[pkhuong]

https://github.com/sbcl/sbcl/blob/master/src/compiler/x86-64... has

0x66 0x0f 0x1f 0x84 0x00 0x00 0x00 0x00 0x00

That's a size override prefix, followed by the dedicated NOP instruction (0x0f 0x1f), and finally 6 bytes to encode an effective address with offset.

[makomk]

Multi-byte nops have compatibility issues on some of the more obscure 32-bit x86 CPUs, unfortunately: https://sourceware.org/bugzilla/show_bug.cgi?id=13675

[pkhuong]

Right… you have to check cpuid for the long nop feature. I believe 0x66 0x90 is compatible (but slow, I would expect) with older CPUs.