Comparison instructions perform an arithmetic operation for the purpose of guiding the conditional execution of a program. Table 1 lists the basic instructions associated with conditional control.

The cmp instruction compares the value of two operands, O1 and O2. Specifically, it subtracts O2 from O1. The tst instruction performs bitwise AND. It is common to see an instruction like:

In this example, the bitwise AND of x0 with itself is zero only when x0 contains zero. In other words, this is a test for a zero value and is equivalent to:

Unlike the arithmetic instructions covered thus far, cmp and tst do not modify a destination register. Instead, both instructions modify a series of single-bit values known as condition code flags. For example, cmp will modify condition code flags based on whether the value O1 - O2 results in a positive (greater), negative (less), or zero (equal) value. Recall that condition code values encode information about an operation in the ALU. The condition code flags are part of the ARM processor state (PSTATE), which replaces the current program status register (CPSR) from ARMv7-A systems.

Table 2 depicts the common flags used for condition code operations. Revisiting the cmp O1, O2 instruction:

While an in-depth discussion of condition code flags is beyond the scope of this book, the setting of these registers by cmp and tst enables the next set of instructions we cover (the branch instructions) to operate correctly.

A branch instruction enables a program’s execution to "jump" to a new position in the code. In the assembly programs we have traced through thus far, pc always points to the next instruction in program memory. The branch instructions enable pc to be set to either a new instruction not yet seen (as in the case of an if statement) or to a previously executed instruction (as in the case of a loop).

Table 3 lists the set of common branch instructions; L refers to a symbolic label, which serves as an identifier in the program’s object file. All labels consist of some letters and digits followed by a colon. Labels can be local or global to an object file’s scope. Function labels tend to be global and usually consist of the function name and a colon. For example, main: (or <main>:) is used to label a user-defined main function. In contrast, labels whose scope are local are preceded by a period. For example, .L1: is a label one may encounter in the context of an if statement or loop.

All labels have an associated address (addr in Table 3). When the CPU executes a b instruction, it sets the pc register to addr. The b instruction enables the program counter to change within 128 MB of its current location; a programmer writing assembly can also specify a particular address to branch to by using the br instruction. Unlike the b instruction, there are no restrictions on the address range of br.

Sometimes, local labels also are shown as an offset from the start of a function. Therefore, an instruction whose address is 28 bytes away from the start of main may be represented with the label <main+28>. For example, the instruction b 0x7d0 <main+28> indicates a branch to address 0x7d0, which has the associated label <main+28>, meaning that it is 28 bytes away from the starting address of the main function. Executing this instruction sets pc to 0x7d0.

The last three instructions are conditional branch instructions. In other words, the program counter register is set to addr only if the given condition evaluates to true. The cbz and cbnz instructions require a register in addition to an address. In the case of cbz, if R is zero, the branch is taken and pc is set to addr. In the case of cbnz, if R is nonzero, the branch is taken and pc is set to addr.

The most powerful of the conditional branch instructions are the b.c instructions, which enable the compiler or assembly writer to pick a custom suffix that indicates the condition on which a branch is taken.

Table 4 lists the set of common conditional branch suffixes (c). When used in conjunction with a branch, each instruction starts with the letter b and a dot, denoting that it is a branch instruction. The suffix of each instruction (c) indicates the condition for the branch. The branch instruction suffixes also determine whether to interpret numerical comparisons as signed or unsigned. Note that conditional branch instructions have a much more limited range (1 MB) than the b instruction. These suffixes are also used for the conditional select instruction (csel), which is covered in the next section.

In the following subsections, we look at conditionals and loops in assembly and reverse engineer them back to C. When translating assembly code of conditionals and loops back into C, it is useful to understand their corresponding C language goto forms. The goto statement is a C primitive that forces program execution to switch to another line in the code. The assembly instruction associated with the goto statement is b.

The goto statement consists of the goto keyword followed by a goto label, a type of program label that indicates that execution should continue at the corresponding label. So, goto done means that the program execution should branch to the line marked by label done. Other examples of program labels in C include the switch statement labels previously covered in Chapter 2.

The following code listings depict a function getSmallest written in regular C code (left) and its associated goto form in C (right). The getSmallest function compares the value of two integers (x and y), and assigns the smaller value to variable smallest.

The goto form of this function may seem counterintuitive, but let’s discuss what exactly is going on. The conditional checks to see whether variable x is less than or equal to y.

While goto statements were commonly used in the early days of programming, their use in modern code is considered bad practice, because it reduces the overall readability of code. In fact, computer scientist Edsger Dijkstra wrote a famous paper lambasting the use of goto statements called Go To Statement Considered Harmful¹.

In general, well-designed C programs do not use goto statements and programmers are discouraged from using it to avoid writing code that is difficult to read, debug, and maintain. However, the C goto statement is important to understand, as GCC typically changes C code with conditionals into a goto form prior to translating it to assembly, including code that contains if statements and loops.

The following subsections cover the assembly representation of if statements and loops in greater detail.