[PATCH] D134461: [Clang] Warn when trying to deferencing void pointers in C

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added a comment.

In D134461#3815458 <https://reviews.llvm.org/D134461#3815458>, @aaron.ballman wrote:

> In D134461#3815298 <https://reviews.llvm.org/D134461#3815298>, @nathanchance wrote:
>
>> This warning is quite noisy for the Linux kernel due to a couple of places where a `void *` is dereferenced as part of compile time checking.
>
> Thank you for letting us know!

In particular offsetof and __is_constexpr are _heavily_ used throughout the codebase.

> This one is interesting to me -- I was on the fence about whether we want to diagnose this in an unevaluated context or not. The `clang/test/Analysis/misc-ps.m` test was why I was considering it, and I eventually convinced myself that the dereference there was a harmless UB when we support pointer arithmetic on void as an extension, but UB nonetheless. Now I wonder whether we want different behavior in the `sizeof` case -- it seems odd to me that we warn by default on `sizeof(*vp)` but not `sizeof(void)`: https://godbolt.org/z/PcMrW7b5W so I think we might want to silence `sizeof(*vp)` diagnostics.
>
> WDYT?

SGTM

> All this said, I think in both cases we want `-pedantic` to warn on these situations even if we would silence by default. e.g.,
>
>   void func(void *vp) {
>     sizeof(void); // No warning by default, does get pedantic warning
>     sizeof(*vp); // No warning by default, does get pedantic warning
>     sizeof(*(vp))`; // No warning by default, does get pedantic warning
>     void inner(typeof(*vp)); // No warning by default, does get pedantic warning
>   
>     (void)*vp; // Warning by default
>   }
>
> What do folks think of that idea?

SGTM; either warn via -Wpedantic or silenced with -Wno-gnu.


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