Optimal no-ops

[gaul]

It is better to use fewer but longer no-ops to achieve padding. Some examples:

1:  90                             nop
2:  66 90                          data16 nop
3:  0F 1F 00                       nopl  %eax, (%rax)
4:  0F 1F 40 00                    nopl  %eax, (%rax)
5:  0F 1F 44 00 00                 nopl  %eax, (%rax,%rax,1)
6:  66 0F 1F 44 00 00              nopw  %ax,  (%rax,%rax,1)
7:  0F 1F 80 00 00 00 00           nopl  %eax, (%rax)
8:  0F 1F 84 00 00 00 00 00        nopl  %eax, (%rax,%rax,1)
9:  66 0F 1F 84 00 00 00 00 00     nopw  %ax,  (%rax,%rax,1)
10: 66 2E 0F 1F 84 00 00 00 00 00  nopw  %ax,  (%rax,%rax,1)

More recent processors support efficient 15-byte no-ops as well. References:

• https://stackoverflow.com/a/12564044/2800111
• https://bugzilla.mozilla.org/show_bug.cgi?id=1568450#c8
• https://github.com/llvm/llvm-project/blob/2463239777bfb41f63bb50d3eb931719612ddd0a/llvm/lib/Target/X86/MCTargetDesc/X86AsmBackend.cpp#L341-L347