This switch enables optimization. It can have the following possible values:
- Switches on optimizations, corresponding to level 2 optimizations.
- Switches of all kinds of optimizations.
- Returns to default (i.e. command-line or config file) specified optimizations.
- Parses the string and switches on the optimizations found in the string as if they were
passed using the -Oo command line option. The optimizations must be separated by
The following strings are supported:
- Use register variables.
- Use uncertain optimizations.
- Optimize for size.
- Skip stackframes.
- Peephole optimizations.
- Use common subexpression elimination at the assembler level.
- Unroll loops
- change tail recursion to regular while
- Reorder fields if this results in better alignment.
- Fast math operations
- remove calls to empty procedures.
- Use common subexpression elimination
- Use DFA (Data Flow Analysis).
- Try to automatically inline procedures where appropriate.
- Level 1 optimizations.
- Level 2 optimizations.
- Level 3 optimizations.
- Level 4 optimizations.
- ARM CPU only: perform instruction scheduling. Enabled by default at -O3
- i8086 and i386 CPUs only: use the (E)BP register for register variables when it is
not used to hold the frame pointer. Enabled by default when using level 4 optimizations.
- x86-64 CPU only: use the RBP register for register variables when it is not used
to hold the frame pointer. Enabled by default when using level 4 optimizations.
- Allow removing expressions whose result is not used, even when this
can change program behaviour. For example, range check errors can disappear when
indexing an array and not doing anything with the loaded value, or access violations
due to invalid pointer dereferences can disappear.
Note: it does not (and must not) remove expressions that have explicit side-effects.
Enabled by default when using level 4 optimizations.
- Perform constant propagation across statements. Enabled by default when
using level 3 optimizations or higher.
- Remove stores to variables if the value of that variable is no longer used
afterwards. This optimization is not enabled by default at any optimization level.
Requires DFA (Data Flow Analysis) to be active (otherwise it won’t do anything),
which itself is enabled by default when using level 3 optimizations or higher.
- ARM and Xtensa CPU only (currently the switch is not yet
used there). Do not use a separate frame pointer register if a procedure/function has no
nested pure assembler procedures/functions. This requires the compiler to estimate the
size of the stack frame in advance, and if that estimate is wrong this will result in an
internal compiler error. For this reason it is not enabled by default at any optimization
- Optimize expressions of the form ”x := x ¡op¿ y” by
directly modifying the value of x rather than loading, modifying and then storing it.
This is not enabled by default at any optimization level.
You can disable a certain optimization by prefixing the value with NO.
is equivalent to
Multiple optimizations can be specified with:
Disable loop unrolling with:
This switch is also activated by the -Ooxxx command line switch. Note the small ’o’: it is -Oo
followed by the switch name.