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. Nevertheless, the Intel compilers have been accused of producing sub-optimal code with malicious intent. For example, Steve Westfield wrote in a 2005 article at the AMD website:
| “ | Intel 8.1 C/C++ compiler uses the flag -xN (for Linux) or -QxN (for Windows) to take advantage of the SSE2 extensions. For SSE3, the compiler switch is -xP (for Linux) and -QxP (for Windows). With the -xN/-QxN and -xP/-QxP flags set, it checks the processor vendor string—and if it's not "GenuineIntel," it stops execution without even checking the feature flags. Ouch! | ” |
The Danish developer and scholar Agner Fog wrote in 2009:
| “ | The Intel compiler and several different Intel function libraries have suboptimal performance on AMD and VIA processors. The reason is that the compiler or library can make multiple versions of a piece of code, each optimized for a certain processor and instruction set, for example SSE2, SSE3, etc. The system includes a function that detects which type of CPU it is running on and chooses the optimal code path for that CPU. This is called a CPU dispatcher. However, the Intel CPU dispatcher does not only check which instruction set is supported by the CPU, it also checks the vendor ID string. If the vendor string is "GenuineIntel" then it uses the optimal code path. If the CPU is not from Intel then, in most cases, it will run the slowest possible version of the code, even if the CPU is fully compatible with a better version. | ” |
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. A legal battle between AMD and Intel over this and other issues has been settled in November 2009. In late 2010, AMD settled an US Federal Trade Commission antitrust investigation against Intel.
The FTC settlement included a disclosure provision where Intel must:
| “ | ...publish clearly that its compiler discriminates against non-Intel processors (such as AMD's designs), not fully utilizing their features and producing inferior code. | ” |
In compliance with this rule, Intel added an "optimization notice" to its compiler descriptions stating that they "do not optimize equally for non-Intel microprocessors" and that "certain compiler options for Intel compilers, including some that are not specific to Intel micro-architecture, are reserved for Intel microprocessors". It says that:
| “ | Intel® compilers, associated libraries and associated development tools may or may not optimize to the same degree for non-Intel microprocessors for optimizations that are not unique to Intel microprocessors. These optimizations include Intel® Streaming SIMD Extensions 2 (Intel® SSE2), Intel® Streaming SIMD Extensions 3 (Intel® SSE3), and Supplemental Streaming SIMD Extensions 3 (Intel® SSSE3) instruction sets and other optimizations. | ” |
Read more about this topic: Intel C++ Compiler
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