Computed gotos on Reddit

[bearophile]

> Do you have a proof? (Show me asm code)

Just to be more clear, what I mean is that given a D program 
equivalent to the C code shown in the article:


int interp_cgoto(unsigned char* code, int initval) {
     static const void* dispatch_table[] = {
         &&do_halt, &&do_inc, &&do_dec, &&do_mul2,
         &&do_div2, &&do_add7, &&do_neg};

     #define DISPATCH() goto *dispatch_table[code[pc++]]

     int pc = 0;
     int val = initval;

     DISPATCH();

     do_halt:
         return val;
     do_inc:
         val++;
         DISPATCH();
     do_dec:
         val--;
         DISPATCH();
     do_mul2:
         val *= 2;
         DISPATCH();
     do_div2:
         val /= 2;
         DISPATCH();
     do_add7:
         val += 7;
         DISPATCH();
     do_neg:
         val = -val;
         DISPATCH();
}

int main() {return 0;}


Is a 32 bit D compiler producing asm (with performance) similar 
to:


_interp_cgoto:
     movl    4(%esp), %edx
     movzbl  (%edx), %eax
     addl    $1, %edx
     movl    _dispatch_table.1363(,%eax,4), %ecx
     movl    8(%esp), %eax
     jmp *%ecx
     .p2align 4,,7
L3:
     rep
     ret
     .p2align 4,,7
L4:
     movzbl  (%edx), %ecx
     addl    $1, %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     .p2align 4,,7
L5:
     addl    $1, %edx
     jmp *%ecx
     .p2align 4,,7
L6:
     movzbl  (%edx), %ecx
     subl    $1, %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     addl    $1, %edx
     jmp *%ecx
     .p2align 4,,7
L7:
     movzbl  (%edx), %ecx
     addl    %eax, %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     addl    $1, %edx
     jmp *%ecx
     .p2align 4,,7
L8:
     movl    %eax, %ecx
     shrl    $31, %ecx
     addl    %ecx, %eax
     movzbl  (%edx), %ecx
     sarl    %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     addl    $1, %edx
     jmp *%ecx
     .p2align 4,,7
L9:
     movzbl  (%edx), %ecx
     addl    $7, %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     addl    $1, %edx
     jmp *%ecx
     .p2align 4,,7
L10:
     movzbl  (%edx), %ecx
     negl    %eax
     movl    _dispatch_table.1363(,%ecx,4), %ecx
     addl    $1, %edx
     jmp *%ecx

     .section .rdata,"dr"
     .align 4
_dispatch_table.1363:
     .long   L3
     .long   L4
     .long   L6
     .long   L7
     .long   L8
     .long   L9
     .long   L10


Bye,
bearophile