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The impact of compilers on x86/x64 CPU architectures
by Guillaume Louel
Published on September 27, 2012



Introduction
Do compilers bias processor performance one way or another? This is a legitimate question and something we sometimes raise ourselves when we do our processor testing. The fact that Intel and AMD both make their own compilers is clearly a clue to the answer and certain past practices have contributed to the debate.

More recently, the US Federal Trade Commission looked into, among other things, the workings of Intel compilers, highlighting the fact that they don’t offer optimisations for a given level of support (inclusion of SSE 4 for example), but rather for precise processor models (second gen Core i7 and so on). The inquiry resulted in an agreement that, in the end, didn’t change much with respect to compilers.


One of the few consequences of the agreement between Intel
and the FTC was the incluseion of this note in the documentation.

The question of optimisations still holds however and we wanted to take a closer look at the subject to see whether there is an impact or not and, if so, how extensive it is. In the interests of thoroughness, we’ll briefly go back over the different types of optimisation that are offered in compilers and the impact these optimisations have.

For the purposes of the test we used applications for which the source code was available to us. This is important as it allows us both to put into context applications for which the source code isn't available and to gauge the complexity of the problem: while you can always recompile open source software, this isn’t the case for commercial applications. It’s impossible to recompile Adobe Photoshop for a precise processor model and the user simply has to put up with the choices made by developers in terms of the compiler – and compilation options. An important point to keep in mind!


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