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Chapter 7 Linear Assembly

Chapter 7 Linear Assembly. Learning Objectives. Comparison of programming techniques. How to write Linear Assembly. Interfacing Linear Assembly with C. Assembly optimiser tool. Introduction. With the assembly optimiser, optimisation for loops can be made very simple.

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Chapter 7 Linear Assembly

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  1. Chapter 7 Linear Assembly

  2. Learning Objectives • Comparison of programming techniques. • How to write Linear Assembly. • Interfacing Linear Assembly with C. • Assembly optimiser tool.

  3. Introduction • With the assembly optimiser, optimisation for loops can be made very simple. • Linear assembly takes care of the pipeline structure and generates highly parallel assembly code automatically. • The performance of the assembly optimiser can easily reach the performance of hand written assembly code.

  4. Comparison of Programming Techniques C C ++ Optimising Compiler 80 - 100% Low LinearASM Assembly Optimiser 95 - 100% Med ASM Hand Optimised 100% High Source Efficiency* Effort * Typical efficiency vs. hand optimized assembly.

  5. Writing in Linear Assembly ZERO sum loop LDH *p_to_a, a LDH *p_to_b, b MPY a, b, prod ADD sum, prod, sum SUB B0, 1, B0 B loop • Linear assembly is similar to hand assembly, except: • Does not require NOPs to fill empty delay slots. • The functions units do not need to be specified. • Grouping of instructions in parallel is performed automatically. • Accepts symbolic variable names.

  6. How to Write Code in Linear Assembly .cproc defines the beginning of the code _sa_Function .cproc ZERO sum loop LDH *pm++, m LDH *pn++, n MPY m, n, prod ADD sum, prod, sum [count] SUB count, 1, count B loop .return sum .endproc .return specifies the return value .endproc defines the end of the linear assembly code • File extension: • Use the “.sa” extension to specify the file is written in linear assembly. • How to write code: NO NOPs required NO parallel instructions required NO functional units specified NO registers required

  7. Passing and Returning Arguments int y = dotp (short* a, short* x, int count) _dotp .cproc pm, pn, count ... .return y .endproc • “pm” and “pn” are two pointers declared in the C code that calls the linear assembly function. • The following function prototype in C calls the linear assembly function: • The linear assembly function receives the arguments using .cproc:

  8. Declaring the Symbolic Variables .reg pm, pn, m, n, prod, sum • All the symbolic registers except those used as arguments are declared as follows: • The assembly optimiser will attempt to assign all these values to registers.

  9. Complete Linear Assembly Code _dotp .cproc pm, pn, count .reg m, n, prod, sum ZERO sum loop LDH *pm++, m LDH *pn++, n MPY m, n, prod ADD sum, prod, sum [count] SUB count, 1, count B loop .return sum .endproc • Note: Linear assembly performs automatic return to the calling function.

  10. Function calls in Linear Assembly Function1.sa _function1 .cproc a, b .reg y, float1 MPY a,b,y .call float1 = _fix_to_float(y) .return .endproc Fix_to_float.sa _fix_to_float .cproc fix .reg float1 INTSP fix, float1 .return float1 .endproc • In linear assembly you can call other functions written in C, linear assembly or assembly. • To do this the .call directive is used: • Note: Branch out of a linear assembly routine is not allowed.

  11. Invoking the Assembly Optimiser • The development tools recognise the linear assembler code by the file extension “.sa”. • The assembly optimiser uses the same options as the optimising C compiler. • Note: In CCS you can change the options of each file individually by right clicking on the file in the project view and selecting “File Specific Options…”.

  12. Linear Assembly Examples • The following chapters have code written in linear assembly: • \Code\Chapter 15 - Infinite Impulse Response Filters • \Code\Chapter 17 - Goertzel Algorithm • For more details on Interfacing C and Assembly see Chapter 11.

  13. Chapter 7 Linear Assembly - End -

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