Sometimes, we need to use embedded assembly in C/C++ code because certain operations or instructions aren't available through standard C functions. For example, when I was working on an FIR filter implementation for ARM, I needed to saturate the final result, but GCC didn’t provide a function like `ssat`. As a result, I had to embed assembly instructions directly into the C code.
### 1. Getting Started
The main challenge when embedding assembly in C is associating C variables with the operands of the assembly instructions. Fortunately, GCC provides mechanisms to handle this. Here's a simple example:
```c
asm("fsinx %1, %0" : "=f"(result) : "f"(angle));
```
In this example, the `fsinx` instruction requires two floating-point registers as operands. Since GCC doesn’t know what specific registers are needed for `fsinx`, it’s up to the programmer to specify this information using operand constraints.
The `"=f"` and `"f"` after the instruction indicate that these are floating-point register operands. The `"="` before the constraint indicates that this is an output operand, while without it, it’s an input operand. The variables inside the parentheses are linked to those registers.
This tells GCC how to translate the embedded assembly into actual machine instructions:
- `fsinx`: the assembly instruction.
- `%1, %0`: the operands used in the instruction.
- `"=f"(result)`: operand `%0` is a floating-point register associated with the variable `result` (output).
- `"f"(angle)`: operand `%1` is a floating-point register associated with the variable `angle` (input).
So, the embedded assembly would likely be translated into at least three instructions (in non-optimized code):
- Load the value of `angle` into register `%1`.
- Execute the `fsinx` instruction, using `%1` as the source and `%0` as the destination.
- Store the result from `%0` into the `result` variable.
Of course, this might change at higher optimization levels—like if `angle` is already in a floating-point register.
The `"="` symbol is crucial here: it tells GCC whether to load the variable into the register before execution or store the result back after execution.
### 2. Common Operand Constraints
GCC supports several operand constraints. Some commonly used ones include:
- `m`: memory operand.
- `r`: register operand.
- `i`: immediate integer.
- `f`: floating-point register.
- `F`: immediate floating point.
The general format of an embedded assembly statement is:
```c
asm("assembly instruction" : "=output constraint"(variable) : "input constraint"(variable));
```
The output operand must be an lvalue, which makes sense since it’s where the result will be stored.
### 3. Multiple Operands or No Output Operands
If an instruction has multiple input or output operands, you can separate them with commas:
```c
asm("add %0, %1, %2" : "=r"(sum) : "r"(a), "r"(b));
```
Here, `%0`, `%1`, and `%2` correspond to `sum`, `a`, and `b`, respectively.
When there are no output operands, you just omit the output part, resulting in two consecutive colons:
```c
asm("some instruction" : : "r"(a), "r"(b));
```
### 4. Read-Write Operands
Some instructions use the same register for both input and output. For example, in x86:
```c
asm("add %1, %0" : "+r"(a) : "r"(b));
```
This corresponds to `a = a + b`. In this case, the `"+"` before the constraint indicates that the operand is both read and written. You cannot use `"="` here, as it would tell GCC that the operand is only an output, not an input.
Another approach is to split the read-write operand into two parts:
```c
asm("add %2, %0" : "=r"(c) : "0"(a), "r"(b));
```
Here, `"0"` means that the second operand uses the same register as the first one (`a`). This allows you to associate different C variables with the same physical register.
### 5. Specifying Registers
Sometimes, you need to explicitly assign a variable to a specific register, especially for system calls or low-level operations:
```c
register int a asm("%eax") = 1;
register int b asm("%ebx") = 2;
asm("add %1, %0" : "+r"(a) : "r"(b));
```
Note that only during the execution of the assembly instruction will `a` be in `%eax` and `b` in `%ebx`. At other times, their storage locations may be unknown.
Also, be cautious about overwriting registers. For instance, if `b` is assigned the return value of a function, it might overwrite `%eax`, which could interfere with the initial assignment of `a`.
To avoid this, you can store the function result in a temporary variable first:
```c
int t = func();
register int a asm("%eax") = 1;
register int b asm("%ebx") = t;
asm("add %1, %0" : "+r"(a) : "r"(b));
```
### 6. Implicitly Modified Registers
Some assembly instructions modify registers that aren’t explicitly listed as operands. To inform GCC of this, you can list them after the input operands:
```c
asm volatile("movc3 %0,%1,%2"
: /* no outputs */
: "g"(from), "g"(to), "g"(count)
: "r0", "r1", "r2", "r3", "r4", "r5");
```
Here, `movc3` is a VAX instruction for moving characters. The registers `r0` through `r5` are implicitly modified, so they must be listed in the last section.
It’s important that the implicit registers do not overlap with the input/output registers. Also, if you explicitly use a register in your instruction, you must include it in the implicit list. Additionally, when writing explicit registers in the assembly code, you must use double percent signs (e.g., `%%eax`).
### 7. Volatile Keyword
The `volatile` keyword ensures that GCC does not optimize away the assembly code. For example:
```c
asm volatile("some_instruction");
```
If your assembly code performs side effects (like modifying hardware registers), you should always use `volatile`. Otherwise, GCC may remove or reorder the instructions, leading to unexpected behavior.
In summary, embedded assembly in C offers powerful control over low-level operations, but it requires careful handling of operand constraints, register assignments, and compiler optimizations. Understanding how to correctly map C variables to assembly operands is key to writing reliable and efficient code.
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