I have recently learned that nop instruction is actually xchg eax, eax... what it does is basically exchanges eax with itself.
As far as CPU goes, does the exchange actually happen?
2 Answers
There are several instructions, which could be used depending on the compiler. xchg eax, eax is byte code 90. It is a legit instruction, which takes up a single processing cycle. In addition, there are several other instructions, which could be used in place of xchg eax, eax:
lea eax, [eax + 0x00] byte code 8D 40 00
mov eax, eax byte code 89 C0
Since all of those instructions are different length, compiler chooses one of the most appropriate versions depending on alignment requirements.
Regarding compilers' choices, a few pointers:
GAS (GNU Assembler, used by GCC) x86 NOP operations can be found in function i386_align_code()
similarly, for LLVM it is in X86AsmBackend::writeNopData()
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1but they are probably less "NOP-like" than the xchg is. Besides which, the documented multi-byte NOP is the "0f 1f /0" set. Commented Jun 20, 2013 at 19:16
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The only other
nopinstruction is0F 1F /0which isNOP r/m16andNOP r/m32on supported CPUs. It may take anywhere from 3 to 9 bytes in x86-64, as per Intel doc (page 4-163). So even though you can usemov eax, eaxand others an an alternative, it has a downside vs. realnopin that it blocks CPU pipeline and lacks other hardware optimizations that are implemented in thenopinstruction.– ahmd0Commented May 17, 2018 at 21:45
The short answer is "Yes." In fact, if you experiment by generating machine language op codes directly you will discover that there is a whole range of operations that are effectively NOPs, all of which take a single processor cycle to execute.
While they are not technically "Documented," you will find that very close to the 0x90,
XCHG EAX, EAXXCHG EBX, EBXXCHG ECX, ECXXCHG EDX, EDX
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1I don't think this is is true. I think the non-EAX take more cycles (3 and not 1) and I think all the XCHG versions actually 0 the higher order 32 bits on a x86_64. stackoverflow.com/a/25053039/124486 Commented Feb 6, 2018 at 1:41
xchg EAX, EAXwas actually executed... Most 32 bit instructions in x64 mode zero out the upper 332 bits of their operands , and this instruction did too.