Inline Assembly

Why would we want to write Assembler within C?

• Outsmart gcc logic 

• Specific optimisations 

• Close to the hardware programming (drivers)

• Embed existing code fragments 

• Performance

Compilation Example

Breaking Down Function Execution

• Caller stores arguments in registers or memory

• Function call: caller transfers flow control to the callee

• Callee acquires/allocates memory for doing work

• Callee executes the function body

• Callee stores the result in ‘some’ register

• Callee deallocates memory

• Function return: callee returns control the caller

Inline Assembly

• GCC allows inline assembler 

• The compiler inserts assembler code in the code of its caller 

• This removes function call overheads (recall, function prolog)

• The key word asm or asm is used to include code: 

  • asm("assembly code"); 

• Assembly is architecture-specific

  • You ARM assembly will not be transcoded into x86

• You can also compile an assembly file into an obj file

Basic Inline Assembler

Extended Inline Assembler

The embedded assembler code must interact with C code 

• Parameters must be passed on to the assembler program

• Results must be passed on to C code

Input/Output Operands

Register Allocation

A compiler must assign variables into processor register

• Any two variable must not be assigned to the same register at any point

• Use Spilling to store variable values

• Coalescing will aim to optimize register allocation to reduce value copying

• Graph colouring problem and liveness analysis

Volatile

Used for instructions with processor side-effects

• Disable compiler optimizations, which might lead code block removal (compiling with flag -O1 or higher)

  • volatile asm("assemler"); 

  • __volatile__ asm(“assembler”);

Clobbered Register

• r0 after the third colon tells GCC that the value of r0 is to be modified inside "asm", so GCC won’t use this register to store any other value

• the list can also contain special arguments:

  • "cc": The instruction modifies the condition code flags (save psr)

  • "memory": The instruction accesses unknown memory addresses

Usage Example - MACRO

Usage Example - Interrupts