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Comment by obl | Hacker News Reader

That's fine if you're writing assembly.

Compilers are free to use UB from the standard to optimize and absolutely do not guarantee you to emit machine loads and store naively as specified in the C source.

At least for clang I'm pretty sure they decided _not_ to give knowledge of, e.g., the zero low bits of an int pointer to the optimizer since many people are relying on it. That might not be the case in the future or another compiler.

Here is a thread discussing that http://lists.llvm.org/pipermail/llvm-dev/2016-January/094012...

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Sure. And I love compilers generating very fast code. My point was that there is a ton of code written for x86 on assumptions that are no longer true when compiled with -O3 flags. Fortunately, open source code is less affected because usually target the generic case. However, for private code is a huge problem and risk. In my opinion, code intended to operate on x86 processors should be compiled with -O3 only in the case of high quality software, and if the quality can not be assured, it should be compiled with -O2 (at least when compiled with GCC and CLang).

The problem is the same as it's always been. C made no guarantees about a specific behavior in some cases, but people learned that a specific compiler and platform combination exhibited specific behavior in those cases, so took advantage of that to write code that used that exhibited behavior. Then the chickens came home to roost as the platforms added new capabilities and the compilers changed, in completely spec compliant ways, mind you, to take advantage of those capabilities.

There seems to be a lot of C code written not to spec, but to empirically observed behavior in the wild. People writing "incorrect" programs that function by happenstance isn't new, and the assumptions made when they were written were never true, they just happened to conform to (wrong) expectations for a while.

You still see this a lot nowadays, specially those that only use gcc and clang, without experience writing actual portable C code.

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