Are Const expressions like 1+3 computed at compile time or at execution time?

The answer is:" Const expressions are computed at compile time".
Let see together a practical demonstartion.
If we consider the following code:

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int main() { int a=1+3; }

And compile it using  gcc with -S option we obtain:

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.file "ottimo.c" .def ___main; .scl 2; .type 32; .endef .text .globl _main .def _main; .scl 2; .type 32; .endef _main: pushl %ebp movl %esp, %ebp subl $8, %esp andl $-16, %esp movl $0, %eax addl $15, %eax addl $15, %eax shrl $4, %eax sall $4, %eax movl %eax, -8(%ebp) movl -8(%ebp), %eax call __alloca call ___main movl $4, -4(%ebp) leave ret

As we can see, the compiler, even without the selection of optimization options, optimize the code and translate a= 1+3 as movl $4, -4(%ebp), that is a =4.

Q.E.D. :)