• Eliminates redundant code
• Basic loop optimization
• Can structure code to take advantage of -qarch and -qtune settings

The optimizations provided include:

• In-depth memory access analysis
• Better loop scheduling
• High-order loop analysis and transformations (-qhot=level=0)
• Inlining of small procedures within a compilation unit by default
• Eliminating implicit compile-time memory usage limits
• Widening, which merges adjacent load/stores and other operations
• Pointer aliasing improvements to enhance other optimizations

`-O3 is equivalent to the following flags :

• -O2
• -qhot=level=0

• -O3
• -qipa=level=1
• -qarch=auto
• -qtune=auto
• -qsimd=auto

-O5 is equivalent to the following flags :

• -O4
• -qipa=level=2

qalias=std |nostd :

• bit_packed : The compiler uses the bit_packed alignment rules.
• full : The compiler uses the RISC System/6000 alignment rules. This is the same as power.
• mac68k : The compiler uses the Macintosh alignment rules. This suboption is valid only for 32- bit compilations.
• natural : The compiler maps structure members to their natural boundaries.
• packed : The compiler uses the packed alignment rules.
• power : The compiler uses the RISC System/6000 alignment rules.
• twobyte : The compiler uses the Macintosh alignment rules. This suboption is valid only for 32- bit compilations.

The default is -qalign=full.

Supported values for this flag are :

• auto - Use the processor on which the program is compiled.
• pwr8 - The POWER8 processor based systems.
• pwr7 - The POWER7 processor based systems.
• pwr6e - The POWER6 processor in "Enhanced" mode based systems.
• pwr6 - The POWER6 processor based systems.
• pwr5x - The POWER5+ processor based systems.
• pwr5 - The POWER5 processor based systems.
• pwr4 - The POWER4 processor based systems.
• ppc970 - The PPC970 processor based systems.

• refalign specifies that all pointers inside the compilation unit only point to data that is naturally aligned according to the length of the pointer types.
• contig specifies the compiler can perform optimizations according to the memory layout of the objects occupying contiguous blocks of memory.

The supported values for suboption are :

• arraypad - The compiler will pad any arrays where it infers that there may be a benefit.
• level=0 - The compiler performs a limited set of high-order loop transformations.
• level=1 - The compiler performs its full set of high-order loop transformations.
• simd - Replaces certain instruction sequences with vector instructions.
• vector - Replaces certain instruction sequences with calls to the MASS library.

Specifying -qhot without suboptions implies -qhot=nosimd, -qhot=noarraypad, -qhot=vector and -qhot=level=1. The -qhot option is also implied by -O4 and -O5 .

• level=0 does only minimal interprocedural analysis and optimization.
• level=1 turns on inlining , limited alias analysis, and limited call-site tailoring.
• level=2 turns on full interprocedural data flow and alias analysis.

Specifying nothreads does not run any parallel threads; this is equivalent to running one serial thread. This option does not affect the code in the final binary created.

• -qprefetch=aggressive : Aggressively prefetch data.
• -qprefetch=dscr option causes the Data Streams Control Register to be set to the value specified when executing this program.

Example : -qprefetch=dscr=42

The -qstrict=all, -qstrict=precision, -qstrict=exceptions, -qstrict=ieeefp, and -qstrict=order suboptions and their negative forms are group suboptions that affect multiple, individual suboptions. Group suboptions act as if either the positive or the no form of every suboption of the group is specified.

Default:

• Always -qstrict or -qstrict=all when the -qnoopt or -O0 optimization level is in effect.
• -qstrict or -qstrict=all is the default when the -O2 or -O optimization level is in effect.
• -qnostrict or -qstrict=none is the default when -O3 or a higher optimization level is in effect.

• all | none : all disables all semantics-changing transformations, including those controlled by the ieeefp, order, library, precision, and exceptions suboptions. none enables these transformations.
• precision | noprecision : precision disables all transformations that are likely to affect floating-point precision, including those controlled by the subnormals, operationprecision, association, reductionorder, and library suboptions. noprecision enables these transformations.
• exceptions | noexceptions : exceptions disables all transformations likely to affect exceptions or be affected by them, including those controlled by the nans, infinities, subnormals, guards, and library suboptions. noexceptions enables these transformations.
• ieeefp | noieeefp : ieeefp disables transformations that affect IEEE floating-point compliance, including those controlled by the nans, infinities, subnormals, zerosigns, and operation precision suboptions. noieeefp enables these transformations.
• nans | nonans : nans disables transformations that may produce incorrect results in the presence of, or that may incorrectly produce IEEE floating-point signaling NaN (not-a-number) values. nonans enables these transformations.
• infinities | noinfinities : infinities disables transformations that may produce incorrect results in the presence of, or that may incorrectly produce floating-point infinities. noinfinities enables these transformations.
• subnormals | nosubnormals : subnormals disables transformations that may produce incorrect results in the presence of, or that may incorrectly produce IEEE floating-point subnormals (formerly known as denorms). nosubnormals enables these transformations.
• zerosigns | nozerosigns : zerosigns disables transformations that may affect or be affected by whether the sign of a floating-point zero is correct. nozerosigns enables these transformations.
• operationprecision | nooperationprecision : operationprecision disables transformations that produce approximate results for individual floating-point operations. nooperationprecision enables these transformations.
• order | noorder : order disables all code reordering between multiple operations that may affect results or exceptions, including those controlled by the association, reductionorder, and guards suboptions. noorder enables code reordering.
• association | noassociation : association disables reordering operations within an expression. noassociation enables reordering operations.
• reductionorder | noreductionorder: reductionorder disables parallelizing floating-point reductions. noreductionorder enables these reductions.
• guards | noguards : guards disables moving operations past guards or calls which control whether the operation should be executed or not. noguards, enables these moving operations.
• library | nolibrary : library disables transformations that affect floating-point library functions. nolibrary enables these transformations.

• auto - Use the processor on which the program is compiled.
• pwr8 - The POWER8 processor based systems.
• pwr7 - The POWER7 processor based systems.
• pwr6e - The POWER6 processor in "Enhanced" mode based systems.
• pwr6 - The POWER6 processor based systems.
• pwr5x - The POWER5+ processor based systems.
• pwr5 - The POWER5 processor based systems.
• pwr4 - The POWER4 processor based systems.
• ppc970 - The PPC970 processor based systems.

• signedzero | nosignedzero : Determines how the SIGN(A,B) function handles signed real 0.0. In addition, determines whether negative internal values will be prefixed with a minus when formatted output would produce a negative sign zero.
• autodealloc | noautodealloc : Determines whether the compiler deallocates allocatable arrays that are declared locally without either the SAVE or the STATIC attribute and have a status of currently allocated when the subprogram terminates.
• oldpad | nooldpad : When the PAD=specifier is present in the INQUIRE statement, specifying -qxlf90=nooldpad returns UNDEFINED when there is no connection, or when the connection is for unformatted I/O. This behavior conforms with the Fortran 95 standard and above. Specifying -qxlf90=oldpad preserves the Fortran 90 behavior.
• Default: signedzero, autodealloc and nooldpad for the xlf95, xlf95_r, xlf95_r7 and f95 invocation commands. nosignedzero, noautodealloc and oldpad for all other invocation commands.