[PATCH] D100581: [Clang] -Wunused-but-set-parameter and -Wunused-but-set-variable

[Nick Desaulniers via Phabricator via cfe-commits]

nickdesaulniers added a comment.

Testing Diff 342071 on the mainline Linux kernel, just building x86_64 defconfig triggers 19 instances of this warning; all look legit. ;)

In D100581#2730708 <https://reviews.llvm.org/D100581#2730708>, @dblaikie wrote:

> In D100581#2730557 <https://reviews.llvm.org/D100581#2730557>, @nickdesaulniers wrote:
>
>> I see lots of instances from the kernel that look like this when reduced:
>>
>>   $ cat foo.c
>>   int page;
>>   int put_page_testzero(int);
>>   void foo (void) {
>>     int zeroed;
>>     zeroed = put_page_testzero(page);
>>     ((void)(sizeof(( long)(!zeroed))));
>>   }
>>   $ clang -c -Wall foo.c
>>   foo.c:4:7: warning: variable 'zeroed' set but not used [-Wunused-but-set-variable]
>>     int zeroed;
>>         ^
>
> Any idea what the purpose of this code is/why it's not a reasonable thing to warn on?

include/linux/build_bug.h:

  25 /*                                                                                                                                                                          
  26  * BUILD_BUG_ON_INVALID() permits the compiler to check the validity of the                                                                                                 
  27  * expression but avoids the generation of any code, even if that expression                                                                                                
  28  * has side-effects.                                                                                                                                                        
  29  */                                                                                                                                                                         
  30 #define BUILD_BUG_ON_INVALID(e) ((void)(sizeof((__force long)(e))))

include/linux/mmdebug.h:

  57 #define VM_BUG_ON(cond) BUILD_BUG_ON_INVALID(cond)                                                                                                                          

mm/internal.h:

  410 static inline unsigned long                                                                                                                                                 
  411 vma_address(struct page *page, struct vm_area_struct *vma)                                                                                                                  
  412 {                                                                                                                                                                           
  413   unsigned long start, end;                                                                                                                                                 
  414                                                                                                                                                                             
  415   start = __vma_address(page, vma);                                                                                                                                         
  416   end = start + thp_size(page) - PAGE_SIZE;                                                                                                                                 
  417                                                                                                                                                                             
  418   /* page should be within @vma mapping range */                                                                                                                            
  419   VM_BUG_ON_VMA(end < vma->vm_start || start >= vma->vm_end, vma);

The warning (in previous revisions was triggering on `end` in `vma_address`.


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https://reviews.llvm.org/D100581