The xlc utility optimization option levels

You can use the xlc utility options to specify five base optimization levels, which map to the z/OS® batch options as follows:

-O0 or NOOPT, almost no optimization, best for getting the most debugging information
-O2 or OPTIMIZE(2), strong low-level optimization that benefits most programs
-O3 or OPTIMIZE(3), intense low-level optimization analysis
-O4 or OPTIMIZE(3), HOT, IPA(LEVEL(1)), all of -O3 plus detailed loop analysis and basic whole-program analysis at link time
-O5 or OPTIMIZE(3), HOT, IPA(LEVEL(2)), all of -O4 and detailed whole-program analysis at link time

Note: -O1 level is not supported.

Optimization progression

Table 1 details options you should use with each level and some useful additional options.

Table 1. Optimization levels and options
The xlc utility optimization option level	Additional batch options implied by optimization level	Additional recommended batch options
-O0	None	ARCH(n)
-O2	None	ARCH (n) INLINE (to tune inlining) TUNE(n)
-O3	NOSTRICT	ARCH(n) TUNE(n)
-O4	All of OPTIMIZE(3) plus: HOT IPA(LEVEL(1))	ARCH(n) TUNE(n) PDF
-O5	All of OPTIMIZE(3) plus: HOT IPA(LEVEL(2))	ARCH(n) TUNE(n) PDF

While Table 1 provides a list of the most common compiler options for optimization, the z/OS XL C/C++ compiler offers optimization facilities for almost any application. For more information, see Additional options that affect performance.