Hacker News: Symmetry

New comment by Symmetry in "Starship V3"

Symmetry — Wed, 13 May 2026 23:53:43 +0000

Not if you first absorb heat with your solar panels. Conservation of energy and all that.

New comment by Symmetry in "Starship V3"

Symmetry — Wed, 13 May 2026 13:08:54 +0000

Yeah. I don't have any doubts that this is something that can be done. But doing it cheaply enough to be worth while is the difficult bit. Elon does have reputation for delivering impressive things, but not for finishing them on the deadlines he sets.

New comment by Symmetry in "Starship V3"

Symmetry — Wed, 13 May 2026 12:49:59 +0000

Nobody (sane) is talking about putting nuclear reactors on Satellites in close Earth orbit so we don't have to worry about them generating heat. They've got solar panels that move some of the solar energy they absorb to a central location which presents problems in moving the waste heat back out so that spot doesn't get too hot. But that doesn't change the overall equilibrium temperature.

New comment by Symmetry in "Why Not Venus?"

Symmetry — Fri, 24 Apr 2026 11:49:32 +0000

Selenian Boondocks did a whole series on Venus and what could be usefully extracted from the atmosphere a decade back. https://selenianboondocks.com/category/venus/

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 19:07:29 +0000

.03ns is a frequency of 33 GHz. The chip doesn't actually clock that fast. What I think you're seeing is the front end detecting the idiom and directing the renamer to zero that register and just remove that instruction from the stream hitting the execution resources.

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 17:02:56 +0000

There's been a lot of churn over the years but additions being done in the same timeframe as XORs has been pretty constant. The Pentium 4 double pumped its ALU but both XORs and ADDs could happen in a half cycle latency. The POWER 6 cut the FO4s of latency in stage from 16 to 10 and kept that parity as well. When you need 2 FO4s for latching between stages and 2 to handle clock jitter at high frequencies the difference between what a XOR needs and what an ADD need start looking smaller, particularly when you include the circuitry to move the data and select the instruction. Maybe if we move to asynchronous circuits?

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 15:25:08 +0000

For a 32 bit number you're looking at going from using 256 to ~1800 transistors in the operation itself. A modern core will have roughly 1,000,000,000 transistors. Some of those are for vector operations that aren't involved in a xor or sub, but most of them are for allowing the core to extract more parallelism from the instruction stream. It's really just a dust mote compared to the power reduction you could get by, e.g., targeting a 10 MHz lower clock rate.

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 15:18:13 +0000

Right. Keeping down the number of slots the scheduler and bypass network need to worry about is an important design pressure.

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 15:03:54 +0000

There's a structure called a carry-bypass adder[1] that lets you add two numbers in O(√n) time for only O(n) gates. That or a similar structure is what modern CPUs use and they allow you two add two numbers in a single clock cycle which is all you care about from a software perspective.

There are also tree adders which add in O(log(n)) time but use O(n^2) gates if you really need the speed, but AFAIK nobody actually does need to.

[1]https://en.wikipedia.org/wiki/Carry-skip_adder

New comment by Symmetry in "XOR'ing a register with itself is the idiom for zeroing it out. Why not sub?"

Symmetry — Wed, 22 Apr 2026 14:55:35 +0000

That's all true, but on any modern x86 processor both the single pair of gates for the xor and the 10 or so for a carry-bypass 64 bit wide subtraction both happen with a single clock cycle of latency so from a programmer's perspective they're the same in that sense. There's still an energy difference but its tiny compared to what even the register file and bypass network for the operation use, let along the OoO structures.

New comment by Symmetry in "Fusion Power Plant Simulator"

Symmetry — Tue, 21 Apr 2026 17:29:55 +0000

There's only a few grams of hydrogen in the reactor's plasma, it's reaction with oxygen wouldn't be much more exciting than just losing containment. There are engineering challenges that have to be addressed but no worse than the 6 MW research reactor I used to walk by every day to my college classes in the middle of a dense city.

The proliferation risk of someone using the neutron flux to produce an atomic or dirty bomb are real but that exists no matter where it is.

New comment by Symmetry in "Laws of Software Engineering"

Symmetry — Tue, 21 Apr 2026 17:18:25 +0000

"Any problem in computer science can be solved with another level of indirection...except for the problem of too many levels of indirection."

New comment by Symmetry in "Anthropic takes $5B from Amazon and pledges $100B in cloud spending in return"

Symmetry — Tue, 21 Apr 2026 15:10:52 +0000

With NVidia/OpenAI actual graphics cards did change hands. Vendor financing, like when a car dealership gives you a loan to buy a new car, is actually pretty normal.

New comment by Symmetry in "The abandoned war: Why no one is stopping the genocide in Sudan"

Symmetry — Tue, 21 Apr 2026 14:04:04 +0000

It does have oil. And the reason the UAE is backing the RSF is that they have gold interests there.

New comment by Symmetry in "Laws of Software Engineering"

Symmetry — Tue, 21 Apr 2026 12:51:22 +0000

On my laptop I have a yin-yang with DRY and YAGNI replacing the dots.

New comment by Symmetry in "Tesla concealed fatal accidents to continue testing autonomous driving"

Symmetry — Mon, 20 Apr 2026 20:37:47 +0000

I meant to include both SAE 2 and 3. I think having both lane keeping and cruise control on at the same time will tend to cause people to lose focus in a way they wouldn't if they had to do one or the other.

New comment by Symmetry in "Tesla concealed fatal accidents to continue testing autonomous driving"

Symmetry — Mon, 20 Apr 2026 13:16:29 +0000

SAE level 2 is just a bad idea. People can't be expected to carefully monitor a car and take over at a moment's notice when it's doing all the driving. My adaptive cruise control is great and I hope to have a future car where I can zone out while it drives and take over after after a few seconds heads up, but the zone between shouldn't be a valid feature.

New comment by Symmetry in "Claude Token Counter, now with model comparisons"

Symmetry — Mon, 20 Apr 2026 12:59:55 +0000

If they wanted they could always just double the $/token. They don't seem to be able to keep up with their current demand and that's what companies normally do in that circumstance if they're looking to money grab, no need for the bankshot approach.

New comment by Symmetry in "All 12 moonwalkers had "lunar hay fever" from dust smelling like gunpowder (2018)"

Symmetry — Sat, 18 Apr 2026 10:54:26 +0000

"Poison" can also refer to a substance toxic to other animals. We say that chocolate is poisonous to dogs for instance. And a good fraction of Earth's biosphere was killed off by oxygen poisoning in the first of Earth's great mass extinctions.

Also, the dose makes the poison and excess oxygen actually can poison humans. Deep sea divers have to worry about excess oxygen inducing seizures if they mess up their breathing gasses enough. And even 100% oxygen at regular pressure will slowly damage the lungs, something ICUs have to worry about.

Nick Lane had a great book about oxygen, Oxygen, which maybe isn't as good as his book about mitochondria but is well worth reading.

New comment by Symmetry in "CRISPR takes important step toward silencing Down syndrome’s extra chromosome"

Symmetry — Sat, 18 Apr 2026 10:46:43 +0000

I was misremembering, it's actually cats where hetrochromatic eyes is associated with females. In humans there isn't any difference.