Thread: clang warning: -Watomic-implicit-seq-cst

  1. #1
    Registered User
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    Mar 2020
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    clang warning: -Watomic-implicit-seq-cst

    I have this piece of code:

    Code:
    atomic_int Vector[2];
    
    void Load(int *v)
    {
       v[0]=atomic_load(&Vector[0]);
       v[1]=atomic_load(&Vector[1]);
    }
    
    void Store(int *v)
    {
       atomic_store(&Vector[0],v[0]);
       atomic_store(&Vector[1],v[1]);
    }
    clang give this warning: implicit use of sequentially-consistent atomic may incur stronger memory barriers than necessary.
    How can this warning be solved?

  2. #2
    and the hat of int overfl Salem's Avatar
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    You need to be more specific.
    Code:
    #include <stdatomic.h>
    
    atomic_int Vector[2];
     
    void Load(int *v)
    {
       v[0]=atomic_load(&Vector[0]);
       v[1]=atomic_load(&Vector[1]);
    }
     
    void Store(int *v)
    {
       atomic_store(&Vector[0],v[0]);
       atomic_store(&Vector[1],v[1]);
    }
    
    int main()
    {
        return 0;
    }
    I tried this over on Compiler Explorer for several versions of clang with several sets of flags and saw nothing unusual.

    State:
    - your actual clang version
    - your actual compiler command line
    - your actual compilable program (not some snippet)
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.

  3. #3
    Registered User
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    Mar 2020
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    Ok, this is the code for compiler explorer try with flag -Weverything

    Code:
    #include <stdatomic.h>
    
    
    typedef struct
    {
       atomic_ullong V[2];
    } vector_t;
    
    vector_t *Vector;
    
    
    void Load(int index,vector_t *vec)
    {  
       vec->V[0] = atomic_load(&Vector[index].V[0]);
       vec->V[1] = atomic_load(&Vector[index].V[1]);
       vec->V[0]^=vec->V[1];
    }
    
    
    void Store(int index,vector_t *vec)
    {  
       vec->V[0]^=vec->V[1];
       atomic_store(&Vector[index].V[0],vec->V[0]);
       atomic_store(&Vector[index].V[1],vec->V[1]);
    }
    
    
    

  4. #4
    Registered User
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    Mar 2020
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    13
    I have solved in this way:

    Code:
    #include <stdatomic.h>
    
    typedef struct
    {
       atomic_ullong V[2];
    } vectoratomic_t;
    
    typedef struct
    {
        unsigned long long int V[2];
    } vector_t;
    
    vectoratomic_t *Vector;
    
    void Load(int index,vector_t *vec)
    {  
       vec->V[0] = atomic_load(&Vector[index].V[0]);
       vec->V[1] = atomic_load(&Vector[index].V[1]);
       vec->V[0]^=vec->V[1];
    }
    
    void Store(int index,vector_t *vec)
    {  
       vec->V[0]^=vec->V[1];
       atomic_store(&Vector[index].V[0],vec->V[0]);
      atomic_store(&Vector[index].V[1],vec->V[1]);
    }
    
    I'm sorry for the question.

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