nathanchance added subscribers: nickdesaulniers, nathanchance. nathanchance added a comment.
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. My original commentary is available on our GitHub issue tracker (https://github.com/ClangBuiltLinux/linux/issues/1720) but I will summarize it here: The first major instance of this warning comes from the `__is_constexpr` macro, which is used a lot in common macros do fancy things at compile time (change that added this: https://git.kernel.org/linus/3c8ba0d61d04ced9f8d9ff93977995a9e4e96e91): /* * This returns a constant expression while determining if an argument is * a constant expression, most importantly without evaluating the argument. * Glory to Martin Uecker <martin.uec...@med.uni-goettingen.de> */ #define __is_constexpr(x) \ (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) Sample warnings: In file included from scripts/mod/devicetable-offsets.c:3: In file included from ./include/linux/mod_devicetable.h:13: In file included from ./include/linux/uuid.h:12: In file included from ./include/linux/string.h:253: ./include/linux/fortify-string.h:159:10: warning: ISO C does not allow indirection on operand of type 'void *' [-Wvoid-ptr-dereference] q_len = strlen(q); ^~~~~~~~~ ./include/linux/fortify-string.h:131:24: note: expanded from macro 'strlen' __builtin_choose_expr(__is_constexpr(__builtin_strlen(p)), \ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/const.h:12:25: note: expanded from macro '__is_constexpr' (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ In file included from arch/x86/kernel/asm-offsets.c:9: In file included from ./include/linux/crypto.h:20: In file included from ./include/linux/slab.h:15: In file included from ./include/linux/gfp.h:7: In file included from ./include/linux/mmzone.h:8: In file included from ./include/linux/spinlock.h:55: In file included from ./include/linux/preempt.h:78: In file included from ./arch/x86/include/asm/preempt.h:7: In file included from ./include/linux/thread_info.h:60: In file included from ./arch/x86/include/asm/thread_info.h:53: In file included from ./arch/x86/include/asm/cpufeature.h:5: In file included from ./arch/x86/include/asm/processor.h:22: In file included from ./arch/x86/include/asm/msr.h:11: In file included from ./arch/x86/include/asm/cpumask.h:5: In file included from ./include/linux/cpumask.h:12: In file included from ./include/linux/bitmap.h:9: ./include/linux/find.h:40:17: warning: ISO C does not allow indirection on operand of type 'void *' [-Wvoid-ptr-dereference] val = *addr & GENMASK(size - 1, offset); ^~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/bits.h:38:3: note: expanded from macro 'GENMASK' (GENMASK_INPUT_CHECK(h, l) + __GENMASK(h, l)) ^~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/bits.h:25:3: note: expanded from macro 'GENMASK_INPUT_CHECK' __is_constexpr((l) > (h)), (l) > (h), 0))) ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/const.h:12:25: note: expanded from macro '__is_constexpr' (sizeof(int) == sizeof(*(8 ? ((void *)((long)(x) * 0l)) : (int *)8))) ^ ./include/linux/build_bug.h:16:62: note: expanded from macro 'BUILD_BUG_ON_ZERO' #define BUILD_BUG_ON_ZERO(e) ((int)(sizeof(struct { int:(-!!(e)); }))) ^ The second major instance is the type checking in `container_of` when ptr is a `void *` (change that added this: https://git.kernel.org/linus/c7acec713d14c6ce8a20154f9dfda258d6bcad3b): /** * container_of - cast a member of a structure out to the containing structure * @ptr: the pointer to the member. * @type: the type of the container struct this is embedded in. * @member: the name of the member within the struct. * */ #define container_of(ptr, type, member) ({ \ void *__mptr = (void *)(ptr); \ static_assert(__same_type(*(ptr), ((type *)0)->member) || \ __same_type(*(ptr), void), \ "pointer type mismatch in container_of()"); \ ((type *)(__mptr - offsetof(type, member))); }) #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b)) Sample warning: In file included from sound/soc/sof/core.c:14: In file included from ./include/sound/sof.h:14: ./include/linux/pci.h:600:9: warning: ISO C does not allow indirection on operand of type 'void *' [-Wvoid-ptr-dereference] return container_of(priv, struct pci_host_bridge, private); ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/container_of.h:20:21: note: expanded from macro 'container_of' __same_type(*(ptr), void), \ ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/compiler_types.h:295:63: note: expanded from macro '__same_type' #define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b)) ^ ./include/linux/build_bug.h:77:50: note: expanded from macro 'static_assert' #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr) ~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~ ./include/linux/build_bug.h:78:56: note: expanded from macro '__static_assert' #define __static_assert(expr, msg, ...) _Static_assert(expr, msg) ^~~~ I am aware these are quite specialized cases so warning is likely appropriate. If so, we can just disable this warning for the kernel but I figured it was worth a comment in case the diagnostic can be improved in any way. Repository: rG LLVM Github Monorepo CHANGES SINCE LAST ACTION https://reviews.llvm.org/D134461/new/ https://reviews.llvm.org/D134461 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
