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7.4.1. Preliminaries

Conditional Comparison Instructions

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.

Table 1. Conditional Control Instructions

The cmp instruction compares the values of two registers, R2 and R1. Specifically, it subtracts R1 from R2. The test instruction performs bitwise AND. It is common to see an instruction like:

test %rax, %rax

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

cmp $0, %rax

Unlike the arithmetic instructions covered thus far, cmp and test do not modify the 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 R2 - R1 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 FLAGS register on x86 systems.

Table 2. Common Condition Code Flags.

Table 2 depicts the common flags used for condition code operations. Revisiting the cmp R1, R2 instruction:

• The ZF flag is set to 1 if R1 and R2 are equal.
• The SF flag is set to 1 if R2 is less than R1 (R2 - R1 results in a negative value).
• The OF flag is set to 1 if the operation R2 - R1 results in an integer overflow (useful for signed comparisons).
• The CF flag is set to 1 if the operation R2 - R1 results in a carry operation (useful for unsigned comparisons).

The SF and OF flags are used for comparison operations on signed integers, whereas the CF flag is used for comparisons on unsigned integers. Although an in-depth discussion of condition code flags is beyond the scope of this book, the setting of these registers by cmp and test enables the next set of instructions we cover (the jump instructions) to operate correctly.

The Jump Instructions

A jump 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, %rip always points to the next instruction in program memory. The jump instructions enable %rip 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).

Direct jump instructions

Table 3. Direct Jump Instructions

Table 3 lists the set of direct jump 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 local label one may encounter in the context of an if statement or loop.

All labels have an associated address. When the CPU executes a jmp instruction, it modifies %rip to reflect the program address specified by label L. A programmer writing assembly can also specify a particular address to jump to using the jmp * instruction. Sometimes, local labels 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 jmp 0x8048427 <main+28> indicates a jump to address 0x8048427, which has the associated label <main+28>, representing that it is 28 bytes away from the starting address of the main function. Executing this instruction sets %rip to 0x8048427.

Conditional jump instructions

The behavior of conditional jump instructions depends on the condition code registers set by the cmp instruction. Table 4 lists the set of common conditional jump instructions. Each instruction starts with the letter j denoting that it is a jump instruction. The suffix of each instruction indicates the condition for the jump. The jump instruction suffixes also determine whether to interpret numerical comparisons as signed or unsigned.

Table 4. Conditional Jump Instructions; Synonyms Shown in Parentheses

Instead of memorizing these different conditional jump instructions, it is more helpful to sound out the instruction suffixes. Table 5 lists the letters commonly found in jump instructions and their word correspondence.

Table 5. Jump Instruction Suffixes.

Sounding it out, we can see that jg corresponds to jump greater and that its signed synonym jnl stands for jump not less. Likewise, the unsigned version ja stands for jump above, whereas its synonym jnbe stands for jump not below or equal.

If you sound out the instructions, it helps to explain why certain synonyms correspond to particular instructions. The other thing to remember is that the terms greater and less instruct the CPU to interpret the numerical comparison as a signed value, whereas above and below indicate that the numerical comparison is unsigned.

The goto Statement

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 the 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 jmp.

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

Table 6. Comparison of a C function and its associated goto form.

Table 6 depicts a function getSmallest() written in regular C code and its associated goto form in C. 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.

• If x is less than or equal to y, the program transfers control to the label marked by else_statement, which contains the single statement smallest = x. Since the program executes linearly, the program continues on to execute the code under the label done, which returns the value of smallest (x).
• If x is greater than y, smallest is assigned the value y. The program then executes the statement goto done, which transfers control to the done label, which returns the value of smallest (y).

While goto statements were commonly used in the early days of programming, the use of goto statements in modern code is considered bad practice, as 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 Harmful_1.

In general, well-designed C programs do not use goto statements and programmers are discouraged from using them 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.

• If Statements
• Loops

References

1. Edsger Dijkstra. "Go To Statement Considered Harmful". Communications of the ACM 11(3) pp. 147—​148. 1968.