https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110202
Alexander Monakov <amonakov at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |amonakov at gcc dot gnu.org
--- Comment #6 from Alexander Monakov <amonakov at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #3)
> And I must say I don't immediately see easy rules how to find out from the
> immediate value which set is which, so unless we find some easy rule for
> that, we'd need to hardcode the mapping between the 256 values to a bitmask
> which inputs are actually used.
Well, that's really easy. The immediate is just a eight-entry look-up table
from any possible input bit triple to the output bit. The leftmost operand
corresponds to the most significant bit in the triple, so to check if the
operation vpternlog(A, B, C, I) is invariant w.r.t A you check if nibbles of I
are equal. Here we have 0x55, equal nibbles, and the operation is invariant
w.r.t A.
Similarly, to check if it's invariant w.r.t B we check if two-bit groups in I
come in pairs, or in code: (I & 0x33) == ((I >> 2) & 0x33). For 0x55 both sides
evaluate to 0x11, so again, invariant w.r.t B.
Finally, checking invariantness w.r.t C is (I & 0x55) == ((I >> 1) & 0x55).
