x86 prefixes and escape opcodes flowchart
soc.me75 points by gaul 11 hours ago
75 points by gaul 11 hours ago
This 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 choice to reject invalid combinations, rather than silently accept them.
Also, the (E)VEX prefix doesn't behave like the other prefixes: it must be placed last, and can therefore only appear once. All other prefixes can be repeated.
> 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.
I wonder whether there are some prefixes that cause (some) CPUs to execute the instruction a lot slower.
This 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
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);
}
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.
This is an interesting way to prevent the hug of death. I wonder what the author's reasoning is, also would it really be effective?
I doubt it, the redirect is client-side, I got a flash of the page before the redirect.
If anything, it's going to at least double its traffic this way when people click again assuming they hit back somehow.
Wow, I didn't even notice because I have extensions that strip the referrer header. Excellent.
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.
Fun 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.
On that page, there is a link to another interesting page on the same site:
https://soc.me/interfaces/intels-original-64bit-extensions-f...
where there are links to a couple of patents filed by Intel in 2000, about a 64-bit extension of the x86 ISA, which had been implemented in Pentium 4, but which had been nonetheless disabled and hidden from the users, in order to not compete with Itanium.
The page explains the content of the patents.
As already mentioned by another poster, at least on Firefox you have to open a tab and then copy this link there, to avoid being identified as an "undesirable" :-)
There's EVEX2 now. It's hard to keep up...
I respect the disobedience.
This 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 are dead.
Technically, code density still matters - because both L1 cache memory and L1 instruction fetch misses are very expensive.
But as you point out, code density gets far less attention in tech circles these days. And higher-level decision makers rightfully focus on higher-level system performance metrics.