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* [Bug c/112840] New: feature request: warn on incorrect tagged union value access @ 2023-12-03 21:36 matheus.a.m.moreira at gmail dot com 2023-12-03 21:39 ` [Bug c/112840] " pinskia at gcc dot gnu.org ` (3 more replies) 0 siblings, 4 replies; 5+ messages in thread From: matheus.a.m.moreira at gmail dot com @ 2023-12-03 21:36 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840 Bug ID: 112840 Summary: feature request: warn on incorrect tagged union value access Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: matheus.a.m.moreira at gmail dot com Target Milestone: --- It would be useful if GCC could warn the programmer if the value of a tagged union that doesn't correspond to its type tag is accessed. Here's an example that illustrates the kind of mistake such a warning would prevent: #include <stdio.h> enum T { I, F }; union U { int i; float f; }; struct S { enum T t; union U u; }; int main(void) { struct S s = { .t = F, .u.f = 12345.67890f }; switch (s.t) { case I: printf("%d\n", s.u.i); break; case F: // copied the above case // but neglected to update the code printf("%d\n", s.u.i); break; } } I understand that unions are typically used for type punning and that such accesses are often intended by the programmer but compiler checks would still be beneficial when that's not the case. A compiler mechanism to establish a relationship between the union values and their corresponding enum tags would be extremely useful. Something like this, perhaps: struct S { enum T t; union U { int i __attribute__((tag(t, I))); float f __attribute__((tag(t, F))); } u; }; Then gcc would be able to warn when union values are accessed in a context where their specified tags are not known to be the correct value: switch (s.t) { case I: // i is accessed // the tag of i is t // t is supposed to equal I // compiler knows t equals I because of switch case // correct, no warning is emitted printf("%d\n", s.u.i); break; case F: // i is accessed // the tag of i is t // t is supposed to equal I // compiler knows t equals F because of switch case // incorrect, a warning is emitted printf("%d\n", s.u.i); break; } Such a feature would make C less error prone. I've also seen support for safe tagged unions in newer languages like Zig. People have created C preprocessor solutions to use tagged unions safely in C due to the lack of this safety: https://github.com/Hirrolot/datatype99 Relevant clang issue: https://github.com/llvm/llvm-project/issues/74205 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/112840] feature request: warn on incorrect tagged union value access 2023-12-03 21:36 [Bug c/112840] New: feature request: warn on incorrect tagged union value access matheus.a.m.moreira at gmail dot com @ 2023-12-03 21:39 ` pinskia at gcc dot gnu.org 2023-12-03 21:40 ` pinskia at gcc dot gnu.org ` (2 subsequent siblings) 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-12-03 21:39 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840 Andrew Pinski <pinskia at gcc dot gnu.org> changed: What |Removed |Added ---------------------------------------------------------------------------- Keywords| |diagnostic Severity|normal |enhancement See Also| |https://github.com/llvm/llv | |m-project/issues/74205 ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/112840] feature request: warn on incorrect tagged union value access 2023-12-03 21:36 [Bug c/112840] New: feature request: warn on incorrect tagged union value access matheus.a.m.moreira at gmail dot com 2023-12-03 21:39 ` [Bug c/112840] " pinskia at gcc dot gnu.org @ 2023-12-03 21:40 ` pinskia at gcc dot gnu.org 2023-12-17 19:38 ` uecker at gcc dot gnu.org 2024-01-30 0:40 ` matheus.a.m.moreira at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: pinskia at gcc dot gnu.org @ 2023-12-03 21:40 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840 --- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> --- This seems more like something for analyzier rather than a generic warning due to it requires keeping track of the path where the access is located. ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/112840] feature request: warn on incorrect tagged union value access 2023-12-03 21:36 [Bug c/112840] New: feature request: warn on incorrect tagged union value access matheus.a.m.moreira at gmail dot com 2023-12-03 21:39 ` [Bug c/112840] " pinskia at gcc dot gnu.org 2023-12-03 21:40 ` pinskia at gcc dot gnu.org @ 2023-12-17 19:38 ` uecker at gcc dot gnu.org 2024-01-30 0:40 ` matheus.a.m.moreira at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: uecker at gcc dot gnu.org @ 2023-12-17 19:38 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840 uecker at gcc dot gnu.org changed: What |Removed |Added ---------------------------------------------------------------------------- CC| |uecker at gcc dot gnu.org --- Comment #2 from uecker at gcc dot gnu.org --- I would go for a more generic feature where one can specify some invariant / condition that needs be true when a member of a struct / union is accessed. struct S { enum T t; union U { int i [[gnu::guard(.t == I)]]; float f [[gnu::guard(.t == f)]]; } u; }; ^ permalink raw reply [flat|nested] 5+ messages in thread
* [Bug c/112840] feature request: warn on incorrect tagged union value access 2023-12-03 21:36 [Bug c/112840] New: feature request: warn on incorrect tagged union value access matheus.a.m.moreira at gmail dot com ` (2 preceding siblings ...) 2023-12-17 19:38 ` uecker at gcc dot gnu.org @ 2024-01-30 0:40 ` matheus.a.m.moreira at gmail dot com 3 siblings, 0 replies; 5+ messages in thread From: matheus.a.m.moreira at gmail dot com @ 2024-01-30 0:40 UTC (permalink / raw) To: gcc-bugs https://gcc.gnu.org/bugzilla/show_bug.cgi?id=112840 --- Comment #3 from Matheus Afonso Martins Moreira <matheus.a.m.moreira at gmail dot com> --- (In reply to uecker from comment #2) > I would go for a more generic feature where one can specify some invariant / > condition that needs be true when a member of a struct / union is accessed. So the proposed solution is a guard(expr) attribute that causes the compiler to warn the developer if it can't statically determine that expr is true in the appropriate context. This would be great!! Much more flexible than the tag attribute I proposed. How can we make it happen? I've never hacked on GCC before. ^ permalink raw reply [flat|nested] 5+ messages in thread
end of thread, other threads:[~2024-01-30 0:40 UTC | newest] Thread overview: 5+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2023-12-03 21:36 [Bug c/112840] New: feature request: warn on incorrect tagged union value access matheus.a.m.moreira at gmail dot com 2023-12-03 21:39 ` [Bug c/112840] " pinskia at gcc dot gnu.org 2023-12-03 21:40 ` pinskia at gcc dot gnu.org 2023-12-17 19:38 ` uecker at gcc dot gnu.org 2024-01-30 0:40 ` matheus.a.m.moreira at gmail dot com
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