The Strict Aliasing Situation is Pretty Bad – Embedded in Academia
I’ll start with a quick review of the strict aliasing rules in C and C++ and then present some less well-known material. Compiler optimizations are often shot down by the potential for pointers to be aliases. For example, although we might naively expect a compiler to optimize this function to return zero, that cannot happen because x and y might refer to the same location: int foo(int *x, int *y)
A Tourist’s Guide to the LLVM Source Code – Embedded in Academia
In my Advanced Compilers course last fall we spent some time poking around in the LLVM source tree. A million lines of C++ is pretty daunting but I found this to be an interesting exercise and at least some of the students agreed, so I thought I’d try to write up something similar. We’ll be using LLVM 3.9, but the layout isn’t that different for previous (and probably subsequent) releases. I don’t
Efficient Integer Overflow Checking in LLVM – Embedded in Academia
(Here’s some optional background reading material.) We want fast integer overflow checking. Why? First, if the undefined behavior sanitizers go faster then testing goes faster. Second, when overhead drops below a certain point people will become willing to use UBSan to harden production code against integer overflows. This is already being done in parts of Android. It isn’t the kind of thing to do
How to Debug – Embedded in Academia
One of the painful parts of teaching a lab-based embedded systems course is that over and over I have to watch a team with a relatively simple bug in their code, but who is trying to fix it by repeatedly making random changes. Generally they start with code that’s pretty close to working and break it worse and worse. By the end of the lab they’re frustrated, aren’t any closer to finding the bug, a
Undefined Behavior Consequences Contest Winners – Embedded in Academia
The contest that I posted the other day received some very nice entries. I decided to pick multiple winners since the best entries illustrate consequences of several kinds of undefined behavior. First, here’s the runner up, submitted by Octoploid: int main (void) { int x = 1; while (x) x <<= 1; return x; } This program is undefined by C99, where signed left shifts must not push a 1 bit into or pas
Embedded in Academia : A Quiz About Integers in C
The C language's rules for integer operations have some quirks that can make even small programs behave in confusing ways. This post is a review of these rules in the form of a quiz containing 20 questions. It is not intended to be extremely tricky, and I expect serious C programmers to get high scores. I recommend going through the questions in order. You should assume C99. Also assume that x86 o
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