sha3.c: permute_n_scalar(): simplify num_rounds logic

bench results (samish across the board)
-------------
x86-64-scalar-gcc before:
  > make clean all BACKEND=1 && perf record ./bench 10000
  ...
  info: cpucycles: version=20240318 implementation=amd64-pmc persecond=4800000000
  info: backend=scalar num_trials=10000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,19.5,10.0,10.0,9.0,8.8
  sha3_256,32,19.5,10.0,9.9,9.5,9.3
  sha3_384,48,20.1,14.7,12.3,12.2,12.0
  sha3_512,64,19.5,19.5,18.2,17.2,17.1
  shake128,32,19.6,10.0,8.7,7.8,7.6
  shake256,32,19.6,10.1,10.0,9.6,9.3

x86-64-scalar-clang before:
  > make clean all CC=clang BACKEND=1 && perf record ./bench 10000
  ...
  info: cpucycles: version=20240318 implementation=amd64-pmc persecond=4800000000
  info: backend=scalar num_trials=10000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,19.7,9.9,9.7,8.8,8.6
  sha3_256,32,19.7,9.8,9.7,9.4,9.1
  sha3_384,48,19.7,14.5,12.0,11.9,11.8
  sha3_512,64,19.7,19.1,17.7,16.8,16.8
  shake128,32,20.4,10.0,8.7,7.7,7.5
  shake256,32,20.3,10.0,9.8,9.4,9.1

x86-64-scalar-gcc after:
  > make clean all BACKEND=1 && perf record ./bench 10000
  ...
  info: cpucycles: version=20240318 implementation=amd64-pmc persecond=4800000000
  info: backend=scalar num_trials=10000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,19.5,10.0,9.9,9.0,8.8
  sha3_256,32,19.5,10.0,9.9,9.5,9.3
  sha3_384,48,19.5,14.7,12.3,12.2,12.0
  sha3_512,64,19.7,19.5,18.2,17.2,17.1
  shake128,32,19.6,10.0,8.7,7.8,7.6
  shake256,32,19.6,10.1,10.0,9.6,9.3

x86-64-scalar-gcc after:
  > make clean all CC=clang BACKEND=1 && perf record ./bench 10000
  ...
  info: cpucycles: version=20240318 implementation=amd64-pmc persecond=4800000000
  info: backend=scalar num_trials=10000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,19.7,9.9,9.8,8.8,8.7
  sha3_256,32,19.6,9.9,9.8,9.4,9.2
  sha3_384,48,19.7,14.6,12.1,12.0,11.9
  sha3_512,64,19.7,19.3,17.9,16.9,16.9
  shake128,32,19.7,9.9,8.7,7.7,7.5
  shake256,32,19.7,9.9,9.8,9.5,9.2

a76-scalar-gcc before:
  > make clean all BACKEND=1 && ./bench 5000
  info: cpucycles: version=20240318 implementation=arm64-vct persecond=2400000000
  info: backend=scalar num_trials=5000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,20.2,10.1,10.2,9.3,9.2
  sha3_256,32,20.2,10.3,10.3,10.0,9.7
  sha3_384,48,20.9,15.3,12.8,12.7,12.5
  sha3_512,64,20.9,20.3,18.9,18.0,17.9
  shake128,32,20.2,10.3,9.0,8.1,7.9
  shake256,32,20.2,10.1,10.3,9.9,9.7

a76-scalar-clang before:
  > make clean all CC=clang BACKEND=1 && ./bench 5000
  info: cpucycles: version=20240318 implementation=arm64-vct persecond=2400000000
  info: backend=scalar num_trials=5000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,18.8,9.6,9.5,8.7,8.5
  sha3_256,32,18.8,9.6,9.5,9.2,9.0
  sha3_384,48,18.8,14.1,11.8,11.8,11.6
  sha3_512,64,18.8,18.6,17.5,16.6,16.6
  shake128,32,18.8,9.6,8.4,7.5,7.4
  shake256,32,18.8,9.6,9.6,9.2,9.0

a76-scalar-gcc after:
  > make clean all BACKEND=1 && ./bench 5000
  info: cpucycles: version=20240318 implementation=arm64-vct persecond=2400000000
  info: backend=scalar num_trials=5000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,20.2,10.1,10.3,9.3,9.2
  sha3_256,32,20.2,10.3,10.3,10.0,9.7
  sha3_384,48,20.9,15.3,12.8,12.7,12.5
  sha3_512,64,20.9,20.3,18.9,18.0,17.9
  shake128,32,20.2,10.3,9.0,8.1,7.9
  shake256,32,20.2,10.3,10.3,9.9,9.7

a76-scalar-clang after:
  > make clean all CC=clang BACKEND=1 && ./bench 5000
  info: cpucycles: version=20240318 implementation=arm64-vct persecond=2400000000
  info: backend=scalar num_trials=5000 src_lens=64,256,1024,4096,16384 dst_lens=32
  function,dst_len,64,256,1024,4096,16384
  sha3_224,28,18.8,9.4,9.3,8.5,8.3
  sha3_256,32,18.8,9.4,9.3,9.0,8.8
  sha3_384,48,18.8,13.9,11.6,11.6,11.4
  sha3_512,64,18.8,18.3,17.1,16.3,16.3
  shake128,32,18.8,9.4,8.3,7.4,7.2
  shake256,32,18.8,9.4,9.4,9.1,8.9

1 files changed, 2 insertions, 2 deletions

diff --git a/sha3.c b/sha3.c
index 14d518e..b846719 100644
--- a/sha3.c
+++ b/sha3.c
@@ -290,12 +290,12 @@ static inline void iota(uint64_t a[static 25], const int i) {
  */
 static inline void permute_n_scalar(uint64_t a[static 25], const size_t num_rounds) {
   uint64_t tmp[25] = { 0 };
-  for (size_t i = 0; i < num_rounds; i++) {
+  for (size_t i = (SHA3_NUM_ROUNDS - num_rounds); __builtin_expect(i < SHA3_NUM_ROUNDS, 1); i++) {
     theta(a);
     rho(a);
     pi(tmp, a);
     chi(a, tmp);
-    iota(a, (SHA3_NUM_ROUNDS - num_rounds + i));
+    iota(a, i);
   }
 }
 #endif /* (BACKEND == BACKEND_SCALAR) || defined(TEST_SHA3) */