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John Regehr
Works at University of Utah
Attended University of Virginia
Lives in Salt Lake City
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John Regehr

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In case you have not heard through other social media outlets: Bit9 and I have parted ways, which leaves me looking for my next opportunity! Check out my resume . "Experienced software engineer and leader. Ported flagship Bit9 security product to Linux, imp...
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Group work is overused in schools, and stifles good ideas

Says that teacher.
What happened to quiet reflection? Teacher Diana Senechal talks to Holly Welham about how too much group work and teaching to exams crushes intellectual thought
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In other news, avoiding off-by-one errors is hard.
 
I was gasping for air after this -- two guys (in complete sincerity) arguing about how many days are in the week:

http://forum.bodybuilding.com/showthread.php?t=107926751
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African or European leap weeks?
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Nice blog post from Dan Luu about the overhead of integer overflow checking:
http://danluu.com/integer-overflow/

One thing to keep in mind is that the overflow checking that we did for Clang is really not tuned for performance. Our implementation replaces each operation with a checked version: the LLVM x.with.overflow intrinsics. No problem so far! But then, every check is immediately followed by a branch to error handler. This is nice because it gives very precise information about what overflowed, but it is not the performant implementation. To do this right, you want to OR together a whole expressions' worth (or more) of overflow flags and then put a branch at the end. But now you have to go back and figure out what happened if you want to print a good diagnostic. Since our primary focus was diagnostics and not performance, we didn't do that stuff. So Clang integer overflow checking is not that fast.

Of course there are other problems too, such as the poor register allocation decision in Dan's post. Another major problem is that LLVM lacks a solid value range analysis that could be used to remove provably unnecessary overflow checks. So basically there's a lot of room for improvement.

Anyhow Dan's blog doesn't allow comments so I wanted to leave this somewhere.

Also I am psyched that Souper (our LLVM superoptimizer) will soon understand the x.with.overflow intrinsics and sooner or later we can expect it to do a somewhat decent job of getting rid of unnecessary checks.
How much overhead should we expect from enabling integer overflow checks? Using a compiler flag or built-in intrinsics, we should be able to do the …
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Dan Luu
 
+Richard Smith Whoa! Interesting. Thanks for letting me know about that.
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+Carlos Scheidegger Check out the photos near the bottom of this page. I think this location isn't that far from Tucson -- you should check it out if you haven't already.

http://dirtanddogs.blogspot.com/2014/11/feb-water-stops.html
Words by Greg Photos by Greg and Trina Arizona is known for its heat. For its dryness. For its prickly plants, sand, dust and generally rugged desert landscape. And for good reason, as much of the state is just that. Back in ...
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82 is crazy. But then again, I was hiking in short sleeves today.
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Hell yes, I can go to space now.
 
The last barrier for human's conquest of space has fallen: we can now make espresso in zero gravity.
Specially designed ‘ISSpresso’ machine overcomes absence of gravity by firing pressurised water through capsule of coffee
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John Regehr

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A hypercube of bits

This is the kind of thing mathematicians know about almost instinctively - but most ordinary folks don't.  It's a 4-dimensional cube drawn in a very nice way, with each corner labeled by a string of 4 bits. 

If you haven't ever thought about this stuff, try these puzzles!

Puzzle 1: The same thing happens when you start at any number and move west-northwest.  For example, when you go from 0110 to 1110.  What always happens to the number when you move this way?

Puzzle 2: What happens when you move east-northeast?   For example, when you go from 0110 to 1110.

Puzzle 3: What happens when you move north-northwest?  For example, when you go from 1001 to 1101.

Puzzle 4: What happens when you move north-northeast?  For example, when you go from 1001 to 1011.

Puzzle 5: There are 8 bit strings on the outside of this picture.  What happens when you go from one of these to the one directly opposite?

Puzzle 6: There are also 8 bit strings on the inside of this picture.  What happens when you go from one of these to the one directly opposite?

Puzzle 7: How many pictures of cubes can you find in this picture?  The cubes will be a bit slanted.

This picture is a tiny part of a huge subject called coding theory, which about efficiently sending messages as strings of bits, while making it hard for one message to get mistaken for another when an error occurs.

http://en.wikipedia.org/wiki/Coding_theory

#geometry  
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proofcraft.org: new blog about proof and Isabelle.

This week: Isabelle syntax highlighting in markdown and HTML.
Wouldn't it be nice to just include snippets of Isabelle code in markdown or html and have them show up rendered with symbols and highlighting? For instance on a blog? Or a list like the Top 100 theorems? Or on github? Set your quick_and_dirty flags and read on.
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Best thing ever.
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+John Regehr There is an easier method - buy an unwrapped spring just like the one in the video.
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"The entire scam seems to be possible because there is no attempt at confirming the credibility of the people presuming to actually do the peer review. This type of decentralized authentication is precisely what PGP is particularly good at."

https://news.ycombinator.com/item?id=8664293

+Carlos Scheidegger 
The entire scam seems to be possible because there is no attempt at confirming the credibility of the people presuming to actually do the peer review. This type of decentralized authentication is precisely what PGP is particularly good at. I have to imagine that it would be trivial to design a ...
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+Suresh Venkatasubramanian I am sick and tired of being asked to review papers for which I have no trace of expertise.
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He does have a way with words.
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Have him in circles
920 people
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Education
  • University of Virginia
    CS, 1995 - 2001
  • Kansas State University
    CS and math, 1990 - 1995
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Male
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Employment
  • University of Utah
    prof, 2001 - present
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Salt Lake City
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I've been here a number of times both without and with kids. It's a wonderful restaurant either way. The steak frites is completely fantastic as are several other items on the menu. I've loved most of the sides that I've tried. There's usually a nice soup. They're happy to let us split things when visiting with the kids. Their own beers are perfectly fine (though underpowered) but they have a decent selection of bottles also.
Public - 8 months ago
reviewed 8 months ago
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