id	summary	reporter	owner	description	type	status	priority	component	version	resolution	keywords	cc
316	Const initialization from computation is ignored within same compile unit	mlbrooks		"CFA allows a constant to have a nontrivial expression as its initializer (like C++), though C does not.  Current lowering works for access between compile units, but fails within one compile unit, leaving the value 0 in place of the correct one.

{{{
// $ cfa -c -DSPLIT_CU=1 demo.cfa -o demo.1.o
// $ cfa -c -DSPLIT_CU=2 demo.cfa -o demo.2.o
// $ cfa demo.1.o demo.2.o 
// $ ./a.out
#if SPLIT_CU == 1

    const unsigned int x = 25;
    const unsigned int y = 5 * 5;

    void in_cu() {
        printf( ""in-compunit access: %u %u\n"", x, y );
    }

#elif SPLIT_CU == 2

    extern const unsigned int x;
    extern const unsigned int y;
    extern void in_cu();

    void cross_cu() {
        printf( ""cross-compunit access: %u %u\n"", x, y );
    }

    int main() {
        in_cu();
        cross_cu();
    }

#else
    #error Bad SPLIT_CU value
#endif
}}}

Expected:
{{{
in-compunit access: 25 25
cross-compunit access: 25 25
}}}

Actual:
{{{
in-compunit access: 25 0
cross-compunit access: 25 25
}}}

We understand the reason is the CU-1 lowering produces
{{{
const unsigned int x = 25;
const unsigned int y;
}}}
along with startup-time code to initialize `y`.  There, C semantics say, ""`y` is defined to be 'constexpr' with value 0.""  So, at the usage (`in_cu`'s print), GCC does not emit loads for (`x` and) `y`.  Rather, it uses the statically ""known"" values.  This effect occurs even at -O0.  As a result, even though the initialization code correctly writes the evaluation of of `5 * 5` into the location of `y` (via const cast, before `main` runs), `in_cu` never reads it.

We believe our current lowering ""of constants as constants"" is fundamentally incorrect, due to the above phenomenon.  Rather, when lowering a constant that requires (early) runtime initialization, it should be lowered as a mutable global variable.
"	defect	new	major	cfa-cc	1.0		const init compunit