Inline Assembly

One of the most notable features provided by armcc is the ability to write inline assembly code directly within C functions. This feature allows developers to leverage the full power of the underlying ARM architecture by directly accessing assembly instructions and registers. By using inline assembly, performance-critical sections of code can be finely optimized to achieve maximum efficiency.

To illustrate the usage of inline assembly, consider the following example where we want to count the number of leading zeros in a 32-bit integer using the CLZ (Count Leading Zeros) instruction:

int count_leading_zeros(uint32_t value)
{
    int leading_zeros;
    __asm
    {
        CLZ leading_zeros, value
    }
    return leading_zeros;
}

In this example, the __asm keyword is used to indicate the start of inline assembly code. The CLZ instruction is then used to perform the leading zero count and store the result in the leading_zeros variable. Finally, the function returns the computed number of leading zeros.

Compiler Hints

Another useful extension provided by armcc is the ability to use compiler hints to provide additional information to the compiler about code optimization. These hints can help the compiler in making informed decisions during the optimization process, leading to improved performance or reduced code size.

One common use case for compiler hints is to provide alignment information for variables or data structures. This can be achieved using the __align attribute as follows:

struct __align(4) aligned_struct
{
    int value1;
    float value2;
};

In this example, the __align(4) attribute is used to specify that the aligned_struct should be 4-byte aligned. This can help the compiler in generating more efficient code when accessing members of the structure, as it eliminates potential alignment-related penalties.

Conclusion

ARM C Compiler-specific extensions provide a powerful set of tools for optimizing code targeting ARM processors. The use of inline assembly allows direct access to assembly instructions and registers, enabling fine-grained performance optimizations. Compiler hints, on the other hand, assist the compiler in making better decisions during the optimization process. By utilizing these extensions, developers can write highly optimized code for ARM platforms and achieve better performance or reduced code size.

#ARM #CompilerExtensions