John Regehr

A Tourist’s Guide to the LLVM Source Code

Hi, would you allow me to translate your article to French, preferably under a free lincense like CC-by-sa?

Teaching C

I was hoping you would mention the dynamic checkers, ASAN etc. My fear is people will start using these but not understand how to really use them properly. Since they’re dynamic checkers you really need a good testing harness and test cases to go with it. The bugs ASAN/TSAN etc catch are usually strange boundary…

The Little C Function From Hell

Does the standard say whether or not “char” is “unsigned char” or “signed char”? (I guess not from what you wrote above.)

How LLVM Optimizes a Function

This is very nice summary of IR Passes. I found “InstCombine” pass to be easier to start with to understand the code flow in LLVM IR Pass. Maybe the beginners can start with that first.

Proposal for a Friendly Dialect of C

What implications exist around linking friendly-c code with machiavellian-c? I.e. put the performance critical kernels in there own module and work realy hard to make it correct.

A Quiz About Integers in C

How Clang Compiles a Function

Minor nit: printing the predecessors of a basic block is a feature of the IL printer, not Clang. It is just visualizing information that is implicit in the IR anyway.

The Problem with Friendly C

What’s the difference between an integer and a pointer?

The Standard does not require that implementations define intptr_t nor uintptr_t. It also does not require that implementations define integer-to-pointer conversions in any meaningful fashion unless they define those types. Conforming implementations are certainly allowed to document some integer-to-pointer…

The Strict Aliasing Situation Is Pretty Bad

One thing that works well in practice in cases where you really don’t want to process single bytes at a time is to use a union for going from one type to another. Granted, writing one member of a union and reading from a different one also isn’t allowed per the standard, but every compiler I’ve come across allows it…

Undefined Behavior in 2017

Since you talk about the sanitizers and tools like Checked-C, I’ll note there’s also SaferCPlusPlus[1], which is essentially a memory-safe (and data race safe) subset of C++. The idea is that there are a finite number of C++ elements that contribute to memory unsafety, and since the advent of C++11, it is now…

Finding Compiler Bugs by Removing Dead Code

What has caused you to slack off on the bug reporting? The cost of computer time or the human effort side? (I’m guessing the second.)

57 Small Programs that Crash Compilers

I noticed there are no MSVC examples in there — was it not a targeted compiler? If so, I’m curious as to why.

Taming Undefined Behavior in LLVM

Interesting post, but you lost me at “Let’s try to do that using undef”. Why bother with notions of an “undef value” (or “deferred UB”), when plain UB is all you need?

Undefined Behavior != Unsafe Programming

Are you arguing that undefined behaviour should not be a thing in, e.g., C? It sounds like you’re claiming that the fabled sufficiently smart compiler could take code in a C-like language with no UB, and emit something equivalent to what a C compiler would produce given equivalent input in C that correctly avoids UB…

C No Evil

C and C++ Aren't Future Proof

My hope would be option 1 for application code (as opposed to the “systems code” that you have there): fewer and fewer people use these languages to write application code. Moreover, I think that’s probably happening to some extent.

A Guide to Undefined Behavior in C and C++ (2010)

The Future of Compiler Optimization

Hello, My name is Nadav and I am a PhD candidate, researching compilers for embedded systems and high-level synthesis. This is an excellent post and I enjoyed reading it. I have a few comments. 1. I agree about verification and optimization joining forces. We already see it to some extent with compiler warnings. 2…

Do C compilers disprove Fermat's Last Theorem?

(You probably already have realized this.) The reason the compiler produced code that terminated was based upon the fact that the values being altered (i.e., a, b and c) within the “while” loop are never referenced again after the loop. But, once you added code which printed the resulting values it was forced to…