SPEC MPI2007 Flag Descriptions for HP-MPI

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 Linux64 platforms, -O1 disable software pipelining and global code scheduling. On Intel Itanium processors, this option also enables optimizations for server applications
(straight-line and branch-like code with a flat profile).

-unroll0, -fbuiltin, -mno-ieee-fp, -fomit-frame-pointer (same as -fp), -ffunction-sections

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

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 Intel Itanium processors, the O3 option enables optimizations for technical computing applications (loop-intensive code):
loop optimizations and data prefetch. 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.

-i-static prevents linking intel libraries as shared libraries.

This option enables additional interprocedural optimizations for single file compilation. These optimizations are a subset of full intra-file interprocedural optimizations. One of these optimizations enables the compiler to perform inline function expansion for calls to functions defined within the current source file.

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

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)

-static (link libraries statically)

To override one of the options set by /fast, specify that option after the -fast option on the command line. The options set by /fast may change from release to release.

Instrument program for profiling for the first phase of two-phase profile guided otimization. This instrumentation gathers information about a program's execution paths and data values but does not gather information from hardware performance counters. The profile instrumentation also gathers data for optimizations which are unique to profile-feedback optimization.

Instructs the compiler to produce a profile-optimized executable and merges available dynamic information (.dyn) files into a pgopti.dpi file. If you perform multiple executions of the instrumented program, -Qprof_use merges the dynamic information files again and overwrites the previous pgopti.dpi file.
Without any other options, the current directory is searched for .dyn files

Tells the compiler the maximum number of times (n) to unroll loops.

Enables inline expansion of all intrinsic functions.

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.

Places each function in its own COMDAT section.

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.

-static prevents linking with shared libraries.

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.

Enable SmartHeap library usage by forcing the linker to ignore multiple definitions

Enable SmartHeap library usage by forcing the linker to ignore multiple definitions

Generates specialized code to run exclusively on processors with the extensions T. This option can generate SSSE3, SSE3, SSE2, and SSE instructions for Intel processors, and it can optimize for the Intel (R) Core (TM) 2 Duo processor family.

Flushes denormal floating point results to zero when the application is in gradual underflow mode.

Enables optimizations that give slightly less precise results than full IEEE division. With some optimizations, such as -xN and -xB, the compiler may change floating-point division compu- tations into multiplication by the reciprocal of the denomina- tor. For example, A/B is computed as A * (1/B) to improve the speed of the computation. The default is -prec-div, which provides fully precise IEEE division. It improves precision of floating-point divides by disabling floating-point division-to-multiplication optimiza- tions, resulting in greater accuracy with some loss of perfor- mance.

Disables the insertion of software prefetching by the compiler. Default is -prefetch.

Specifies the percentage multiplier that should be applied to all inlining options that define upper limits. The value is a positive integer specifying the percentage value. The default value is 100 (a factor of 1).

Enables use of faster but slightly less accurate code sequences for math functions, including sqrt, reciprocal sqrt, divide and reciprocal. When compared to strict IEEE* precision, this option slightly reduces the accuracy of floating-point calculations performed by these functions, usually limited to the least significant digit. This option also performs reassociation transformations, which can alter the order of operations, over a larger scope. The increased reasssociation enables generation of more optimal sequences of floating-point multiply-add instructions than not using this option. Note that use of floating-point multiply-add can cause programs to produce different numerical results due to changes in rounding.

Tells the compiler to assume the program does adhere to to the Fortran 95 Standard type aliasability rules (default).

Tells the compiler to assume the program does adhere to the rules defined in the ISO C Standard. The default is to not assume such adherence. If your C/C++ program adheres to these rules, then -ansi-alias will allow the compiler to optimize more aggressively. If it doesn't adhere to these rules, then assuming so can cause the compiler to generate incorrect code.

Do not assume arguments may be aliased. (DEFAULT = -alias-args).

Tells the compiler not to assume aliasing in the program (DEFAULT = -falias).

Instructs the compiler to analyze and transform the program so that 64-bit pointers are shrunk to 32-bit pointers, and 64-bit longs (on Linux) are shrunk into 32-bit longs wherever it is legal and safe to do so. In order for this option to be effective the compiler must be able to optimize using the -ipo option and must be able to analyze all library or external calls the program makes. This option requires that the size of the program executable never exceeds 2 (to the 32nd power) bytes and all data values can be represented within 32 bits. If the program can run correctly in a 32-bit system, these requirements are implicitly satisfied. If the program violates these size restrictions, unpredictable behavior might occur.

The -Wl option directs the compiler to pass a list of arguments to the linker. In this case, "-z muldefs" is passed to the linker. For the Gnu linker (ld), the "-z keyword" option accepts several recognized keywords. Keyword "muldefs" allows multiple definitions. The muldefs keyword will enable, for example, linking with third party libraries like SmartHeap from Microquill.

MicroQuill SmartHeap Library available from http://www.microquill.com/