[Example] Conditionals in Assembly

Conditionals1

(version 1, compare at each branch similar to what you see in Java)

// Program to show how to use if/else-if/else inside a loop
// Loops 10 times: say if the current loop# is less than, greater than, or exactly 5.

// Define format string for call to printf()
less: .string "Loop #%d: %d is less than 5.\n"
greater:.string "Loop #%d: %d is greater than 5.\n"
equal: .string "Loop #%d: Exactly 5! (Ya I know this program's stupid too.)\n"

// Define the main function for our program
  .balign 4                 // Instructions must be word aligned
  .global main               // Make "main" visible to the OS
main: stp x29, x30, [sp, -16]!     // Save frame pointer (fp, x29) and link register (lr, x30) to stack, allocating 16 bytes, pre-increment SP
  mov x29, sp               // Update frame pointer (fp) to current stack pointer (sp) (after we've incremented sp in the last step)

  mov x19, 1             // Set x19 general purpose register to 0, this will be the loop counter

  // While loop (pre-test, so check before the first iteration)
  // Let's loop 10 times, printing out each number.
test: cmp x19, 10               // Compare loop counter and 10
  b.gt done                 // If [loop counter]>10, exit loop and branch to "done"

// Start of code inside the loop

  // If loop# < 5
  cmp x19, 5
  b.ge elseif             // If loop# >=5, then condition is NOT satifsied, move to next if (elseif)

  adrp x0, less             // Set the 1st argument of printf(fmt, var1, var2...) (high-order bits)
  add x0, x0, :lo12:less       // Set the 1st argument of printf(fmt, var1, var2...) (lower 12 bits)
  mov x1, x19             // Set the 2nd argument of printf().
                        // We are adding one, then saving it to x1 register as 2nd argument
                         // This is just to print 1-10, instead of 0-9
  mov x2, x1               // Set the 3rd argument of printf().
  b next                 // Since

// Else-if loop# > 5
elseif: cmp x19, 5
b.le else // If loop# <=5, then condition is NOT satifised, move to next part (else)

  adrp x0, greater
  add x0, x0, :lo12:greater
  mov x1, x19
  mov x2, x1

b next

  // Else (loop# == 5)
else: adrp x0, equal             // Else catches everything else, if it reaches here then just proceed, no need to compare
  add x0, x0, :lo12:equal
  mov x1, x19

// In the conditionals we set the arguments for printf(), so let's call it now.
next: bl printf // Call the printf() function

// End of code inside the loop

add x19, x19, 1 // Increment loop counter by 1
b test

// Return 0 in main, like we did in C
done: mov w0, 0

  // Restore registers and return to calling code (OS)
  ldp x29, x30, [sp], 16     // Restore fp and lr from stack, post-increment sp
  ret                 // Return to caller

Conditionals2

(version 2, compare once, then either fall through to first case or branch to second/third cases. There are three cases because two operands are either >, <, or =.)

// Program to show how to use if/else-if/else inside a loop
// Loops 10 times: say if the current loop# is less than, greater than, or exactly 5.

  // Define the main function for our program
  .balign 4                 // Instructions must be word aligned
  .global main             // Make "main" visible to the OS
main: stp x29, x30, [sp, -16]!     // Save frame pointer (fp, x29) and link register (lr, x30) to stack, allocating 16 bytes, pre-increment SP
  mov x29, sp             // Update frame pointer (fp) to current stack pointer (sp) (after we've incremented sp in the last step)

// Start of code inside the loop

  // If loop# < 5
  cmp x19, 5
  b.gt elseif             // If loop# > 5, then condition is NOT satifsied, move to elseif (loop# >5)
  b.eq else             // If loop# == 5, then condition is NOT satifsied, move to else (loop# == 5)

  adrp x0, less             // Set the 1st argument of printf(fmt, var1, var2...) (high-order bits)
  add x0, x0, :lo12:less     // Set the 1st argument of printf(fmt, var1, var2...) (lower 12 bits)
  mov x1, x19               // Set the 2nd argument of printf().
                        // We are adding one, then saving it to x1 register as 2nd argument
                        // This is just to print 1-10, instead of 0-9
  mov x2, x1               // Set the 3rd argument of printf().
  b next                   // Since

  // Else-if loop# > 5
elseif: adrp x0, greater
  add x0, x0, :lo12:greater
  mov x1, x19
  mov x2, x1

b next

  // Else (loop# == 5)
else: adrp x0, equal             // Else catches everything else, if it reaches here then just proceed, no need to compare
  add x0, x0, :lo12:equal
  mov x1, x19

// In the conditionals we set the arguments for printf(), so let's call it now.
next: bl printf // Call the printf() function

  // End of code inside the loop
  add x19, x19, 1           // Increment loop counter by 1
  b test

// Return 0 in main, like we did in C
done: mov w0, 0

  // Restore registers and return to calling code (OS)
  ldp x29, x30, [sp], 16         // Restore fp and lr from stack, post-increment sp
  ret                       // Return to caller