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Merged
evmar merged 1 commit into from
Sep 9, 2026
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add jp, jnp, jo, jno #19

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28 changes: 28 additions & 0 deletions runtime/src/ops/control_flow.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -48,6 +48,34 @@ impl Context {
from
}

pub fn jp(&mut self, from: Cont, x: Cont) -> Cont {
if self.cpu.flags.contains(Flags::PF) {

@evmar evmar Sep 9, 2026

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BTW, note that none of the operations that affect parity currently set this flag, so these instructions won't do the right thing 😬

@LinusU LinusU Sep 10, 2026

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Looking into it!

@evmar evmar Sep 10, 2026 •
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Parity is really annoying because in principle all the arithmetic operations compute it, and then it's almost never actually needed.

Originally what I did in retrowin32 was a hack where if the decoder saw a jp/jnp, it would panic if the previous instruction wasn't some specific thing, so then when it hit the jnp it could retroactively compute parity. (In both links I'm talking about the deleted code)

In my ideal world in theseus we could statically analyze the code enough that we would know when we actually need parity computation or not, similar to this hack but principled and covering all the cases. But I think until we hit that world, just computing parity always (like the added code in the above diff) is probably fine, and I can think of the static analysis thing as an optimization. (Lately I've been working on the static analysis bit of things, which is why I haven't committed to Theseus in a bit!)

@LinusU LinusU Sep 12, 2026

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I think that Theseus is currently setting this flag at least in some cases? But I might be missing something. My game is using these instructions and seems to be working fully (which is amazing 🤩) so that would point towards that as well, but I'm not to familiar with the code base to say anything for sure yet 😅

https://github.com/search?q=repo%3Aevmar%2Ftheseus+flags.set%28Flags%3A%3APF&type=code

My agent did find another issue when looking into this, which I believe was already fixed later in my branch as well, sending a follow up PR for that now: #21

return x;
}
from
}

pub fn jnp(&mut self, from: Cont, x: Cont) -> Cont {
if !self.cpu.flags.contains(Flags::PF) {
return x;
}
from
}

pub fn jo(&mut self, from: Cont, x: Cont) -> Cont {
if self.cpu.flags.contains(Flags::OF) {
return x;
}
from
}

pub fn jno(&mut self, from: Cont, x: Cont) -> Cont {
if !self.cpu.flags.contains(Flags::OF) {
return x;
}
from
}

pub fn js(&mut self, from: Cont, x: Cont) -> Cont {
if self.cpu.flags.contains(Flags::SF) {
return x;
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3 changes: 2 additions & 1 deletion tc/src/codegen/control_flow.rs
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Expand Up @@ -132,7 +132,8 @@ impl<'a> CodeGen<'a> {
bitness = self.module.bitness()
));
}
Je | Jne | Jb | Js | Jns | Ja | Jae | Jl | Jg | Jge | Jecxz | Jle | Jbe | Jcxz
Je | Jne | Jb | Js | Jns | Ja | Jae | Jl | Jg | Jge | Jecxz | Jle | Jbe | Jcxz | Jp
| Jnp | Jo | Jno
| Loop | Loopne => {
let next = self.resolve_jmp(instr.next_ip());
let (None, None, cont) = self.jmp_target(instr) else {
Expand Down