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

Using the Compiler on the Command Line
TABLE 3-9: SPECIFIC OPTIMIZATION OPTIONS (CONTINUED)
Option Definition
-freduce-all-givs Forces all general-induction variables in loops to be
strength-reduced.
These options may generate better or worse code; results are
highly dependent on the structure of loops within the source
code.
-frename-registers Attempt to avoid false dependencies in scheduled code by
making use of registers left over after register allocation. This
optimization will most benefit processors with lots of registers.
It can, however, make debugging impossible, since variables
will no longer stay in a “home register”.
-frerun-cse-afterloop
Rerun common subexpression elimination after loop
optimizations has been performed.
-frerun-loop-opt Run the loop optimizer twice.
-fschedule-insns Attempt to reorder instructions to eliminate dsPIC® DSC
Read-After-Write stalls (see the “dsPIC30F Family Reference
Manual” (DS70046) for more details). Typically improves
performance with no impact on code size.
-fschedule-insns2 Similar to -fschedule-insns, but requests an additional
pass of instruction scheduling after register allocation has
been done.
-fstrength-reduce Perform the optimizations of loop strength reduction and
elimination of iteration variables.
-fstrict-aliasing Allows the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C, this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at
the same address as an object of a different type, unless the
types are almost the same. For example, an unsigned int
can alias an int, but not a void* or a double. A character
type may alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than
the one most recently written to (called “type-punning”) is
common. Even with -fstrict-aliasing, type-punning is
allowed, provided the memory is accessed through the union
type. So, the code above will work as expected. However, this
code might not:
int f() {
a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
© 2008 Microchip Technology Inc. DS51284H-page 49