On 03 Feb 2015, at 15:58, leledumbo wrote:
int c ;
int test( int p){
int i;
i = p;
return (i+2+c+2+c+2+c);
}
int main(){
c = test(128);
}
Hmm....yes, from C perspective, there's nothing can modify c before
it's
used the first time (unless you inject the startup code). Therefore,
the
compiler is safe to assume that c is its initial global value (0)
and is
able to use constant propagation, with c assumed as constant to
reduce the
function call into a simple value.
The assembler code generated for the C program does take into account
the initial value of C:
movl _c, %eax
leal 134(%eax,%eax,2), %eax
movl %eax, _c
It could also be modified in a C program before "main" gets executed
by e.g. the initialisation code of a dynamic library.
In the end, it's not so much about constant propagation as it is about
reordering expressions to be able to fold them better.
Jonas
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