On: I found a bug in Intel Skylake processors

[qb45]

TFA says clang actually uses an encoding of AND which operates on RAX but takes only 32b of constant so it is pretty much the solution you propose. And BTW, operating directly on EAX itself fucks up the upper half of RAX.

[yuhong → qb45]

AFAIK it is standard for all 64-bit instructions that use immediates.

[qb45 → yuhong]

Interesting, it seems you are right and no 64b immediates are possible at all except in MOV.