Intel C++ Compiler

Intel C++ Compiler, also known as icc or icl, is a group of C and C++ compilers from Intel available for Windows, OS X, Linux and Intel-based Android devices.

Overview

The compilers generate optimized code for IA-32 and Intel 64 architectures, and non-optimized code for non-Intel but compatible processors, such as certain AMD processors. A specific release of the compiler (11.1) is available for development of Linux-based applications for IA-64 (Itanium 2) processors.

The 14.0 compiler added support for Intel-based Android devices and optimized vectorization and SSE Family instructions for performance. The 13.0 release added support for the Intel Xeon Phi coprocessor. It continues support for automatic vectorization, which can generate SSE, SSE2, SSE3, SSSE3, SSE4, AVX and AVX2 SIMD instructions, and the embedded variant for Intel MMX and MMX 2.^[4] Use of such instruction through the compiler can lead to improved application performance in some applications as run on IA-32 and Intel 64 architectures, compared to applications built with compilers that do not support these instructions.

Intel compilers support Cilk Plus, which is a capability for writing vectorized and parallel code that can be used on IA-32 and Intel 64 processors or which can be offloaded to Xeon Phi coprocessors. They also continue support for OpenMP 4.0, symmetric multiprocessing, automatic parallelization, and Guided Auto-Parallization (GAP). With the add-on Cluster OpenMP capability, the compilers can also automatically generate Message Passing Interface calls for distributed memory multiprocessing from OpenMP directives.

Intel C++ is compatible with Microsoft Visual C++ on Windows and integrates into Microsoft Visual Studio. On Linux and OS X, it is compatible with GNU Compiler Collection (GCC) and the GNU toolchain. Intel C++ Compiler for Android is hosted on Windows, OS X or Linux and is compatible with the Android NDK, including gcc and the Eclipse IDE. Intel compilers are known for the application performance they can enable as measured by benchmarks, such as the SPEC CPU benchmarks.

Optimizations

Intel compilers are optimized to computer systems using processors that support Intel architectures. They are designed to minimize stalls and to produce code that executes in the fewest possible number of cycles. The Intel C++ Compiler supports three separate high-level techniques for optimizing the compiled program: interprocedural optimization (IPO), profile-guided optimization (PGO), and high-level optimizations (HLO). The Intel C++ compiler in the Parallel Studio XE 2016 products also supports tools, techniques and language extensions, such as Cilk Plus, for adding and maintaining application parallelism on IA-32 and Intel 64 processors, and also enables application offloading to Intel coprocessors, such as the Intel Xeon Phi coprocessor.

Cilk Plus adds language extensions to C++ to express data and task-parallelism. _Cilk_spawn and _Cilk_sync are keywords to enable task parallelism and the _Cilk_for keyword enables parallelization of for loops. It also provides vector notation with array notations and elemental function capabilities.

Profile-guided optimization refers to a mode of optimization where the compiler is able to access data from a sample run of the program across a representative input set. The data would indicate which areas of the program are executed more frequently, and which areas are executed less frequently. All optimizations benefit from profile-guided feedback because they are less reliant on heuristics when making compilation decisions.

High-level optimizations are optimizations performed on a version of the program that more closely represents the source code. This includes loop interchange, loop fusion, loop unrolling, loop distribution, data prefetch, and more.^[5]

Interprocedural optimization applies typical compiler optimizations (such as constant propagation) but using a broader scope that may include multiple procedures, multiple files, or the entire program.^[6]

Intel's compiler has been criticized for applying, by default, floating-point optimizations not allowed by the C standard and that require special flags with other compilers such as gcc.^[7]

Architectures

• IA-32
• x86-64 (Intel 64 and AMD64)
• Intel Xeon Phi coprocessor
• IA-64 (Itanium 2)

Description of packaging

Except for the Intel Bi-Endian C++ Compiler, Intel C++ compilers are not available in standalone form. They are available in suites:

• Intel Parallel Studio XE for development of technical, enterprise, and high-performance computing applications on Windows, Linux and OS X
• Intel System Studio for development of system and app software for embedded systems or devices running Windows, Linux or Android

The suites include other build tools, such as libraries, and tools for threading and performance analysis.

History Since 2003

Flags and manuals

Documentation can be found at the Intel Software Technical Documentation site.

Debugging

The Intel compiler provides debugging information that is standard for the common debuggers (DWARF 2 on Linux, similar to gdb, and COFF for Windows). The flags to compile with debugging information are /Zi on Windows and -g on Linux. Debugging is done on Windows using the Visual Studio debugger and, on Linux, using gdb.

While the Intel compiler can generate a gprof compatible profiling output, Intel also provides a kernel level, system-wide statistical profiler called Intel VTune Amplifier. VTune can be used from a command line or thru an included GUI on Linux or Windows. It can also be integrated into Visual Studio on Windows, or Eclipse on Linux).

Intel also offers a tool for memory and threading error detection called Intel Inspector XE. Regarding memory errors, it helps detect memory leaks, memory corruption, allocation/de-allocation of API mismatches and inconsistent memory API usage. Regarding threading errors, it helps detect data races (both heap and stack), deadlocks and thread and synch API errors.

Criticism

Intel and third parties have published benchmark results to substantiate performance leadership claims over other commercial, open source and AMD compilers and libraries on Intel and non-Intel processors. Intel and AMD have documented flags to use on the Intel compilers to get optimal performance on Intel and AMD processors.^[12][13] Nevertheless, the Intel compilers have been known to produce sub-optimal code for processors from other vendors than Intel. For example, Steve Westfield wrote in a 2005 article at the AMD website:^[14]

The Danish developer and scholar Agner Fog wrote in 2009:^[15]

This vendor-specific CPU dispatching decreases the performance on non-Intel processors of software built with an Intel compiler or an Intel function library - possibly without the knowledge of the programmer. This has allegedly led to misleading benchmarks.^[15] A legal battle between AMD and Intel over this and other issues has been settled in November 2009.^[16] In late 2010, AMD settled a US Federal Trade Commission antitrust investigation against Intel.^[17]

The FTC settlement included a disclosure provision where Intel must:^[18]

In compliance with this rule, Intel added an "optimization notice" to its compiler descriptions stating that they "may or may not optimize to the same degree for non-Intel microprocessors" and that "certain optimizations not specific to Intel microarchitecture are reserved for Intel microprocessors." It says that:^[19]

As reported by The Register^[20] in July, 2013, Intel was suspected of "benchmarksmanship", when it was shown that the object code produced by the Intel compiler for the AnTuTu Mobile Benchmark omitted portions of the benchmark which showed increased performance compared to ARM platforms.

See also

• Intel Parallel Studio XE
• Intel Integrated Performance Primitives (IPP)
• Intel Data Analytics Acceleration Library (DAAL)
• Intel Math Kernel Library (MKL)
• Intel Threading Building Blocks (TBB)
• Cilk Plus
• VTune Amplifier
• Intel Fortran Compiler
• Intel Developer Zone (Intel DZ; support and discussion)

References

External links

• Intel C++ Compiler for Android
• Compilers in Parallel Studio XE 2013
• Cilk Plus Open Source Site
• TBB Open Source Site
• Free download of Intel compilers for non-commercial use