MPLAB C Compiler for PIC24 MCUs and dsPIC DSCs ... - Microchip

Using the Compiler on the Command Line
TABLE 3-8: GENERAL OPTIMIZATION OPTIONS (CONTINUED)
Option Definition
-O2 Optimize even more. The compiler performs nearly all
supported optimizations that do not involve a space-speed
trade-off. -O2 turns on all optional optimizations except for
loop unrolling (-funroll-loops), function inlining
(-finline-functions), and strict aliasing optimizations
(-fstrict-aliasing). It also turns on force copy of
memory operands (-fforce-mem) and Frame Pointer elimination
(-fomit-frame-pointer). As compared to -O, this
option increases both compilation time and the performance of
the generated code.
-O3 Optimize yet more. -O3 turns on all optimizations specified by
-O2 and also turns on the inline-functions option.
-Os Optimize for size. -Os enables all -O2 optimizations that do
not typically increase code size. It also performs further
optimizations designed to reduce code size.
The following options control specific optimizations. The -O2 option turns on all of
these optimizations except -funroll-loops, -funroll-all-loops and
-fstrict-aliasing.
You can use the following flags in the rare cases when “fine-tuning” of optimizations to
be performed is desired.
TABLE 3-9: SPECIFIC OPTIMIZATION OPTIONS
Option Definition
-falign-functions
-falign-functions=n
-falign-labels
-falign-labels=n
-falign-loops
-falign-loops=n
Align the start of functions to the next power-of-two greater
than n, skipping up to n bytes. For instance,
-falign-functions=32 aligns functions to the next
32-byte boundary, but -falign-functions=24 would align
to the next 32-byte boundary only if this can be done by
skipping 23 bytes or less.
-fno-align-functions and -falign-functions=1 are
equivalent and mean that functions will not be aligned.
The assembler only supports this flag when n is a power of
two; so n is rounded up. If n is not specified, use a
machine-dependent default.
Align all branch targets to a power-of-two boundary, skipping
up to n bytes like -falign-functions. This option can
easily make code slower, because it must insert dummy
operations for when the branch target is reached in the usual
flow of the code.
If -falign-loops or -falign-jumps are applicable and
are greater than this value, then their values are used instead.
If n is not specified, use a machine-dependent default which is
very likely to be 1, meaning no alignment.
Align loops to a power-of-two boundary, skipping up to n bytes
like -falign-functions. The hope is that the loop will be
executed many times, which will make up for any execution of
the dummy operations.
If n is not specified, use a machine-dependent default.
-fcaller-saves Enable values to be allocated in registers that will be
clobbered by function calls, by emitting extra instructions to
save and restore the registers around such calls. Such
allocation is done only when it seems to result in better code
than would otherwise be produced.
© 2008 Microchip Technology Inc. DS51284H-page 47