This setting implies -qstrict_induction unless -qnostrict_induction is explicitly specified.

The optimizations may change from product release to release. If you need a specific level of optimization, specify the appropriate numeric value.

This setting implies -qstrict_induction unless -qnostrict_induction is explicitly specified.

This level is the compiler's highest and most aggressive level of optimization. -O3 performs optimizations that have the potential to slightly alter the semantics of your program. It also applies the -O2 level of optimization with unbounded time and memory. The compiler guards against these optimizations at -O2.

You can use the -qstrict option with -O3 to turn off the aggressive optimizations that might change the semantics of a program. -qstrict combined with -O3 invokes all the optimizations performed at -O2 as well as further loop optimizations. Note that the -qstrict compiler option must appear after the -O3 option, otherwise it is ignored.

The aggressive optimizations performed when you specify -O3 are:

1. Aggressive code motion, and scheduling on computations that have the potential to raise an exception, are allowed.

   Loads and floating-point computations fall into this category. This optimization is aggressive because it may place such instructions onto execution paths where they will be executed when they may not have been according to the actual semantics of the program.

   For example, a loop-invariant floating-point computation that is found on some, but not all, paths through a loop will not be moved at -O2 because the computation may cause an exception. At -O3, the compiler will move it because it is not certain to cause an exception. The same is true for motion of loads. Although a load through a pointer is never moved, loads off the static or stack base register are considered movable at -O3. Loads in general are not considered to be absolutely safe at -O2 because a program can contain a declaration of a static array a of 10 elements and load a[60000000003], which could cause a segmentation violation.

   The same concepts apply to scheduling.

   Example: In the following example, at -O2, the computation of b+c is not moved out of the loop for two reasons:

   1. it is considered dangerous because it is a floating-point operation
   2. it does not occur on every path through the loop

   At -O3, the code is moved.

         ...
       int i ;
       float a[100], b, c ;
       for (i = 0 ; i < 100 ; i++)
        {
        if (a[i] < a[i+1])
         a[i] = b + c ;
        }
         ...

2. Conformance to IEEE rules are relaxed.

   With -O2 certain optimizations are not performed because they may produce an incorrect sign in cases with a zero result, and because they remove an arithmetic operation that may cause some type of floating-point exception.

   For example, X + 0.0 is not folded to X because, under IEEE rules, -0.0 + 0.0 = 0.0, which is -X. In some other cases, some optimizations may perform optimizations that yield a zero result with the wrong sign. For example, X - Y * Z may result in a -0.0 where the original computation would produce 0.0.

   In most cases the difference in the results is not important to an application and -O3 allows these optimizations.

3. Floating-point expressions may be rewritten.

   Computations such as a*b*c may be rewritten as a*c*b if, for example, an opportunity exists to get a common subexpression by such rearrangement. Replacing a divide with a multiply by the reciprocal is another example of reassociating floating-point computations.

Notes

• -qfloat=fltint:rsqrt are on by default in -O3.
• Built-in functions do not change errno at -O3.
• Aggressive optimizations do not include the following floating-point suboptions: -qfloat=hsflt, hssngl, and -qfloat=rndsngl, or anything else that affects the precision mode of a program.
• Integer divide instructions are considered too dangerous to optimize even at -O3.
• The default maxmem value is -1 at -O3.
• Refer to flttrap to see the behavior of the compiler when you specify optimize options with the flttrap option.
• You can use the -qstrict and -qstrict_induction compiler options to turn off effects of -O3 that might change the semantics of a program. Reference to the -qstrict compiler option can appear before or after the -O3 option.
• The -O3 compiler option followed by the -O option leaves -qignerrno on.

This option is the same as -O3, except that it also:

• Sets the -qipa option
• Sets the -qarch and -qtune options to the architecture of the compiling machine

Note: Later settings of -O, -qcache-qipa, -qarch, and -qtune options will override the settings implied by the -O4 option.

• Sets the -qipa=level=2 option to perform full interprocedural data flow and alias analysis.

Note: Later settings of -O, -qcache, -qipa, -qarch, and -qtune options will override the settings implied by the -O5 option.