5.3 What You Need to Know to Write Linear Assembly

By using the C6000 profiling tools, you can identify the time-critical sections of your code that need to be rewritten as linear assembly. The source code that you write for the assembly optimizer is similar to assembly source code. However, linear assembly code does not need to be partitioned, scheduled, or register allocated. The intention is for you to let the assembly optimizer determine this information for you. When you are writing linear assembly code, you need to know about these items:

• Assembly optimizer directives

Your linear assembly file can be a combination of linear assembly code segments and regular assembly source. Use the assembly optimizer directives to differentiate the assembly optimizer code from the regular assembly code and to provide the assembly optimizer with additional information about your code. The assembly optimizer directives are described in Section 5.4.

• Options that affect what the assembly optimizer does

The compiler options in Table 5-1 affect the behavior of the assembly optimizer.

Table 5-1 Options That Affect the Assembly Optimizer

Option	Effect	See
--ap_extension	Changes the default extension for assembly optimizer source files	Section 3.3.10
--ap_file	Changes how assembly optimizer source files are identified	Section 3.3.8
--disable_software_pipelining	Turns off software pipelining	Section 4.6.1
--debug_software_pipeline	Generates verbose software pipelining information	Section 4.6.2
--interrupt_threshold=n	Specifies an interrupt threshold value	Section 3.12
--keep_asm	Keeps the assembly language (.asm) file	Section 3.3.2
--no_bad_aliases	Presumes no memory aliasing	Section 4.12.3
--opt_for_space=n	Controls code size on four levels (n=0, 1, 2, or 3)	Section 4.9
--opt_level=n	Increases level of optimization (n=0, 1, 2, or 3)	Section 4.1
--quiet	Suppresses progress messages	Section 3.3.2
--silicon_version=n	Select target version	Section 3.3.5
--skip_assembler	Compiles or assembly optimizes only (does not assemble)	Section 3.3.2
--speculate_loads=n	Allows speculative execution of loads with bounded address ranges	Section 4.6.3

• TMS320C6000 instructions

When you are writing your linear assembly, your code does not need to indicate the following:

• Pipeline latency
• Register usage
• Which unit is being used

As with other code generation tools, you might need to modify your linear assembly code until you are satisfied with its performance. When you do this, you will probably want to add more detail to your linear assembly. For example, you might want to partition or assign some registers.

NOTE

Do Not Use Scheduled Assembly Code as Source

The assembly optimizer assumes that the instructions in the input file are placed in the logical order in which you would like them to occur (that is, linear assembly code). Parallel instructions are illegal.

If the compiler cannot make your instructions linear (non-parallel), it produces an error message. The compiler assumes instructions occur in the order the instructions appear in the file. Scheduled code is illegal (even non-parallel scheduled code). Scheduled code may not be detected by the compiler but the resulting output may not be what you intended.

• Linear assembly source statement syntax

The linear assembly source programs consist of source statements that can contain assembly optimizer directives, assembly language instructions, and comments. See Section 5.3.1 for more information on the elements of a source statement.

• Specifying registers or register sides

Registers can be assigned explicitly to user symbols. Alternatively, symbols can be assigned to the A-side or B-side leaving the compiler to do the actual register allocation. See Section 5.3.2 for information on specifying registers.

• Specifying the functional unit

The functional unit specifier is optional in linear assembly code. Data path information is respected; unit information is ignored.

• Source comments

The assembly optimizer attaches the comments on instructions from the input linear assembly to the output file. It attaches the 2-tuple <x, y> to the comments to specify which iteration and cycle of the loop an instruction is on in the software pipeline. The zero-based number x represents the iteration the instruction is on during the first execution of the kernel. The zero-based number y represents the cycle the instruction is scheduled on within a single iteration of the loop. See Section 5.3.4, for an illustration of the use of source comments and the resulting assembly optimizer output.