SPEC OMP2012 Flag Description for GNU Fortran/C/C++ Compilers 6.2 for 64 Bit Linux

Enables optimizations for speed and disables some optimizations that
increase code size and affect speed.
To limit code size, this option:
- Enables global optimization; this includes data-flow analysis, code motion, strength reduction and test replacement, split-lifetime analysis, and instruction scheduling.
- Disables intrinsic recognition and intrinsics inlining.
The O1 option may improve performance for applications with very large code size, many branches, and execution time not dominated by code within loops.
On IA-32 Windows platforms, -O1 sets the following:

/Qunroll0, /Oi-, /Op-, /Oy, /Gy, /Os, /GF (/Qvc7 and above), /Gf (/Qvc6 and below), /Ob2, and /Og

Enables optimizations for speed. This is the generally recommended optimization level. This option also enables:
- Inlining of intrinsics
- Intra-file interprocedural optimizations, which include:
- inlining
- constant propagation
- forward substitution
- routine attribute propagation
- variable address-taken analysis
- dead static function elimination
- removal of unreferenced variables
- The following capabilities for performance gain:
- constant propagation
- copy propagation
- dead-code elimination
- global register allocation
- global instruction scheduling and control speculation
- loop unrolling
- optimized code selection
- partial redundancy elimination
- strength reduction/induction variable simplification
- variable renaming
- exception handling optimizations
- tail recursions
- peephole optimizations
- structure assignment lowering and optimizations
- dead store elimination

On IA-32 Windows platforms, -O2 sets the following:

/Og, /Oi-, /Os, /Oy, /Ob2, /GF (/Qvc7 and above), /Gf (/Qvc6 and below), /Gs, and /Gy.

Enables O2 optimizations plus more aggressive optimizations, such as prefetching, scalar replacement, and loop and memory access transformations. Enables optimizations for maximum speed, such as:
- Loop unrolling, including instruction scheduling
- Code replication to eliminate branches
- Padding the size of certain power-of-two arrays to allow more efficient cache use.
On IA-32 and Intel EM64T processors, when O3 is used with options -ax or -x (Linux) or with options /Qax or /Qx (Windows), the compiler performs more aggressive data dependency analysis than for O2, which may result in longer compilation times.
The O3 optimizations may not cause higher performance unless loop and memory access transformations take place. The optimizations may slow down code in some cases compared to O2 optimizations.
The O3 option is recommended for applications that have loops that heavily use floating-point calculations and process large data sets. On IA-32 Windows platforms, -O3 sets the following:

/GF (/Qvc7 and above), /Gf (/Qvc6 and below), and /Ob2

Tells the compiler the maximum number of times to unroll loops.

Disables conformance to the ANSI C and IEEE 754 standards for floating-point arithmetic.

Allows use of EBP as a general-purpose register in optimizations.

This option enables most speed optimizations, but disables some that increase code size for a small speed benefit.

This option enables global optimizations.

Specifies the level of inline function expansion.

Ob0 - Disables inlining of user-defined functions. Note that statement functions are always inlined.

Ob1 - Enables inlining when an inline keyword or an inline attribute is specified. Also enables inlining according to the C++ language.

Ob2 - Enables inlining of any function at the compiler's discretion.

This option tells the compiler to separate functions into COMDATs for the linker.

This option enables read only string-pooling optimization.

This option enables read/write string-pooling optimization.

This option disables stack-checking for routines with 4096 bytes of local variables and compiler temporaries.

Multi-file ip optimizations that includes:
- inline function expansion
- interprocedural constant propogation
- dead code elimination
- propagation of function characteristics
- passing arguments in registers
- loop-invariant code motion

enable floating point model variation [no-]except - enable/disable floating point semantics fast[=1|2] - enables more aggressive floating point optimizations precise - allows value-safe optimizations source - enables intermediates in source precision strict - enables -fp-model precise -fp-model except, disables contractions and enables pragma stdc fenv_access double - rounds intermediates in 53-bit (double) precision extended - rounds intermediates in 64-bit (extended) precision

The -fast option enhances execution speed across the entire program by including the following options that can improve run-time performance:

-O3   (maximum speed and high-level optimizations)

-ipo (enables interprocedural optimizations across files)

-xT  (generate code specialized for Intel(R) Core(TM)2 Duo processors, Intel(R) Core(TM)2 Quad processors and Intel(R) Xeon(R) processors with SSSE3)

-static (disable -prec-div) Statically link in libraries at link time

-no-prec-div (disable -prec-div) where -prec-div improves precision of FP divides (some speed impact)

To override one of the options set by /fast, specify that option after the -fast option on the command line. The exception is the xT or QxT option which can't be overridden. The options set by /fast may change from release to release.

Code is optimized for Intel(R) Core(TM)2 Duo processors, Intel(R) Core(TM)2 Quad processors and Intel(R) Xeon(R) processors with SSSE3. The resulting code may contain unconditional use of features that are not supported on other processors. This option also enables new optimizations in addition to Intel processor-specific optimizations including advanced data layout and code restructuring optimizations to improve memory accesses for Intel processors.

Do not use this option if you are executing a program on a processor that is not an Intel processor. If you use this option on a non-compatible processor to compile the main program (in Fortran) or the function main() in C/C++, the program will display a fatal run-time error if they are executed on unsupported processors.

Compiler option to statically link in libraries at link time

Link Intel provided libraries dynamically

Tells the compiler to generate instructions for the highest instruction set available on the compilation host processor.

(disable/enable[default] -[no-]prec-div)

-prec-div improves precision of floating-point divides. It has a slight impact on speed. -no-prec-div disables this option and enables optimizations that give slightly less precise results than full IEEE division.

(disable/enable[default] -[no-]prec-sqrt)

-prec-sqrt improves precision of floating-point square root. It has a slight impact on speed. -no-prec-sqrt disables this option and enables optimizations that give slightly less precise results than full IEEE division.

Enable/disable(DEFAULT) use of ANSI aliasing rules in optimizations; user asserts that the program adheres to these rules.

Enables the parallelizer to generate multi-threaded code based on OpenMP* directives. Option -qopenmp is the replacement option for -openmp, which is deprecated.

Link Intel provided libraries statically

specify source files are in free format. Same as -FR. -nofree indicates fixed format

-mcmodel= use a specific memory model to generate code and store data small - Restricts code and data to the first 2GB of address space (DEFAULT) medium - Restricts code to the first 2GB; it places no memory restriction on data large - Places no memory restriction on code or data

enable language support for , as described below c99 enable C99 support for C programs c++0x enable preliminary support for some C++0x features

Invoke the Intel C compiler for IA32 and Intel 64 applications.

Invoke the Intel C compiler for IA32 and Intel 64 applications.

Invoke the Intel Fortran compiler for IA32 and Intel 64 applications.