Pro Android Apps Performance Optimization

Data Types

You already got an aperçu of the first point, how the CPU can manipulate a certain data type, in Chapter 2 with the native code version of computeIterativelyFaster(), which used two add instructions to add two 64–bit integers, as shown in Listing 4–1.

Listing 4–1. Adding Two 64–bit Integers

 448:    e0944002         adds    r4, r4, r2  44c:    e0a55003         adc     r5, r5, r3

Because the ARM registers are 32-bit wide, two instructions are needed to add two 64–bit integers; the lowest 32-bit integers are stored in one register (r4), and the highest 32-bit are stored in another register (r5). Adding two 32-bit values would require a single instruction.

Let's now consider the trivial C function, shown in Listing 4–2, which simply ...