Comment by peterfirefly
1 month ago
> The CPU also doesn't care if you use prefixes that aren't valid for a specific instruction, for example a REP on a non-repeatable instruction.
This is one of the reasons why the x86 could be extended so much. PAUSE is just REP NOP, for example. Segment prefixes in front of conditional branches were used as static branch prediction hints (which I believe have returned in some newer Intel CPUs). Useful if you want to make a hint on newer CPUs that is harmless on older CPUs.
Some prefixes have become part of the encoding for certain SIMD instructions, but that is a different case because those prefixes aren't hints.
Not at all.
The correct behavior for allowing future extensions has already been introduced by Intel with 80186, in 1982, which has introduced an invalid instruction exception, to be used for all undefined instruction opcodes.
This behavior was unlike 8086/8088, which happily executed any undefined instructions, most of them being aliases to defined instructions.
For any opcode where current CPUs generate invalid instruction exceptions, it is very easy to define them in future CPUs to encode useful instructions. Had REP NOP generated exceptions in old CPUs, it would have been still fine for it to become PAUSE in current CPUs. Unfortunately, the designers of Intel CPUs have not always followed their own documentation, so not all invalid opcodes generate the exception, as they should. The non-enforcing of this condition has led to the existence of even commercial programs that are invalid or of compilers that generate officially invalid instructions.
It is true that there are a few cases when Intel has exploited the fact that some encodings were equivalent with a NOP on old CPUs, by reusing them for some instruction on new CPUs, where this allowed the execution of a program compiled for new CPUs on old CPUs. However this has been possible only for very few instructions, e.g. for branch direction hints, when not executing them on old CPUs does not change the result of a program.
In general the reuse of an opcode for a new instruction, when that opcode does not generate exceptions on old CPUs, is very dangerous, because the execution on old CPUs of a program compiled for new CPUs will have unpredictable consequences, like destroying some property of the user.
Your example with PAUSE is also one of the very few examples, besides branch hints, where the execution of a new program on old computers is not dangerous, despite the reassignment of the opcode.
Some time ago there was a discussion about a bug in some CPU, but I do not remember in which one, where the bug was triggered when the order of the REP prefix and of the 64-bit REX prefix was invalid, but the invalid order was ignored by the older CPUs instead of generating the appropriate exception, which allowed the execution of invalid programs, which did not have any bad effects on old CPUs, but they triggered the bug on that specific new CPU.
The new CPU should have been bug-free, but also the programs that triggered the bug should not have existed, as they should have crashed immediately on any older CPU.
There is utility for having a reserved set of opcode space for "NOP if you don't know what the semantics are, but later ISAs may attach semantics for it," because this allows you to add various instructions that merely do nothing on processors that don't support them. The ENDBR32/ENDBR64 instructions for CET, XACQUIRE/XRELEASE hints for LOCK, the MPX instructions, the PREFETCH instructions all use reserved NOP space (0F0D and 0F18-0F1F opcode space).
This is true, but the encoding space reserved for future extensions that is interpreted as NOP should be many times smaller than the space for encodings that generate the invalid instruction exception.
The reason is that the number of useful instructions that are only performance hints or checks for some exceptional conditions, so that if they are ignored that does not have bad consequences, is very limited.
For the vast majority of instruction set extensions, not executing the new instructions completely changes the behavior of the program, which is not acceptable, so the execution of such programs must be prevented on older CPUs.
Regarding the order of prefixes, Intel has made mistakes in not specifying it initially in 8086 and in allowing redundant prefixes. The latter has been partially corrected in later CPUs by imposing a limit for the instruction length.
Because of this lack of specification, the various compilers and assemblers have generated any instruction formats that were accepted by an 8088, so it became impossible to tighten the specification.
However, what is really weird is why Intel and AMD have continued to accept incrorrect instruction encodings even after later extensions of the ISA that clearly specified only a certain encoding to be valid, but in reality the CPUs also accept other encodings and now there are programs that use those alternative encodings that were supposed to be invalid.
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