https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113235
--- Comment #6 from Jan Hubicka <hubicka at gcc dot gnu.org> --- The internal loops are: static const unsigned keccakf_rotc[24] = { 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, 2, 14, 27, 41, 56, 8, 25, 43, 62, 18, 39, 61, 20, 44 }; static const unsigned keccakf_piln[24] = { 10, 7, 11, 17, 18, 3, 5, 16, 8, 21, 24, 4, 15, 23, 19, 13, 12, 2, 20, 14, 22, 9, 6, 1 }; static void keccakf(ulong64 s[25]) { int i, j, round; ulong64 t, bc[5]; for(round = 0; round < SHA3_KECCAK_ROUNDS; round++) { /* Theta */ for(i = 0; i < 5; i++) bc[i] = s[i] ^ s[i + 5] ^ s[i + 10] ^ s[i + 15] ^ s[i + 20]; for(i = 0; i < 5; i++) { t = bc[(i + 4) % 5] ^ ROL64(bc[(i + 1) % 5], 1); for(j = 0; j < 25; j += 5) s[j + i] ^= t; } /* Rho Pi */ t = s[1]; for(i = 0; i < 24; i++) { j = keccakf_piln[i]; bc[0] = s[j]; s[j] = ROL64(t, keccakf_rotc[i]); t = bc[0]; } /* Chi */ for(j = 0; j < 25; j += 5) { for(i = 0; i < 5; i++) bc[i] = s[j + i]; for(i = 0; i < 5; i++) s[j + i] ^= (~bc[(i + 1) % 5]) & bc[(i + 2) % 5]; } s[0] ^= keccakf_rndc[round]; } } I suppose with complete unrolling this will propagate, partly stay in registers and fold. I think increasing the default limits, especially -O3 may make sense. Value of 16 is there for very long time (I think since the initial implementation).