[cfe-dev] Warnings are different on preprocessed file

[Volker Diesel]

  Hi David. 
Great to hear that clang development reserves it's right to be different and to ignore objections regarding C++ standard  Let's wait and see to where this policy leeds. I have enough professional experience to know what will come out of this attitude. 
BR 
Volker 

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"David Blaikie [via Clang Developers]" <ml-node+s42468n4039196h92 at n3.nabble.com> schrieb: 

    On Wed, Mar 12, 2014 at 7:13 AM, Richtarsky, Martin 
    < [hidden email] > wrote: 
    > Hi, 
    > 
    > is this expected behavior? 
    
    To answer this original question: Yes, it is expected behavior that 
    preprocessing source and compiling it does not produce the same 
    results as compiling the preprocessed source directly. 
    
    Certain warnings produce (too many) false positives without taking 
    into account whether they were expanded from a macro, etc. 
    
    Take, for example: 
    
    #define FUNC_TEST(arg) (func(arg) ? GOOD : BAD) 
    
    Perhaps this macro is the idiomatic way to call 'func' in this 
    library? (ie: 'func' is an internal name, only ever meant to be 
    accessed via the macro) 
    
    Sometimes the user wants the result: 
    
    if (FUNC_TEST(42) == GOOD) { ... } 
    
    but sometimes it's not needed: 
    
    FUNC_TEST(42); 
    
    Which all seems fine. But if the user explicitly wrote: 
    
    func(arg) ? GOOD : BAD; 
    
    we would like to warn the user that they have unused-values there? 
    (Why would anyone /ever/ write that code? The whole conditional 
    operator and the values GOOD and BAD are discarded, the user might as 
    well just write "func(arg);") 
    
    So using the presence/absence of a macro helps diagnose the bad cases 
    while not needlessly warning on reasonable cases. 
    
    To your later comment about "no other compiler does this" - I suspect 
    some do, but also one of Clang's engineering novelties is that it 
    keeps track of all this macro expansion information so it /can/ reason 
    about warnings using this information. Many other compilers don't have 
    this information available to make such choices about when to emit and 
    suppress warnings. 
    
    - David 
    
      
      > 
      > cat preproc_warning.sh 
      > #!/bin/bash 
      > 
      > echo "-- direct compilation:" 
      > clang++ -c preproc_warning.cpp -o /dev/null 
      > echo "-- preprocessing:" 
      > clang++ -E preproc_warning.cpp -o preproc_out.cpp 
      > echo "-- compilation of preprocessed source:" 
      > clang++ -c preproc_out.cpp -o /dev/null 
      > 
      > 
      > cat preproc_warning.cpp 
      > #include <stddef.h> 
      > 
      > #define CHECK(a) ( a ? 1 : NULL) 
      > 
      > int func1() 
      > { 
      >     int a; 
      >     return NULL; // produces warning during direct compilation and compilation of preprocessed source 
      > } 
      > 
      > int func2() 
      > { 
      >     int a; 
      >     return (CHECK(a)); // produces warning only on preprocessed source 
      > } 
      > 
      > 
      > shows this output: 
      > -- direct compilation: 
      > preproc_warning.cpp:8:12: warning: implicit conversion of NULL constant to 'int' [-Wnull-conversion] 
      >     return NULL; // produces warning during direct compilation and compilation of preprocessed source 
      >     ~~~~~~ ^~~~ 
      >            0 
      > 1 warning generated. 
      > -- preprocessing: 
      > -- compilation of preprocessed source: 
      > preproc_warning.cpp:8:12: warning: implicit conversion of NULL constant to 'int' [-Wnull-conversion] 
      >     return __null; 
      >     ~~~~~~ ^~~~~~ 
      >            0 
      > preproc_warning.cpp:14:23: warning: implicit conversion of NULL constant to 'int' [-Wnull-conversion] 
      >     return (( a ? 1 : __null)); 
      >     ~~~~~~            ^~~~~~ 
      >                       0 
      > 2 warnings generated. 
      > 
      > 
      > Best regards, 
      > Martin 
      > 
      > _______________________________________________ 
      > cfe-dev mailing list 
      >  [hidden email] 
      >  http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev 
    
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