memory rather than keep it in registers,
which results in poor performance.

So instead, let's rephrase the algorithm using fixed array indexes only.
This helps the compiler keep the state matrix in registers, resulting
in the following speedup (SHA3-256 performance in cycles per byte):

                                            before   after   speedup
  Intel Core i7 @ 2.0 GHz (2.9 turbo)        100.6    35.7     2.8x
  Cortex-A57 @ 2.0 GHz (64-bit mode)         101.6    12.7     8.0x
  Cortex-A53 @ 1.0 GHz                       224.4    15.8    14.2x
  Cortex-A57 @ 2.0 GHz (32-bit mode)         201.8    63.0     3.2x

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>