0    Turn off all optimizations.

1    Turn on local optimizations that can be done quickly. Do peephole optimizations and instruction scheduling.

2    Turn on extensive optimization. This is the default.
The optimizations at this level are generally conservative, in the sense that they are virtually always beneficial and avoid changes which affect such things as floating point accuracy. In addition to the level 1 optimizations, do inner loop unrolling, if-conversion, two passes of instruction scheduling, global register allocation, dead store elimination, instruction scheduling across basic blocks, and partial redundancy elimination.

3    Turn on aggressive optimization.
The optimizations at this level are distinguished from -O2 by their aggressiveness, generally seeking highest-quality generated code even if it requires extensive compile time. They may include optimizations that are generally beneficial but may hurt performance.
This includes but is not limited to turning on the Loop Nest Optimizer, -LNO:opt=1, and setting -OPT:roundoff=1:IEEE_arithmetic=2:Olimit=9000:reorg_common=ON.

s    Specify that code size is to be given priority in tradeoffs with execution time.

NOTE: -Ofast enables -ipa (inter-procedural analysis), which places limitations on how libraries and .o files are built.

The default number of threads used at run-time is the number of CPUs available in the machine. This number of threads can also be controlled by setting the OMP_NUM_THREADS environment variable.

During the training run, the instrumented executable produces information regarding execution paths and data values, but does not generate information by using hardware performance counters.

During the -fb_opt compilation phase, information regarding execution paths and data values are used to improve the information available to the optimizer. FDO enables some optimizations which are only performed when the feedback data file is available. The safety of optimizations performed under FDO is consistent with the level of safety implied by the other optimization flags (outside of fb_create and fb_opt) specified on the compile and link lines.

-CG:flow : OFF disables control flow optimization in the code generation. Default is ON.

-IPA:callee_limit=N : Functions whose size exceeds this limit will never be automatically inlined by the compiler. The default is 500.

0    Disable procedure reordering. This is the default for non-C++ programs.

1    Reorder based on the frequency in which different procedures are invoked. This is the default for C++ programs.

2    Reorder based on caller-callee relationship.

-LNO:blocking : Enable or disable the cache blocking transformation. The default is ON.

Because -LNO:fusion is on by default, turning on fission without turning off fusion may result in their effects being nullified. Ordinarily, fusion is applied before fission. Specifying -LNO:fission=2 will turn on fission and cause it to be applied before fusion.

0 = Prefetch disabled.

1 = Prefetch is done only for arrays that are always referenced in each iteration of a loop.

2 = Prefetch is done without the above restriction. This is the default.

3 = Most aggressive.

0 = Turn off the vectorizer.

1 = (Default) Vectorize only if the compiler can determine that there is no undesirable performance impact due to sub-optimal alignment. Vectorize only if vectorization does not introduce accuracy problems with floating-point operations.

2 = Vectorize without any constraints (most aggressive).

-OPT:alias=<name>
Specify the pointer aliasing model to be used. By specifying one or more of the following for <name>, the compiler is able to make assumptions throughout the compilation:

typed
Assume that the code adheres to the ANSI/ISO C standard which states that two pointers of different types cannot point to the same location in memory. This is ON by default when -OPT:Ofast is specified.

restrict
Specify that distinct pointers are assumed to point to distinct, non-overlapping objects. This is OFF by default.

disjoint
Specify that any two pointer expressions are assumed to point to distinct, non-overlapping objects. This is OFF by default.

1 Adhere to IEEE accuracy. This is the default when optimization levels -O0, -O1 and -O2 are in effect.

2 May produce inexact result not conforming to IEEE 754. This is the default when -O3 is in effect.

3 All mathematically valid transformations are allowed.

0 = Inhibit optimizations that might affect the floating-point behavior. This is the default when optimization levels -O0, -O1, and -O2 are in effect.

1 = Allow simple transformations that might cause limited round-off or overflow differences. Compounding such transformations could have more extensive effects. This is the default when -O3 is in effect.

2 = Allow more extensive transformations, such as the reordering of reduction loops. This is the default level when -OPT:Ofast is specified.

3 = Enable any mathematically valid transformation.

-WOPT:aggstr=N
This controls the aggressiveness of the strength reduction optimization performed by the scalar optimizer, in which induction expressions within a loop are replaced by temporaries that are incremented together with the loop variable. When strength reduction is overdone, the additional temporaries increase register pressure, resulting in excessive register spills that decrease performance. The value specified must be a positive integer value, which specifies the maximum number of induction expressions that will be strength-reduced across an index variable increment. When set at 0, strength reduction is only performed for non-trivial induction expressions. The default is 11.