how to assign multiple values into a struct at once?

Question

I can do this on initialization for a struct Foo:

Foo foo =  {bunch, of, things, initialized};

but, I can\'t do this:

Foo f         


        

Answer 1

Memory Footprint - Here is an interesting i386 addition.

After much hassle, using optimization and memcpy seems to generate the smallest footprint using i386 with GCC and C99. I am using -O3 here. stdlib seems to have all sorts of fun compiler optimizations at hand, and this example makes use of that (memcpy is actually compiled out here).

Do this by:

Foo foo; //some global variable

void setStructVal (void)   {

    const Foo FOO_ASSIGN_VAL = {    //this goes into .rodata
            .bunch       = 1,
            .of          = 2,
            .things      = 3,
            .initialized = 4
    };

    memcpy((void*) &FOO_ASSIGN_VAL, (void*) foo, sizeof(Foo));

    return;
}

Result:

• (.rodata) FOO_ASSIGN_VAL is stored in .rodata
• (.text) a sequence of *movl FOO_ASSIGN_VAL, %registers* occur
• (.text) a sequence of movl %registers, foo occur

Example:

• Say Foo was a 48 field struct of uint8_t values. It is aligned in memory.

• (IDEAL) On a 32-bit machine, this COULD be as quick as 12 MOVL instructions of immediates out to foo's address space. For me this is 12*10 == 120bytes of .text in size.

• (ACTUAL) However, using the answer by AUTO will likely generate 48 MOVB instructions in .text. For me this is 48*7 == 336bytes of .text!!

• (SMALLEST*) Use the memcpy version above. IF alignment is taken care of,

  • FOO_ASSIGN_VAL is placed in .rodata (48 bytes),
  • 12 MOVL into %register
  • 12 MOVL outof %registers are used in .text (24*10) == 240bytes.
  • For me then this is a total of 288 bytes.

So, for me at least with my i386 code,

- Ideal:    120 bytes
- Direct:   336 bytes
- Smallest: 288 bytes

*Smallest here means 'smallest footprint I know of'. It also executes faster than the above methods (24 instructions vs 48). Of course, the IDEAL version is fastest & smallest, but I still can't figure that out.

-Justin

*Does anyone know how to get implementation of 'IDEAL' above? It is annoying the hell out of me!!