Optimization

Optimization

In XGC compilers, the general strategy is to general compact code. Usually this results in the fast code too. But note that at optimization level 3, many functions will be in-lined and the code size will increase. Optimization level 2 is a good project default.

The optimizations performed include the following:

	Automatic register allocation
	Common sub-expression elimination
	Invariant code motion from loops
	Induction variable optimizations
	Constant propagation and copy propagation
	Delayed popping of function call arguments
	Tail recursion elimination
	Integration of in-line functions & frame pointer elimination
	Instruction scheduling
	Loop unrolling
	Leaf function optimization
	Optimized multiplication by constants
	Peephole

The compiler uses a pattern matching technique to select the best instruction for each of hundreds of different cases. For example in the 1750 code generator, setting the value of a one-bit bit field to a known value will nearly always use the set bit or reset bit instructions.

Usually optimization makes the generated code easier to read. We recommend that projects adopt the optimization default of level2.

Updated May 11, 2005