5.7.7 Options for Controlling Optimization

The options tabulated below control compiler optimizations and are described in the sections that follow.

Use of an option is permitted and has an effect with an unlicensed compiler (Free edition) if that option is enabled by default when using optimization level 2 or lower. Any form of option that disables an optimization is available unconditionally.

Table 5-13. Optimization Options
Option	Edition	Controls
`-O0`	All	Do not optimize.
`-O` `-O1`	All	Optimization level 1.
`-O2`	All	Optimization level 2.
`-O3`	PRO only	Speed-orientated optimizations.
`-Os`	PRO only	Size-orientated optimizations.
`-falign-functions[=n]` `-f[no-]align-functions`	All	Alignment of the start of functions.
`-falign-labels[=n]` `-f[no-]align-labels`	All	Alignment of all branch targets.
`-falign-loops[=n]` `-f[no-]align-loops`	All	Alignment of loops.
`-f[no-]caller-saves`	All	Allocation into registers clobbered by function calls.
`-f[no-]cse-follow-jumps`	All	Customization of common subexpression elimination optimizations.
`-f[no-]cse-skip-blocks`	All	Customization of common subexpression elimination optimizations.
`-f[no-]data-sections`	All	Placement of objects into a section in the output file.
`-f[no-]defer-pop`	All	When arguments to function calls are popped.
`-f[no-]expensive-optimizations`	All	Minor optimizations that are relatively expensive.
`-f[no-]function-cse`	All	Placement of function addresses.
`-f[no-]function-sections`	All	Placement of functions intoa section in the output file.
`-f[no-]gcse`	All	The global common subexpression elimination pass.
`-f[no-]gcse-lm`	All	Customization of common subexpression elimination dealing with loads.
`-f[no-]gcse-sm`	All	Customization of common subexpression elimination dealing with stores.
`-f[no-]inline-functions`	All	Integratation of all simple functions into their callers.
`-f[no-]inline-limit=n`	All	The size limit for inlining functions.
`-f[no-]keep-inline-functions`	All	Output of a separate run-time-callable version of inlined functions.
`-f[no-]keep-static-consts`	All	Output of `static const` objects.
`-f[no-]lto`	PRO only	The standard link-time optimizer.
`-f[no-]omit-frame-pointer`	All	Reservation of the Frame Pointer in a register for functions that don't need one.
`-f[no-]optimize-register-move` `-f[no-]regmove`	All	Reassign of register numbers in move and other instructions.
`-f[no-]optimize-sibling-calls`	All	Optimization of sibling and tail recursive calls.
`-f[no-]peephole` `-f[no-]peephole2`	All	Machine-specific peephole optimizations.
`-f[no-]rename-registers`	All	Use of free registers to avoid false dependencies in scheduled code.
`-f[no-]rerun-cse-after-loop`	All	When common subexpression elimination optimizations should be performed.
`-f[no-]rerun-loop-opt`	All	How often to run the loop optimizer.
`-f[no-]schedule-insns` `-f[no-]schedule-insns2`	All	Reordering instructions to eliminate instruction stalls.
`-f[no-]strength-reduce`	All	Loop strength reduction and elimination of iteration variables.
`-f[no-]strict-aliasing`	All	Assumption of the strictest aliasing rules applicable to the language being compiled.
`-f[no-]thread-jumps`	All	Jump-to-branch optimizations.
`-f[no-]toplevel-reorder`	All	Reordering of top-level functions, variables, and `asm` statements.
`-f[no-]unroll-loops` `-f[no-]unroll-all-loops`	All	Loop unrolling.
`-f[no-]whole-program`	PRO only	More aggressive interprocedural optimizations.