n3199 - Improved `__attribute__((cleanup))` Through `defer`

[ThePhD]

Latest draft: https://thephd.dev/vendor/future_cxx/papers/C%20-%20Improved%20__attribute_((cleanup))%20Through%20defer.html

[ThePhD]

It took a while, but the paper was fully written.

Current standard revision: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3199.htm

[jiyuzh]

Thank you for the proposal. I like it very much! I have one question regarding Section 3.8.3: It seems there is no specification over whether break or continue are allowed to be used to jump over a defer. Do you think this type of jump over should be allowed or disallowed?

Pros

Consider the following use case:

for (int i = 0; i < 10; i++) {
    void *ptr = malloc(8);

    if (!ptr)
        break;

    defer { free(ptr); }
}

It seems we should allow such jump over in this example. Also, break is semi-related to switch, allowing break to jump over seems making loops and switch consistent on the constraint.

Cons

Some common goto usages can also be expressed with/converted to loops plus break or continue, hence if goto is banned, so shall break and continue. The wording of the draft also seems to imply return is an exception. Also, if labeled loops are adopted into C standard, the jump over could be even more error prone.

Personal opinion

I implemented a C-based polyfill using __attribute__((cleanup(...))) and used it in several of my projects.

During my usage, I find goto and switch-jump-over are indeed confusing. However, based on my personal feeling, the break/continue use case could be a good thing to have.

#define _DEFER_MERGE(a,b) a##b
#define _DEFER_VARNAME(a) _DEFER_MERGE(____defer_scopevar_, a)
#define _DEFER_FUNCNAME(a) _DEFER_MERGE(____defer_scopefunc_, a)
#define _DEFER(n) \
    auto void _DEFER_FUNCNAME(n)(int *a); \
    __attribute__((cleanup(_DEFER_FUNCNAME(n)))) int _DEFER_VARNAME(n); \
    void _DEFER_FUNCNAME(n)(int *a)

#define defer _DEFER(__COUNTER__)

It closely resembles some behaviors defined by the draft (i.e., example 2, 4, 5 from Section 6.4), which can be tested here: https://godbolt.org/z/3vv5dMMqG

[ThePhD]

Thanks for your support!

Regarding jumps like goto, break, and continue, the latest version of the proposal has the following constraints in the proposed wording:

Constraints

Jumps by means of goto or switch into E shall not jump over a defer statement in E.

Jumps by means of goto or switch shall not jump into any defer statement.

Jumps by means of return, goto, break, or continue shall not exit S.

S here is the scope of the defer statement itself. So the only jumps that are banned are ones that jump out of a defer, not ones that happen to skip its execution. Your example:

for (int i = 0; i < 10; i++) {
    void *ptr = malloc(8);

    if (!ptr)
        break;

    defer { free(ptr); }
}

is perfectly fine and valid. if ptr is NULL/a null pointer, then this code will simply exit the loop and the defer will not be called because it has not been reached in that scope yet. We do not ban any use of goto or friends that is appropriate, only banning if it jumps over a defer statement. You can goto into some block/scope that contains a defer, so long as you don't jump over it:

int handle = get_handle();
goto meow;  // label after defer: using `goto` to jump forwards or backwards over is illegal
defer { release_handle(handle); }
meow0; ;
meow1: ;
printf("hiiii"); 
goto meow1; // okay

I think that covers the cases you were speaking of. Also, even if someone implements break and continue like a goto, that's not of the C Standard's concern. The examples and informative (not normative) explanatory text demonstrating it in terms of gotos is not a mandate for the implementation to follow: a C compiler (or interpreter, or transpiler, or any C implementation) does not have to care about such hints. They have to uphold the Constraints and Semantics sections, as given. That most flow control can be flattened/rewritten in terms of goto is not something the C standard has to care about, so long as the exact use of continue or break follow what is in the standard before your compiler changes it / reorders it / messes with it.

[BatmanAoD]

One small organizational issue: the last paragraph of 3.4 (all of which is duplicated in N3198) states that "for the macros we provide almost every single one will be defined and have the value of 1", which makes sense in the context of N3198 but not N3199. N3198 is linked later, in 3.4.2; if N3198 were mentioned and linked prior to the note about MSVC and SEH, then the note could allude to "the macros provided in N3198."

[tylov]

There are, as far as most reviewers can tell, no errors in the creation or deletion of the various kinds of resources (particularly, repeated memory allocations).

Most reviewers? Each of the returns inside the loop will leak memory on namelist in addition to all namelist[k]'s for k in [i, n). Great proposal, though!

[fuhsnn]

Greetings, if implementation experience from an amateur compiler could help get this through, I wrote a little bit here along with the implementation.

[fuhsnn]

Some feedback on C++ compatibility (hope this is relevant, since the draft does contain sizable words for that):

The lambda based scoped guard implementation under 4.4. The Polyfill/C++ Fix may not behave as expected (as my understanding of the proposed C defer) when copy-elision is in effect, for example EXAMPLE 4 converted to struct may return 5 instead of 4 on all major C++ compilers: https://godbolt.org/z/q41rMo5Ex

My short discussion with LLVM people: https://github.com/llvm/llvm-project/issues/100869

[ThePhD]

The lambda based scoped guard implementation under 4.4. The Polyfill/C++ Fix may not behave as expected (as my understanding of the proposed C defer) when copy-elision is in effect, for example EXAMPLE 4 converted to struct may return 5 instead of 4 on all major C++ compilers: https://godbolt.org/z/q41rMo5Ex

Nothing I can do about that, unfortunately. Maybe if C gets a form of applicable NRVO, but otherwise the intent and the effect is still matching and the same.

[ThePhD]

Greetings, if implementation experience from an amateur compiler could help get this through, I wrote a little bit here along with the implementation.

Thank you for the implementation. At the moment defer is likely headed to a Techincal Specification unless we can convince folks that it should just go straight into the next working draft instead. I might do that but I'm tired of fighting people on this stuff so I'm just writing a Technical Specification; I will link to and include your work in the next revision of the proposal, however.

[fuhsnn]

Thanks, just keep doing what you're doing. I'll keep stalking wg14 documents for fun stuff to implement.