[cfe-dev] [analyzer] Retrieving macro expansions in the plist output

[Artem Dergachev via cfe-dev]

I guess nobody did this before because plists were supposed to be used 
by IDEs, and IDEs were supposed to have their own jump-to-definition 
functionality (in this case, macro definition).

I don't know that much about macros and source locations, but i suspect 
it should indeed be possible to obtain the expanded text by traversing 
spelling and expansion locations, though probably sometimes scanning 
token-by-token is inevitable. To state the obvious, there are a lot of 
useful methods in the SourceManager, but it's often unobvious how to 
combine them correctly :/

On 8/17/18 5:09 AM, Kristóf Umann via cfe-dev wrote:
> Hi!
>
> I'm currently trying to implement a new node in the plist output that 
> would show the expansion of a macro, like this:
>
> void setToNull(int **vptr) {
>   *vptr = nullptr;
> }
>
> void print(void*);
>
> #define TO_NULL(x) \
> setToNull(x)
>
> #define DOES_NOTHING(x) \
> {                     \
>     int b;              \
>     b = 5;              \
> }                     \
>   print(x)
>
> #define DEREF(x)   \
> DOES_NOTHING(x); \
>   *x
>
> void f() {
>   int *a = new int(5);
> TO_NULL(&a);
>   DEREF(a) = 5;
> }
>
> For this code, two PathDiagnosticMacroPieces should be generated, and 
> a message like this should be displayed:
>   Expanding macro 'TO_NULL' to 'print(&a); setToNull(&a)'
>   Expanding macro 'DEREF' to '{ int sajt; sajt = 5; } print(a); *a'
> I've made some progress on this issue, but here are the problems I faced.
>
> Currently, HTML file generation supports macro expansions fairly well, 
> however, the code that achieves this seems to be held together by 
> sheer luck:
> https://clang.llvm.org/doxygen/namespaceclang_1_1html.html#a3283736376c3e436ff65975acdb1d399
> As I understand it, the entire file is re-lexed and preprocessed, 
> during which macro expansions are added to the HTML file. I attempted 
> to reuse the code here without having to re-lex everything, but so far 
> my efforts lead to little success. I also found that any modification 
> of this code quickly leads to instabilities -- although this part of 
> Clang is somewhat of a mystery to me just yet, so I'd concede to this 
> being my fault.
>
> I also fear that since HTML output is not as used as other outputs, 
> and could lack vigorous testing, it could be crash-prone.
>
> Do you know any way of obtaining the expansion of a macro expression 
> that doesn't involve such abusement of the preprocessor? Something 
> like this would be ideal:
>
> SourceLocation Loc = /* PathDiagnosticMacroPiece location */;
> Preproc.getMacroExpansionForExpression(Loc));
>
> Cheers,
> Kristóf
>
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