9.9. Structs in Assembly

A struct is another way to create a collection of data types in C. Unlike arrays, structs enable different data types to be grouped together. C stores structs like single dimension arrays, where data elements (fields) are stored contiguously.

Let’s revisit the studentT struct from Chapter 1:

struct studentT {
    char name[64];
    int  age;
    int  grad_yr;
    float gpa;
};

struct studentT student;

Figure 1 shows how the student struct is laid out in memory. Each ai denotes an offset in memory.

structArray
Figure 1. The memory layout of a struct studentT.

Each field is stored contiguously next to each other in memory in the order in which they are declared. In [structArray6] the age field is allocated at the memory location directly after the name field (at byte offset a64), and is followed by the grad_yr (byte offset a68) and gpa (byte offset a72) fields. This organization enables memory-efficient access to the fields of the struct.

To understand how the compiler generates assembly code to work with structs, consider the function initStudent():

void initStudent(struct studentT *s, char *nm, int ag, int gr, float g) {
    strcpy(s->name, nm);
    s->grad_yr = gr;
    s->age = ag;
    s->gpa = g;
}

The initStudent() function uses the base address of a studentT struct as its first parameter, and desired values for each field as its remaining parameters. The listing below depicts this function in assembly.

Dump of assembler code for function initStudent:
0x7f4 <+0>:   stp  x29, x30, [sp, #-48]! // sp-=48; store x29 and x30 at sp and sp+4
0x7f8 <+4>:   mov  x29, sp               // x29 = sp (frame pointer = stack pointer)
0x7fc <+8>:   str  x0, [x29, #40]        // store s at x29 + 40
0x800 <+12>:  str  x1, [x29, #32]        // store nm at x29 + 32
0x804 <+16>:  str  w2, [x29, #28]        // store ag at x29 + 28
0x808 <+20>:  str  w3, [x29, #24]        // store gr at x29 + 24
0x80c <+24>:  str  s0, [x29, #20]        // store g at x29 + 20
0x810 <+28>:  ldr  x0, [x29, #40]        // x0 = s
0x814 <+32>:  ldr  x1, [x29, #32]        // x1 = nm
0x818 <+36>:  bl   0x6e0 <strcpy@plt>    // call strcpy(s, nm) (s->name)
0x81c <+40>:  ldr  x0, [x29, #40]        // x0 = s
0x820 <+44>:  ldr  w1, [x29, #24]        // w1 = gr
0x824 <+48>:  str  w1, [x0, #68]         // store gr at location (s + 68) (s->grad_yr)
0x828 <+52>:  ldr  x0, [x29, #40]        // x0 = s
0x82c <+56>:  ldr  w1, [x29, #28]        // w1 = ag
0x830 <+60>:  str  w1, [x0, #64]         // store ag at location (s + 64) (s->age)
0x834 <+64>:  ldr  x0, [x29, #40]        // x0 = s
0x838 <+68>:  ldr  s0, [x29, #20]        // s0 = g
0x83c <+72>:  str  s0, [x0, #72]         // store g at location (s + 72) (s->gpa)
0x844 <+80>:  ldp  x29, x30, [sp], #48   // x29 = sp, x30 = sp+4, sp += 48
0x848 <+84>:  ret                        // return (void)

Being mindful of the byte offsets of each field is key to understanding this code. A few things to keep in mind:

  • the strcpy() call takes the base address of the name field of struct s and the address of array nm as its two arguments. Recall that since name is the first field in the studentT struct, the address of s is synonymous with the address of s→name.

0x7fc <+8>:   str  x0, [x29, #40]        // store s at x29 + 40
0x800 <+12>:  str  x1, [x29, #32]        // store nm at x29 + 32
0x804 <+16>:  str  w2, [x29, #28]        // store ag at x29 + 28
0x808 <+20>:  str  w3, [x29, #24]        // store gr at x29 + 24
0x80c <+24>:  str  s0, [x29, #20]        // store g at x29 + 20
0x810 <+28>:  ldr  x0, [x29, #40]        // x0 = s
0x814 <+32>:  ldr  x1, [x29, #32]        // x1 = nm
0x818 <+36>:  bl   0x6e0 <strcpy@plt>    // call strcpy(s, nm) (s->name)

  • The above code snippet contains an undiscussed register (s0). The s0 register is an example of a register reserved for floating point values.

  • The next part of the code (instructions <initStudent+40> thru <initStudent+48>) places the value of the gr parameter at an offset of 68 from the start of s. Revisiting the memory layout of the struct in [structArray6] shows that this address corresponds to s→grad_yr.

0x81c <+40>:  ldr  x0, [x29, #40]        // x0 = s
0x820 <+44>:  ldr  w1, [x29, #24]        // w1 = gr
0x824 <+48>:  str  w1, [x0, #68]         // store gr at location (s + 68) (s->grad_yr)

  • The next section of code (instructions <initStudent+52> thru <initStudent+60>) copies the ag parameter to the s→age field of the struct, which is located at an offset of 64 bytes from the address of s.

0x828 <+52>:  ldr  x0, [x29, #40]        // x0 = s
0x82c <+56>:  ldr  w1, [x29, #28]        // w1 = ag
0x830 <+60>:  str  w1, [x0, #64]         // store ag at location (s + 64) (s->age)

Lastly, the g parameter value is copied to the s→gpa field (byte offset 72) of the struct. Notice the use of the s0 register since the data contained at location x29 + 20 is single precision floating point:

0x834 <+64>:  ldr  x0, [x29, #40]        // x0 = s
0x838 <+68>:  ldr  s0, [x29, #20]        // s0 = g
0x83c <+72>:  str  s0, [x0, #72]         // store g at location (s + 72) (s->gpa)

9.9.1. Data Alignment and Structs

Consider the following modified declaration of our studentT struct:

struct studentTM {
    char name[63]; //updated to 63 instead of 64
    int  age;
    int  grad_yr;
    float gpa;
};

struct studentTM student2;

The size of the name field of the struct is modified to be 63 bytes, instead of the original 64 bytes. Consider how this affects the way the struct is laid out in memory. It may be tempting to visualize it like so:

struct2wrong
Figure 2. An incorrect memory layout for the updated struct studentTM. Note that the struct’s "name" field is reduced from 64 to 63 bytes.

In this depiction, the age field occurs in the byte immediately following the name field. But this is incorrect. Figure 3 depicts the actual layout of the struct in memory:

struct2right
Figure 3. The correct memory layout for the updated struct studentTM. Byte a63 is added by the compiler to satisfy memory alignment constraints, but it doesn’t correspond to any of the struct’s fields.

A64’s alignment policy requires that 4-byte data types (i.e, int) reside at addresses that are a multiple of 4, while 64-bit data types (long, double and pointer data) to reside at addresses that are a multiple of 8. For structs, the compiler adds empty bytes as "padding" between fields to ensure that each field satisfies its alignment requirements. For example, in the struct declared above, the compiler adds a byte of padding at byte a63 to ensure that the age field starts at an address that is at a multiple of 4. Values aligned properly in memory can be read or written in a single operation, enabling greater efficiency.

Consider what happens when the struct is defined as the following:

struct studentTM {
    int  age;
    int  grad_yr;
    float gpa;
    char name[63];
};

struct studentTM student3;

Moving the name array to the end of the struct ensures that age, grad_yr and gpa are 4-byte aligned. Most compilers will remove the filler byte at the end of the struct. However, if the struct is ever used in the context of an array (e.g., struct studentTM courseSection[20];) the compiler will once again add the filler byte as padding between each struct element in the array to ensure that alignment requirements are properly met.