[Bug c++/53792] New: [C++11][constexpr] improving compiler-time constexpr evaluation

[vincenzo.innocente at cern dot ch]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53792

             Bug #: 53792
           Summary: [C++11][constexpr] improving compiler-time constexpr
                    evaluation
    Classification: Unclassified
           Product: gcc
           Version: 4.8.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: vincenzo.innocente@cern.ch


Filed under c++ even if it is most probably an optimization issue.

At the moment it looks like that constexpr are evaluated at compile time only
if explicitly assigned to a constexpr constant. There are cases though where
the compiler can infer that the expression can still be evaluated at compile
time even if used in a run-time context.

Take the following quite realistic example of a consexpr "indexing table" used
to access a non-const array using string literals though an inline function.
In principle foo and bar are equivalent.
At the moment gcc evaluates "getIndex" at compile time for bar (where the marco
expansion explicitly instantiates a constexpr int, while it generates runtime
code for foo that uses the inlined function getV.

Would the compiler be able to transform getV in something like the code in the
macro?



constexpr entry theMap[] = {
 {"a", 0},
 {"b", 1},
 {nullptr,2}
};

// filled at run time 
double v[3];


constexpr bool  same(char const *x, char const *y)   {
 return !*x && !*y ? true : (*x == *y && same(x+1, y+1));
}

constexpr int getIndex(char const *label, entry const *entries)   {
 return !entries->label ? entries->index  : same(entries->label, label) ?
entries->index : getIndex(label, entries+1);
}


inline  double __attribute__((always_inline)) getV(const char * name )  {
 return  v[getIndex(name,theMap)];
}



#define SetV(X,NAME) \
constexpr int i_##X = getIndex(NAME, theMap);\
const double X = v[i_##X]


int foo() {
 const double a = getV("a");
 const double b = getV("b");

 if (a==b) return 1;
 return 0;

}

int bar() {
 SetV(a,"a");
 SetV(b,"b");

 if (a==b) return 1;
 return 0;

}