Earlier quoted context omitted.
open in new tab
That doesn't seem to clear the referrer, at least on Firefox. Gotta go a step further and outright copy/paste the URL into an already-created tab.
Open in private works
x86 prefixes and escape opcodes flowchart
21–30 of 53 posts
Re: x86 prefixes and escape opcodes flowchart
#22This flowchart hides the most awful parts (IMO) of x86 prefixes: some combinations of prefixes are invalid but still parsed and executed, like combining two segment overrides, or placing a legacy prefix after a REX prefix. 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. The LOCK prefix is the only prefix that makes the sane…
> 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.
Re: x86 prefixes and escape opcodes flowchart
#23This site redirects to HN when it notices HN in the referrer.
Copy the URL and manually paste it into a new tab, no referrer then.
Re: x86 prefixes and escape opcodes flowchart
#24This site redirects to HN when it notices HN in the referrer.
The fact sites do evil things with these headers is why I configure Firefox with
network.http.referer.XOriginTrimmingPolicy
Re: x86 prefixes and escape opcodes flowchart
#25This site redirects to HN when it notices HN in the referrer.
If you have JavaScript enabled, that is. JWZ at least does the redirect on the server side. The following is pulled in from ` https://soc.me/assets/js/turnBack.js `: const undesirables = [ "news.ycombinator.com/", // "reddit.com/", // disable temporaily "lobste.rs/" ] ; if (undesirables.find(site => document.referrer.includes(site))) { window.location.replace(document.referrer); } I wonder why Reddit is "temporarily…
Git history doesn't explain it unfortunately
https://github.com/soc/soc.me/blame/main/assets/js/turnBack....
Although, when we inspect author's profile on lobste.rs, we'll see that he's banned:
https://lobste.rs/~soc [Banned 4 years ago by pushcx: Troll.]
Maybe he's banned from HN as well. And this 'undesirables' is a method of taking some kind of revenge.
Re: x86 prefixes and escape opcodes flowchart
#26There's EVEX2 now. It's hard to keep up...
Re: x86 prefixes and escape opcodes flowchart
#27Fun little tidbit: The 0x40-0x4f range used for the REX prefix actually clashes with the single-byte encodings for increment/decrement. When AMD designed the 64 bit extension, they had run out of available single-byte opcodes to use as a prefix and decided to re-use those. The INC/DEC instructions are still available in 64 bit mode, but not in their single-byte encodings.
Which clever code can utilize to determine which mode its running in and branch appropriately depending if the inc/dec were executed or not.
Re: x86 prefixes and escape opcodes flowchart
#28This is in no small part why x86 code density is awful despite variable size encoding.
Awful compared to what? I've seen benchmarks that go both ways in terms of a "winner" but in terms of overall variance there seems to be very little. There are some cases where ARM64 or RISCV do better and there are some cases where x86_64 does better. I can't see code density being a relevant factor when picking one ISA over another. We've got good compilers now anyways.. outside of power consumption.. the ISA wars…
Compared to VAX, of course…
Re: x86 prefixes and escape opcodes flowchart
#29This site redirects to HN when it notices HN in the referrer.
The fact sites do evil things with these headers is why I configure Firefox with network.http.referer.XOriginTrimmingPolicy set to 2. It "breaks" sites, but often in good ways (such as the site in TFA).
Referrer links are a dumb idea. Why the hell do you want to know where I'm coming from other than to track me
Re: x86 prefixes and escape opcodes flowchart
#30This flowchart hides the most awful parts (IMO) of x86 prefixes: some combinations of prefixes are invalid but still parsed and executed, like combining two segment overrides, or placing a legacy prefix after a REX prefix. 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. The LOCK prefix is the only prefix that makes the sane…
> 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…
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.