More space efficient virtual function calls

[Matthew Wilcox]

On Sat, 6 Jul 2002 17:29:11 -0700, Wink Saville wrote:
> I was wondering if there was switch in gcc that would cause it to generate
> smaller code when invoking virtual functions. I'm using GCC 3.1 ARM cross
> compiler with -O3 and -fvtable-thunks=3, and it does the following when
> calling a virtual function with no parameters:
> 
>     LDR    R1,[R4]        /* Fetch the vtable pointer */
>     MOV    R0,R4        /* R0 = this */
>     MOV LR, PC          /* Set of the link register */
>     LDR PC,[R1, #4]  /* Call virtual function 1, the second entry */
> 
> This is good code but it takes 20 bytes, what I was hoping for when I
> enabled -fvtable-thunks=3 the compiler would use intermediate "thunks" to
> invoke functions such as:
> 
>     LDR    R0,R4    /* R0 = this */
>     BL    __VTFunc1     /* Branch and Link to virtual thunk function 1  */
> 
> ...
> 
> __VTFunc0:
>     MOV    R5,[R0]        /* Get vtable pointer */
>     LDR    PC,[R5,#0]    /* Jump to the function */
> __VTFunc1:
>     MOV    R5,[R0]        /* Get vtable pointer */
>     LDR    PC,[R5,#4]    /* Jump to the function */
> __VTFunc2:
>     MOV    R5,[R0]        /* Get vtable pointer */
>     LDR    PC,[R5,#8]    /* Jump to the function */

The current code takes 16 bytes, not 20.  The proposed code above is buggy;
I think what you meant was:

	MOV	r0, r4			/* r0 = this */
	BL	__VTFunc1

...

__VTFunc0:
	LDR	r5, [r0]		/* fetch vtable pointer */
	LDR	pc, [r5, #0]		/* jump to the function */
__VTFunc1:
	LDR	r5, [r0]
	LDR	pc, [r5, #4]
__VTFunc2:
	LDR	r5, [r0]
	LDR	pc, [r5, #8]

if you use a call-clobbered register instead of r5 (i don't remember
the apcs right now), you don't even need to reserve r5.

i have no idea how to change gcc to do what you want though ;-)

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