Nostr notes by John Regehr

actually this file seems like a pretty big tell ...

2026-04-11T16:26:22Z

In reply to nevent1q…0fpf
_________________________

actually this file seems like a pretty big tell https://github.com/serjective/aburiscript/blob/main/docs/debugging.md

wow-- you don't see someone trying to bring up a C++ compiler ...

2026-04-11T16:12:33Z

wow-- you don't see someone trying to bring up a C++ compiler very often

https://github.com/serjective/aburiscript/

I think we're of the same mind overall. in fact I'm ...

2026-02-15T02:04:20Z

In reply to nevent1q…93rv
_________________________

I think we're of the same mind overall. in fact I'm teaching a "vibe coding" class to a bunch of undergrads right now, and the running joke is that everything I teach them about using an LLM better is actually something that makes them a better software engineer for real

the latest coding models are absurdly good, please believe me ...

2026-02-15T01:35:36Z

In reply to nevent1q…6pel
_________________________

the latest coding models are absurdly good, please believe me that I admit this with a gigantic amount of reluctance

program synthesis is a research area that combines searching over ...

2026-02-14T22:13:20Z

program synthesis is a research area that combines searching over program spaces with formal verification. sometimes these are separated, other times we can get an SMT solver to help with the search, as well as doing the verification part.

in any case, it's hard to overstate how cooked the searching part is, due to LLMs. they're just incredibly good at it, and this is nice because our previous solutions were not particularly impressive.

so far LLMs haven't changed the verification part at all

today a sysadmin sort of person in the college of engineering ...

2026-02-11T00:22:40Z

today a sysadmin sort of person in the college of engineering that houses the school of computing where I'm a professor suggested that we might need to take down a long series of technical reports, going back to the start of the 1990s, because the PDFs are not accessible, and that it is imperative that we maximize an accessibility score that is being harmed by these pdfs.

it made me think about "So Long and Thanks for All the Fish"

if you were writing 1 or 2 slides on designing software so that ...

2026-01-14T21:19:43Z

if you were writing 1 or 2 slides on designing software so that it's easy (or at least not impossible) to test, what would you list besides what's on this slide?

does anyone have a pointer to some good reading about practical ...

2026-01-13T01:18:50Z

does anyone have a pointer to some good reading about practical aspects of software architecture? I'm interested in super nuts and bolts aspects of how we structure large software projects -- material for practitioners, not for software engineering researchers.

looking for some material I can have my class read later this spring...

OMG GUYS ...

2025-12-21T03:00:22Z

OMG GUYS

I would be remiss to not show this graph when mentioning ...

2025-12-08T05:03:15Z

In reply to nevent1q…rtlx
_________________________

I would be remiss to not show this graph when mentioning quicksand

in the desert you run into quicksand pretty often, and it's ...

2025-12-08T04:57:45Z

in the desert you run into quicksand pretty often, and it's generally kind of fun, but apparently not always

https://www.reddit.com/r/CampingandHiking/comments/1pgxvt5/quicksand_trapped_me_on_the_hayduke_in_arches_np/

when I used to teach operating systems, sometimes I'd spend ...

2025-11-30T20:52:49Z

when I used to teach operating systems, sometimes I'd spend an entire lecture covering the chain of events in between "you move the mouse" and "cursor on the screen moves." but this guy fits it into 7 minutes!

https://www.youtube.com/watch?v=sZC0Biv_Iyk

go fuck yourself, program ...

2025-11-26T23:24:21Z

go fuck yourself, program

"We conducted a quasi-experimental field study on the Milan ...

2025-11-24T03:29:40Z

"We conducted a quasi-experimental field study on the Milan metro, observing 138 rides. In the control condition, a female experimenter, appearing pregnant, boarded the train with an observer. In the experimental condition, an additional experimenter dressed as Batman entered from another door. Passengers were significantly more likely to offer their seat when Batman was present"

https://www.nature.com/articles/s44184-025-00171-5

I hope that I can ruin your day by getting you to read this map ...

2025-11-10T23:45:08Z

I hope that I can ruin your day by getting you to read this map (courtesy wikipedia) of US town names that are portmanteaus of the two (or sometimes more) states they are near the borders of

at some level I think this depends on having good seminar ...

2025-10-18T17:25:01Z

In reply to nevent1q…chxl
_________________________

at some level I think this depends on having good seminar leadership to set the tone, etc.

but also, if you have the correct invite list, then there's very little of that work that actually needs to be done

I guess you mean in terms of organization? I think there are two ...

2025-10-18T17:23:41Z

In reply to nevent1q…30yj
_________________________

I guess you mean in terms of organization? I think there are two main aspects:

- the invitation list needs to be people who have a lot of interesting things to say, and who you don't mind spending a few days with

- the seminar schedule should not be tightly programmed. there should be opportunities for people to have the floor (if they want this) and then also plenty of time for unstructured discussion

we're not savages

2025-10-17T23:48:31Z

In reply to nevent1q…zjqs
_________________________

we're not savages

anyway eventually I'll tell bro about the secret to clean ...

2025-10-17T23:33:43Z

In reply to nevent1q…wnvz
_________________________

anyway eventually I'll tell bro about the secret to clean shit in the kitchen. but he's gonna suffer a little bit first.

alt: it's "bar keepers friend." that's the secret.

omg having my kids in college is the best thing ever, my IQ has ...

2025-10-17T23:31:23Z

omg having my kids in college is the best thing ever, my IQ has gone up like 40-50 points just due to them leaving home

I was just giving some advice about organizing a Dagstuhl seminar ...

2025-10-17T19:26:06Z

I was just giving some advice about organizing a Dagstuhl seminar to a junior person who's organizing one. I wasn't sure what I was going to say but I'm happy with how it came out:

the job is not to organize a little conference, it's rather to organize the world's best hallway session about whatever your topic is

just noticed that netflix has _Boyz in the Hood_ which I think ...

2025-10-14T04:37:56Z

just noticed that netflix has _Boyz in the Hood_ which I think was the genesis of my Ice Movie Theory, which tells us that movies with Ice Cube are generally good whereas movies with Ice-T are generally bad. I will not be taking questions.

me: I have this recurring nightmare doctor: what is it? ...

2025-10-10T20:36:43Z

me: I have this recurring nightmare

doctor: what is it?

Nostr event nevent1qqs9akpzvxh5ff3x6xgqpvtuc9u895gtsl3355mmk73n3pzntvv736szyrywkp40tayukskvxyra33t9nc7csjd5eetutrm30tp7ns80086n6am9kkp

2025-10-01T19:26:01Z

RIP

did Ken Thompson respond to a question my student asked??? ...

2025-09-23T18:31:49Z

did Ken Thompson respond to a question my student asked???

https://www.tuhs.org/pipermail/tuhs/2025-September/032567.html

is this bullshit? or does ISA not really matter in some ...

2025-09-09T00:57:24Z

is this bullshit? or does ISA not really matter in some fictitious world where we can normalize for process and other factors?

https://www.techpowerup.com/340779/amd-claims-arm-isa-doesnt-offer-efficiency-advantage-over-x86

flew a drone right into a tornado ...

2025-08-21T20:50:10Z

flew a drone right into a tornado

https://www.youtube.com/watch?v=akQbDX-MMlo

icfPOOPsla

2025-08-21T01:19:40Z

In reply to nevent1q…d5py
_________________________

icfPOOPsla

I usually think of big and little endian as being as good as each ...

2025-07-16T15:42:25Z

I usually think of big and little endian as being as good as each other, with the important thing being that we make a consistent choice. but I wonder how many cycles get used each day swapping byte order in optimized lexicographical order tests because we didn't go with big endian

https://gcc.godbolt.org/z/E5h94eq9d

as far as I know it's the only way to initialize a class ...

2025-06-18T18:52:10Z

In reply to nevent1q…pn9c
_________________________

as far as I know it's the only way to initialize a class member that's a reference, for example

I decided to lean into C++ initializer lists alt: the details ...

2025-06-18T17:13:34Z

I decided to lean into C++ initializer lists

alt: the details don't matter that much, just a kind of long C++ initializer list

unfortunately for the AI bros, MCP only means one thing ...

2025-05-26T04:04:19Z

unfortunately for the AI bros, MCP only means one thing

these are my two favorite graphs ...

2025-05-10T22:06:32Z

these are my two favorite graphs

if we're gonna be stuck with vibe coding, might as well try ...

2025-05-03T19:04:31Z

if we're gonna be stuck with vibe coding, might as well try to have some fun with it

("anti-vibe coding" would be my standard schtick: stuff like invariant-based programming, assertions, testing, fuzzing, CI -- all the stuff we do to try to make software actually work)

some time ago I read four books on debugging and wrote a bit ...

2025-04-26T22:51:44Z

In reply to nevent1q…qmrm
_________________________

some time ago I read four books on debugging and wrote a bit about them

https://blog.regehr.org/archives/849

"Why is debugging often overlooked as a critical dev ...

2025-04-26T21:11:22Z

"Why is debugging often overlooked as a critical dev skill?"

think about how much worse this is all going to get with the rise of vibe coding!

https://www.reddit.com/r/ExperiencedDevs/comments/1k8n2c5/why_is_debugging_often_overlooked_as_a_critical/

Nostr event nevent1qqsdakn7c4catmpv50tujcc5ptpq8ag52ckg38htpwm8kuk3lt6jmcszyrywkp40tayukskvxyra33t9nc7csjd5eetutrm30tp7ns80086n6rk6sz2

2025-04-05T20:55:29Z

the protest at the Utah state capitol today felt big!

appreciate this reddit moderator's energy ...

2025-04-04T23:24:39Z

appreciate this reddit moderator's energy

we took advantage of a break between spring storm systems to do a ...

2025-03-16T02:27:28Z

we took advantage of a break between spring storm systems to do a quick backpacking trip in the San Rafael Swell

https://gist.github.com/regehr/ce90534908085c7a933554b0f739ed92

2025-03-13T19:46:40Z

In reply to nevent1q…k7wt
_________________________

https://gist.github.com/regehr/ce90534908085c7a933554b0f739ed92

my colleague Pavel is playing with making binary searches and ...

2025-03-13T19:37:51Z

my colleague Pavel is playing with making binary searches and concludes that modern hardware is very very fast

https://pavpanchekha.com/blog/fast-bsearch.html

I cannot, however, get to cancer.gov

2025-03-02T00:13:32Z

In reply to nevent1q…902c
_________________________

I cannot, however, get to cancer.gov

it's still up for me. weird. (but I get the same result as ...

2025-03-02T00:11:31Z

In reply to nevent1q…lhyc
_________________________

it's still up for me. weird. (but I get the same result as you from "downforeveryoneorjustme")

it works for me at present

2025-03-01T22:49:05Z

In reply to nevent1q…l48e
_________________________

it works for me at present

my younger kid attacked my disastrously disorganized spice ...

2025-02-23T02:02:50Z

my younger kid attacked my disastrously disorganized spice cabinet and made it easy to find stuff ♥️♥️

Nostr event nevent1qqsgwykn0ts8y48tlnzyy6m7e7tl3689majqeylr3nucmgcmp76sy4gzyrywkp40tayukskvxyra33t9nc7csjd5eetutrm30tp7ns80086n6mmgwru

2025-02-22T22:52:40Z

#caturday

just a heads-up for anyone who works with an ABET-accredited ...

2025-02-15T01:17:14Z

just a heads-up for anyone who works with an ABET-accredited university program: ABET is pathetic

https://info.abet.org/abet-important-message-from-abet-president-and-ceo

other day I was showing a student something on my machine and he ...

2025-02-07T05:20:06Z

other day I was showing a student something on my machine and he got all quiet. I figured he was digesting the deep thoughts, then he says "so you really just use emacs all the time?"

been learning and thinking about the Burrows-Wheeler transform ...

2025-02-01T21:01:03Z

been learning and thinking about the Burrows-Wheeler transform and so far my conclusion is "what the fuck?"

https://en.wikipedia.org/wiki/Burrows%E2%80%93Wheeler_transform

not true. they are only legalizing shitty C. in the future ...

2025-01-25T21:14:11Z

In reply to nevent1q…kpah
_________________________

not true. they are only legalizing *shitty* C. in the future please try to be precise.

Orwell was so good!

2025-01-22T04:07:50Z

In reply to nevent1q…dt2v
_________________________

Orwell was so good!

that's definitely a bit of a different first sentence

2025-01-15T00:55:17Z

In reply to nevent1q…65mt
_________________________

that's definitely a bit of a different first sentence

nice!

2025-01-14T23:40:03Z

In reply to nevent1q…5z7a
_________________________

nice!

what’s missing from the index for this textbook about low-level ...

2025-01-13T17:51:07Z

In reply to nevent1q…hszq
_________________________

what’s missing from the index for this textbook about low-level C programming?? 🤔

absolutely indefensible code. I mailed them about this years and ...

2025-01-13T16:27:30Z

In reply to nevent1q…yvja
_________________________

absolutely indefensible code. I mailed them about this years and years ago

https://gcc.godbolt.org/z/W3jdEdnn4

Computer Systems: A Programmer’s Perspective, a textbook by ...

2025-01-13T16:22:31Z

Computer Systems: A Programmer’s Perspective, a textbook by Bryant and O’Halloran, is in its third edition and also its third decade of teaching students falsehoods about C

let's say we have 150 million lines in some language and we ...

2025-01-11T23:58:51Z

In reply to nevent1q…lwfe
_________________________

let's say we have 150 million lines in some language and we want to evolve the language. how does a portable IR help with that problem?

the dilemma facing programming languages, as I see it, is that ...

2025-01-11T21:00:14Z

In reply to nevent1q…j5t2
_________________________

the dilemma facing programming languages, as I see it, is that they fall into two categories:
- languages that can be rapidly and radically changed in response to problems (such as failures of abstraction) encountered in real use. but these are the languages without large load-bearing code bases, so they don't get many resources
- languages that have large, load-bearing code bases, which receive adequate engineering power, but not enough to overcome the legacy code problem

I said it in a reply thread but might as well repeat it here: the ...

2025-01-11T19:39:03Z

In reply to nevent1q…p246
_________________________

I said it in a reply thread but might as well repeat it here: the fix is languages that support proper abstraction of things that you do often, including design pattern sorta things

totally agree with this and there are SO many examples of tools ...

2025-01-11T18:55:59Z

totally agree with this and there are SO many examples of tools where 99% of uses are adapting an existing program/config file instead of creating a new one from scratch

https://blog.yossarian.net/2025/01/10/Be-aware-of-the-Makefile-effect

like think about the last time you started a LaTeX document from a blank file...

meta's AI throwing its creator team under the bus. truly, you ...

2025-01-04T23:43:44Z

meta's AI throwing its creator team under the bus. truly, you could not make this shit up

https://www.theguardian.com/technology/2025/jan/03/meta-ai-powered-instagram-facebook-profiles

oh shit— instagram discovered fencepost bugs ...

2025-01-04T05:14:05Z

oh shit— instagram discovered fencepost bugs

Fondly recalling this gem, which is one of the things that set me ...

2024-12-23T02:57:28Z

Fondly recalling this gem, which is one of the things that set me on the path to integer overflow detection, some time ago

https://research.google/blog/extra-extra-read-all-about-it-nearly-all-binary-searches-and-mergesorts-are-broken/

from @npub1g4g…9ev3: "The only difference with C++ is that ...

2024-12-22T19:54:39Z

from The PhD Place (npub1g4g…9ev3):

"The only difference with C++ is that all fundamental types have the same endianness. Whereas for C, we allow floating point vs. integers to have different endiannesses. That only tends to matter for the tiniest slice of processors and devices, but some people care." 👀 👀

this blog post from 2009 is sort of obviously correct and yet I ...

2024-12-21T19:35:02Z

this blog post from 2009 is sort of obviously correct and yet I don’t know of any web sites that do this

https://www.evanmiller.org/how-not-to-sort-by-average-rating.html

awesome!!

2024-12-17T15:52:28Z

In reply to nevent1q…e7ds
_________________________

awesome!!

yeah his brute forcer was getting nowhere and I told him, ...

2024-12-17T15:43:24Z

In reply to nevent1q…vl2v
_________________________

yeah his brute forcer was getting nowhere and I told him, "look it's just consuming A, three bits at a time" so hopefully he'll get there!

I didn't have much help to give Jonas for part 2, but I ...

2024-12-17T15:37:14Z

In reply to nevent1q…7784
_________________________

I didn't have much help to give Jonas for part 2, but I didn't look at it all that closely.

he did get to be one of the lucky 10,000 who learned about quines yesterday though

very BF! such busy beaver!

2024-12-17T15:36:22Z

In reply to nevent1q…pvxw
_________________________

very BF! such busy beaver!

nevermind, I misunderstood what he did. he just ran ...

2024-12-16T20:09:33Z

In reply to nevent1q…elrp
_________________________

nevermind, I misunderstood what he did. he just ran Dijkstra's rooted at the start and at the finish and that produced enough info that a quick scan of the grid points gave the answer

plenty of repeated work, but whatev!

2024-12-16T18:59:44Z

In reply to nevent1q…jclq
_________________________

plenty of repeated work, but whatev!

he built part 1 from a shortest path code he had sitting around ...

2024-12-16T18:59:08Z

In reply to nevent1q…3h63
_________________________

he built part 1 from a shortest path code he had sitting around from some class, so for part 2 he just ran it in both directions from every grid point

I think we just worked out the right algorithm for part 2 of of ...

2024-12-16T18:39:02Z

In reply to nevent1q…azrw
_________________________

I think we just worked out the right algorithm for part 2 of of last night's problem, he's coding on it now :)

I don't have enough spare brain energy to do Advent of Code ...

2024-12-16T18:32:51Z

I don't have enough spare brain energy to do Advent of Code but my older kid does and I'm having a ton of fun being a sounding board for his ideas. it's a really clever competition!

and now also a CI hook that suggests alternatives when someone ...

2024-12-11T21:28:40Z

In reply to nevent1q…6unh
_________________________

and now also a CI hook that suggests alternatives when someone wants to add an undef to an LLVM unit test 🔥

https://github.com/llvm/llvm-project/pull/118506

this is super nice, Nuno added a guide to undefined behavior in ...

2024-12-11T16:15:09Z

this is super nice, Nuno added a guide to undefined behavior in LLVM:

https://llvm.org/docs/UndefinedBehavior.html

we should have done this years and years ago...

"Memory-safe PNG decoders now vastly outperform C PNG ...

2024-12-10T04:17:22Z

"Memory-safe PNG decoders now vastly outperform C PNG libraries"

https://www.reddit.com/r/rust/comments/1ha7uyi/memorysafe_png_decoders_now_vastly_outperform_c/

Nostr event nevent1qqsweqykxq7swzyqrqgq5su7uasmxkm9p42sft5v2w323t7g57jn4dqzyrywkp40tayukskvxyra33t9nc7csjd5eetutrm30tp7ns80086n6dzy6vr

2024-12-10T01:32:16Z

I often look at the links at hn but seldom look at the comments. ...

2024-12-06T23:04:09Z

I often look at the links at hn but seldom look at the comments. but it's interesting to see their reaction to this Financial Times piece:

"Why America’s economy is soaring ahead of its rivals"

https://archive.is/vt7sY#selection-1757.0-1757.52

basically, even the hn commenters seem to be capable of sensing the enormous divide between the soaring numbers and the on-the-ground experience of being part of this economy. I think that means things are bad.

https://news.ycombinator.com/item?id=42314797

I’m teaching a large systems class in the spring and am trying ...

2024-12-01T21:45:33Z

In reply to nevent1q…m0sz
_________________________

I’m teaching a large systems class in the spring and am trying to think of a pitch that I can use to convince them that actually learning the material is a wise choice, at this point in their lives

now relate this phenomenon to AI coding assistants

2024-12-01T21:35:14Z

In reply to nevent1q…jtfg
_________________________

now relate this phenomenon to AI coding assistants

if you look at what LLVM actually spends its time doing (and I ...

2024-11-30T05:00:44Z

if you look at what LLVM actually spends its time doing (and I assume probably also GCC?), it's pointer chasing. basic blocks are doubly linked lists, etc. for a while I've had the idea that a from-scratch compiler re-implementation for GPGPUs might work out pretty well. and if not, it would at least be a couple years of super fun hacking. anyhow somehow I only now ran across this:

"Compilation On The GPU? A Feasibility Study"

https://dl.acm.org/doi/pdf/10.1145/3528416.3530249

I don't judge but what were the parents thinking here?? ...

2024-11-27T17:22:39Z

I don't judge but what were the parents thinking here??

textual LLVM IR isn't usually produced or parsed other than ...

2024-11-08T16:59:21Z

In reply to nevent1q…q53k
_________________________

textual LLVM IR isn't usually produced or parsed other than for debugging or testing

sure, but, like, we mostly don't write LLVM IR by hand. ...

2024-11-08T16:46:23Z

In reply to nevent1q…rl70
_________________________

sure, but, like, we mostly don't write LLVM IR by hand. it's not a choice I'd make for a programming language.

a design decision about LLVM IR that I like is that it only lets ...

2024-11-08T15:31:55Z

a design decision about LLVM IR that I like is that it only lets you write down a floating point constant if it's representable in the FP type

https://gcc.godbolt.org/z/76cYfsdqW

I can't believe you made me think about this

2024-11-07T15:20:10Z

In reply to nevent1q…q6sv
_________________________

I can't believe you made me think about this

Nostr event nevent1qqs9mdql5w5wjj8yatdpaggd46v0kc67304jxajuaqnphdhgv2ep7uszyrywkp40tayukskvxyra33t9nc7csjd5eetutrm30tp7ns80086n6lxx3e2

2024-11-04T06:14:50Z

Lazarus

not quite #caturday yet but I have this tired girl ...

2024-11-02T02:57:58Z

not quite #caturday yet but I have this tired girl

alt: just a tabbycat in the sun ...

2024-10-26T18:49:26Z

alt: just a tabbycat in the sun

I ran across a thread on reddit about "very inexpensive food ...

2024-10-13T01:42:49Z

I ran across a thread on reddit about "very inexpensive food that is delicious" and I've been thinking a lot about it. I've long believed that many of the world's best foods come out of difficult, constrained situations. anyway, here's my highly incomplete list of delicious, cheap foods
- haluski
- frijoles de la olla
- fried potatoes
- mujadara
- polenta
- fried rice
- red beans and rice

don't tempt me!!

2024-10-03T02:30:32Z

In reply to nevent1q…pgkn
_________________________

don't tempt me!!

that still leaves the problem of synthesizing the closed form, it ...

2024-10-03T02:24:59Z

In reply to nevent1q…e6zr
_________________________

that still leaves the problem of synthesizing the closed form, it doesn't help much

when compiling Brainfuck, optimizing loops that are actually ...

2024-10-03T02:20:46Z

when compiling Brainfuck, optimizing loops that are actually additions is pretty easy. but I'm stuck on a general way to take this a step further and optimize the next level of loops that are actually multiplications. half baked thoughts at the link if anyone wants to take a look. computing a closed form for this kind of loop doesn't seem straightforward.

https://gist.github.com/regehr/4cb11a3a7911775f43b91971197d9cdd

one of the enterprising students in my class is attacking the ...

2024-09-30T23:55:46Z

In reply to nevent1q…t5w2
_________________________

one of the enterprising students in my class is attacking the problem that ~35% of LLVM unit tests trigger UB. 🤪 for real.

hoo boy. I want to say I'd use this in a performance ...

2024-09-30T23:51:56Z

In reply to nevent1q…5xky
_________________________

hoo boy. I want to say I'd use this in a performance comparison but LLVM 2.8 or whatev is pretty distant now

but so far in class we've only targeted native assembly. once ...

2024-09-30T23:39:00Z

In reply to nevent1q…6wtj
_________________________

but so far in class we've only targeted native assembly. once we're done with this, we'll retarget everything to LLVM and pick up for free all of those amazing low-level optimizations that are too numerous for us to do by hand

ah, impossible to say without having worked on the other thing as ...

2024-09-30T23:38:15Z

In reply to nevent1q…wfkj
_________________________

ah, impossible to say without having worked on the other thing as well. what I can say about BF is that it's a bit a sweet spot for a compilers class because it has plenty of low-hanging optimizations, but they mostly require BF-specific knowledge, so for example LLVM alone would not be able to pick up most of these benefits.

it was not my intent for this to become an "optimizing ...

2024-09-30T23:24:07Z

In reply to nevent1q…nwrk
_________________________

it was not my intent for this to become an "optimizing Brainfuck" account. this will stop soon.

I wasn't sure how much it would matter to vectorize scan ...

2024-09-30T23:20:31Z

I wasn't sure how much it would matter to vectorize scan loops such as [<] and [>>] in Brainfuck, but on my M1 Mac I'm seeing dbfi.b going about 10x faster due to just this one optimization

http://www.hevanet.com/cristofd/08.html