Re: Removing bound checks from a switch statement

["Henrik Mannerström" <henrik.mannerstrom@helsinki.fi>]

Hello,

Below is a short program that illustrates my problem. The variable R  
is of type enum reactions so the compiler should know that it does not  
need to check it's value in the switch statement on line 10. The  
assembler output shows that a check is nevertheless inserted. %ebx  
contains the value or R and .L9 the address of the jump table.  
Compiled with g++ -S -g -dA -O2 swloop.cc on a X86_64 machine.

Should not the type system ensure that R is within bounds?

Regards,
Henrik

<from the asm-output>
# swloop.cc:10
.loc 1 10 0
cmpl	$5, %ebx
ja	.L2
# basic block 4
mov	%ebx, %eax
jmp	*.L9(,%rax,8)
</from the asm-output>

enum reactions {R0 = 0, R1, R2, R3, R4, R5};

#include <cstdio>

int main() {
   const enum reactions next_reaction[] = {R1, R2, R3, R4, R5};
   enum reactions R = R0;

   while (R != R5) {
     switch (R) { /* This is line 10 */
     case R0:
       printf("R0\n");
       break;
     case R1:
       printf("R1\n");
       break;
     case R2:
       printf("R2\n");
       break;
     case R3:
       printf("R3\n");
       break;
     case R4:
       printf("R4\n");
       break;
     case R5:
       printf("R5\n");
       break;
     }
     R = next_reaction[R];
   }
}