from Hacker News

The strict aliasing situation is pretty bad (2016)

by pkkm on 10/6/24, 3:16 PM with 58 comments

• by jandrewrogers on 10/10/24, 6:26 AM

  This is an issue that is ignored by just about everyone in practice. The reality is that most developers have subconsciously internalized the compiler behavior and assume that will always hold. And they are mostly right, I’ve only seen a few cases where this has caused a bug in real systems over my entire career. I try, to the extent possible, to always satisfy the requirements of strict aliasing when writing code. It is difficult to determine if I’ve been successful in this endeavor.

  Here is why I don’t blame the developers: writing fast, efficient systems code that satisfies the requirements of strict aliasing as defined by C/C++ is surprisingly difficult. It has taken me years to figure out the technically correct incantations for every weird edge case such that they always satisfy the requirements of strict aliasing. The code gymnastics in some cases are entirely unreasonable. In fairness, recent versions of C++ have been adding ways to express each of these cases directly, eliminating the need to use obtuse incantations. But we still have huge old code bases that assume compiler behavior, as was the practice for decades.

  I am not here to attribute blame, I think it the causes are pretty diffuse honestly. This is just a part of the systems world we failed to do well, and it impacts the code we write every day. I see strict aliasing violations in almost every code base I look at.

• by sapiogram on 10/10/24, 12:19 PM

  Has anyone measured the performance impact of the -fno-strict-aliasing flag? How much real-world performance are we really gaining from all this mess?

• by icedchai on 10/10/24, 1:58 PM

  I learned C on the Amiga, back in the late 80's, and the OS made heavy use of "OO-ish" physical subtyping with structs everywhere. I don't think anybody even thought about strict aliasing violations.

• by RossBencina on 10/10/24, 10:46 AM

  Interesting that the article doesn't even entertain the obvious solution: remove strict aliasing requirements from the standards.

• by Gabriel54 on 10/10/24, 12:07 PM

  Forgive me my ignorance, but if I write

    int foo(int *x) {
      *x = 0;
      // wait until another thread writes to *x
      return *x;
    }
  
  

  Can the C compiler really optimize foo to always return 0? That seems extremely unintuitive to me.

• by dang on 10/11/24, 2:29 AM

  Discussed at the time:

  The Strict Aliasing Situation Is Pretty Bad - https://news.ycombinator.com/item?id=11288665 - March 2016 (67 comments)