[PATCH] D55628: Add support for "labels" on push/pop directives in #pragma clang attribute

[Erik Pilkington via Phabricator via cfe-commits]

erik.pilkington marked an inline comment as done.
erik.pilkington added inline comments.


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Comment at: clang/test/Sema/pragma-attribute-label.c:7
+
+#pragma clang attribute pop // expected-error{{'#pragma clang attribute pop' with no matching '#pragma clang attribute push'}}
+#pragma clang attribute pop NOT_MY_LABEL // expected-error{{'#pragma clang attribute pop NOT_MY_LABEL' with no matching '#pragma clang attribute push NOT_MY_LABEL'}}
----------------
aaron.ballman wrote:
> dexonsmith wrote:
> > aaron.ballman wrote:
> > > Should we really treat this as an error? It seems to me that this should be a warning because pop without a label could be viewed as "I don't care what I'm popping, just pop it". Still worth warning about, but maybe not worth stopping a build over.
> > IMO this is most likely to be an implementation error on the part of a macro author, where the END macro is missing the label used in BEGIN.  This makes the macro pair unsafe to mix with other macros.  If the macro author doesn’t want safety, why use a label in the BEGIN macro at all?
> > 
> > I see you’re envisioning this being used directly by an end-user, which I suppose is plausible, but I think the same logic applies.  Why add a label to push if you don’t want to be precise about pop?
> > Why add a label to push if you don’t want to be precise about pop?
> 
> Why is this important enough to fail everyone's build over it as opposed to warning users that they've done something that could be a bad code smell and let -Werror usage decide whether to fail the build or not? It seems like an extreme measure for something that has explicable "fallback" behavior.
My implicit assumption (which I should have been more clear about!) was that you'd only really ever write a label on a `push` in a BEGIN/END macro. In that world, you'd only ever see this case if 1) you're interacting with another macro that doesn't use the label convention, or 2) if you're interacting with manual push/pop code. In both of those cases, you'd end up popping the wrong attribute group and start applying attributes onto declarations that the programmer didn't intend.

I'm fine with downgrading this to a warning, but IMO an error seems more appropriate. If we wanted to force 1) or 2) through the compiler then we'd also need to downgrade `pop UNPUSHED_LABEL` to a warning, which doesn't seem like the end of the world either.


================
Comment at: clang/test/Sema/pragma-attribute-label.c:15
+// Out of order!
+#pragma clang attribute pop MY_LABEL
+
----------------
aaron.ballman wrote:
> dexonsmith wrote:
> > aaron.ballman wrote:
> > > I feel like this should be diagnosed, perhaps even as an error. The user provided labels but then got the push and pop order wrong when explicitly saying what to pop. That seems more likely to be a logic error on the user's part.
> > On the contrary, the user is using two differently-named and independent macro pairs (A_BEGIN/A_END vs B_BEGIN/B_END) and has no idea they are implemented with _Pragma(“clang attribute ...”) under the hood.  The point is to give the same result as two independent pragma pairs, whose regions do not need to be nested.
> > On the contrary, the user is using two differently-named and independent macro pairs (A_BEGIN/A_END vs B_BEGIN/B_END) 
> 
> I don't think this is a safe assumption to make, and in this case, is false. There are no macros anywhere in this test case.
> 
> > The point is to give the same result as two independent pragma pairs, whose regions do not need to be nested.
> 
> I don't find this to be intuitive behavior. These are stack manipulations with the names push and pop -- pretending that they're overlapping rather than a stack in the presence of labels is confusing. If I saw the code from this test case during a code review, I would flag it as being incorrect because the labels do not match -- I don't think I'd be the only one.
I think the labels only really makes sense if you're writing macros that hide the pragma attribute stack (like ASSUME_X_BEGIN/END, for instance), which for better or for worse people do write, and in fact was the intended use case for #pragma clang attribute. I think if we were to write this feature again, we'd forgo the stack entirly and require every `push` to have a label and be in its own namespace. But this is the best we can do now.

I don't really think that anyone should write a push label outside of a macro definition, since I agree that the semantics are a bit surprising when you're writing the #pragmas yourself without macros. I'll update this test case and the documentation to stress this point more. If you think this is going to be a potential pain point, maybe we can even warn on using a label outside of a macro definition. 


Repository:
  rC Clang

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D55628/new/

https://reviews.llvm.org/D55628