9.4. Conditional Control and Loops

This section covers assembly instructions for conditionals and loops. Recall that conditional statements enable coders to modify program execution based on the result of a conditional expression. The compiler translates conditionals into assembly instructions that modify the instruction pointer (pc) to point to an address that is not the next one in the program sequence.

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
Instruction Translation

cmp O1, O2

Compares O1 with O2 (Computes O1 - O2)

tst O1, O2

Computes O1 & O2

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:

tst x0, x0

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:

cmp x0, #0

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. Common condition code flags.
Flag Translation

Z

Is Equal to Zero (1: yes, 0: no)

N

Is Negative (1: yes, 0: no)

V

Signed overflow has occurred (1:yes, 0: no)

C

arithmetic carry/unsigned overflow has occurred (1: yes, 0:no)

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

  • The Z flag is set to 1 if O1 and O2 are equal.

  • The N flag is set to 1 if O1 is less than O2 (i.e. O1 - O2 results in a negative value).

  • The V flag is set to 1 if the operation O1 - O2 results in overflow (useful for signed comparisons).

  • The C flag is set to 1 if the operation O1 - O2 results in an arithmetic carry operation (useful for unsigned comparisons).

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

Preliminaries: The branch instructions

The branch instruction enables the 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).

Direct branch instructions

Table 3. Common branch Instructions
Instruction Description

b addr L

pc = addr

br A

pc = A

cbz R, addr L

if (` R == 0`), pc = addr (conditional branch)

cbnz R, addr L

if (R != 0), pc = addr (conditional branch)

b.c addr L

if (c), pc = addr (conditional branch).

Table 3 lists the set of common branch instructions. In the above table, 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). 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>, representing 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 non-zero, 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.

Conditional branch instruction suffixes

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 instruct (csel) which is covered in the next section.

Table 4. Conditional branch instruction suffixes. Synonyms are shown in parentheses.
Signed Comparison Unsigned Comparison Description

eq

eq

branch if equal (==) or branch if zero

ne

ne

branch if not equal (!=)

mi

mi

branch if minus (negative)

pl

pl

branch if non-negative (>= 0)

gt

hi

branch if greater than (higher) (>)

ge

cs (hs)

branch if greater than or equal (>=)

lt

lo (cc)

branch if less than (<)

le

ls

branch if less than or equal (<=)

Preliminaries: 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 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.

Table 5. Comparison of a C function and its associated goto form.
Regular C version Goto version
int getSmallest(int x, int y) {
    int smallest;
    if ( x > y ) { //if (conditional)
        smallest = y; //then statement
    }
    else {
        smallest = x; //else statement
    }
    return smallest;
}
int getSmallest(int x, int y) {
    int smallest;

    if (x <= y ) { //if (!conditional)
        goto else_statement;
    }
    smallest = y; //then statement
    goto done;

else_statement:
    smallest = x; //else statement

done:
    return smallest;
}

Table 5 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 counter-intuitive, but let’s discuss what exactly is going on. The conditional checks to see if 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 (or x).

  • If x is greater than y, then smallest is set to y. The program then executes the statement goto done, which transfers control to the done label, which returns the value of smallest (or 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 generally 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:

References

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