[PATCH] D103958: [WIP] Support MustControl conditional control attribute

[Marco Elver via Phabricator via llvm-commits]

melver added a comment.

In D103958#2808967 <https://reviews.llvm.org/D103958#2808967>, @jdoerfert wrote:

>> Please bear with me, I'm updating examples and documentation. I didn't think anybody would look at this while [WIP]. :-)
>
> People try to help so you have early design feedback ;)

Thank you for that. The LKML discussion got a little heated, so I worry slightly that underpresenting the issue will cause a bad first impression.

But while we're here:

There is the consideration to make this a __builtin and not an attribute.

AFAIK a __builtin suffers from the usual problem that the information cannot be propagated between TUs:

  file1.c:
    bool foo(void) { return __builtin_mustcontrol(READ_ONCE(...)); }
  
  file2.c:
    void bar(void) { if (foo()) { WRITE_ONCE(...); } }

Or is a language builtin that gives you an error when used in the wrong context acceptable? It seems a little odd because I'm unaware of other builtins that do that.

GCC devs expressed that GNU attribute syntax may be abused: https://lkml.kernel.org/r/20210609171419.GI18427@gate.crashing.org

The attribute is simpler, and hypothetically, if it were to become part of the language std we'd have to use [[...]] syntax anyway, so the GNU attribute problem seems somewhat artificial to me.

  [[mustcontrol]] if (READ_ONCE(...)) { ... }
  [[mustcontrol]] while (...) { }

Preferences?


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