Texas Instruments TMS320C2x, User Manual

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Performing Program-Level Optimization ( − pm and − o3 Options)
3-6
3.3 Performing Program-Level Optimization ( − pm and − o3 Options)
You can specify program-level optimization by using the − pm option with the
− o3 option. With program-level optimization, all of your source files are
compiled into one intermediate file called a module . The module moves to the
optimization and code generation passes of the compiler. Because the com-
piler can see the entire program, it performs several optimizations that are
rarely applied during file-level optimization:
-
If a particular argument in a function always has the same value, the com-
piler replaces the argument with the value and passes the value instead
of the argument.
-
If a return value of a function is never used, the compiler deletes the return
code in the function.
-
If a function is not called, directly or indirectly, the compiler removes the
function.
To see which program-level optimizations the compiler is applying, use the
− on2 option to generate an information file. See section 3.2.2, Creating an Op-
timization Information File ( − on n Option)
,
on page 3-5 for more information.
3.3.1 Controlling Program-Level Optimization ( − op n Option)
You can control program-level optimization, which you invoke with − pm − o3,
by using the − op option. Specifically, the − op option indicates if functions in
other modules can call a module’s external functions or modify a module’s ex-
ternal variables. The number following
− op indicates the level you set for the
module that you are allowing to be called or modified. The − o3 option com-
bines this information with its own file-level analysis to decide whether to treat
this module’s external function and variable declarations as if they had been
declared static. Use Table 3
− 4 to select the appropriate level to append to the
− op option.