[Bug other/31043] duplicated data in .rodata / .rodata.cst sections.

["pluto at agmk dot net" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #3 from pluto at agmk dot net  2009-10-28 12:03 -------
(In reply to comment #2)
> 4.3.x and newer gcc says:
> 
> pr31043.c:3: warning: 'pi' initialized and declared 'extern'

so, lets get correct testcase:

#ifndef pi_hpp
#define pi_hpp
extern double const pi;
extern double foo();
#endif

#include "pi.hpp"
#include <cmath>
double const pi = M_PI;
double foo() { return pi; }

now, it compiles cleanly with `g++ -Wall -Wextra -O2 -S pi.cpp`

> So, if you don't want to be treated as a constant, write
> 
> double pi = M_PI;
> 
> and this will generate the code you are looking for:

but in this testcase i don't want to drop const.

> As far as double copy when "double const pi = M_PI;" is used, tree optimizers
> already propagate constant to return, as evident from optimized dump:
> 
> foo ()
> {
> <bb 2>:
>   return 3.141592653589793115997963468544185161590576171875e+0;
> 
> }
> 
> And this constant is handled in different way than its shadow in the memory.

could you explain why? they are identicial constants.
-fmerge-constants should do something with this.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=31043