It sets compiler options -O3, -no-prec-div, and -fp-model fast=2.Looking at Intel's page about the techniques used for high-level optimization, it looks like they cannot be enabled à la carte, so with HLO it's all-or-nothing and is enabled using either O2 or O3 (except that O2 uses a subset of O3's techniques).Ĭompare that to -Ofast which does have a long-form equivalent: Ofast So it looks like -O3 activates optimizations that cannot be represented by using other flags (so -O3 does not have a long-form equivalent version).

The bottom of the page lists 'Alternate options' which only lists -Od (which disables all optimizations: probably not what you want).

Many routines in the shared libraries are more highly optimized for Intel® microprocessors than for non-Intel microprocessors.

The O3 option is recommended for applications that have loops that heavily use floating-point calculations and process large data sets.

The optimizations may slow down code in some cases compared to O2 optimizations.

The O3 optimizations may not cause higher performance unless loop and memory access transformations take place.

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 options that are set may change from release to release.

This is determined by the compiler, depending on which operating system and architecture you are using. It accepts a list of command options and file names and.

Performs O2 optimizations and enables more aggressive loop transformations such as Fusion, Block-Unroll-and-Jam, and collapsing IF statements. The ifort command invokes a driver program that is the user interface to the compiler and linker.

The behaviour of -O3 is documented on Intel's website: O3

Intel c compiler flag list -c++