How to compute switch case values from assembly code?

Question

ATT syntax.

I'm trying to understand a practice problem we talked about in class.

We were given the following partial assembly code for a switch statement:

movl  8(%ebp), %eax
addl  $2, %eax
cmpl  $6, %eax
ja    .L2
jmp   *.L8(,%eax,4)

//rest of switch statement would go here

.L8
 .long   .L3
 .long   .L2
 .long   .L4
 .long   .L5
 .long   .L6
 .long   .L6
 .long   .L7

I know that the second portion is the jump table. What I can't figure out is how to calculate the original C-level switch case values. How can I do this?

Answer 1

Analyze the code.

The function's parameter + 2 is used as an index into the table. So, the constants start at -2 and go on to -1, 0, etc.

The maximum constant is 4, see there's a check for parameter + 2 > 6, or, equivalently, for parameter > 4. The number of elements in the table reflects that too.