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Zoltán Kócsi wrote:
> I have various constants. If I define them in a header file like this:
>
> static const int my_thing_a = 3;
> static const int my_thing_b = 12345;
>
> then everything is nice, if I use them the compiler knows their value
> and uses them as literals and it doesn't actually put them into the
> .rodata section (which is important because I have a lot of them and
> code space is at premium).
>
> Now these things are very closely related, so it would make the program
> much clearer is they could be collected in a structure. That is:
>
> struct things { int a; int b; };
>
> and then I could define a global structure
>
> const struct things my_things = { 3, 12345 };
>
> so that I can refer them as my_things.a or my_things.b;
>
> The problem is that I do not want to instantiate the actual "things"
> structure, for the same reason I did not want to instantiate the
> individual const int definitions. So, I tried the GCC extension of
> "compound literals" like this:
>
> #define my_things ((struct things) { 3, 12345 })
>
> int func( int x )
> {
> if ( x )
> return my_things.a;
> else
> return my_things.b;
> }
>
> If I compile the above with -O2 or -Os, then if the target is AVR or
> x86_64 then the result is what I expected, func() just loads 3 or 12345
> then returns and that's all. There is no .rodata generated.
>
> However, compiling for the ARM generates the same function code, but it
> also generates the image of "things" in the .rodata segment. Twice. Even
> when it stores 12345 separatelly. The code never actually references
> any of them and they are not global thus it is just wasted memory:
Works for me, arm-linux-gnueabi-gcc -S t.c -O2 generates:
.file "t.c"
.text
.align 2
.global func
.type func, %function
func:
@ args = 0, pretend = 0, frame = 0
@ frame_needed = 0, uses_anonymous_args = 0
@ link register save eliminated.
ldr r3, .L5
cmp r0, #0
moveq r0, r3
movne r0, #3
bx lr
.L6:
.align 2
.L5:
.word 12345
arm-linux-gnueabi-gcc (GCC) 4.4.0 20080618
Andrew.
