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Comparing G++ and Intel Compilers and Vectorized Code
Nerval's Lobster writes "A compiler can take your C++ loops and create vectorized assembly code for you. It's obviously important that you RTFM and fully understand compiler options (especially since the defaults may not be what you want or think you're getting), but even then, do you trust that the compiler is generating the best code for you? Developer and editor Jeff Cogswell compares the g++ and Intel compilers when it comes to generating vectorized code, building off a previous test that examined the g++ compiler's vectorization abilities, and comes to some definite conclusions. 'The g++ compiler did well up against the Intel compiler,' he wrote. 'I was troubled by how different the generated assembly code was between the 4.7 and 4.8.1 compilers—not just with the vectorization but throughout the code.' Do you agree?"
Asking any audience larger than about 20 to compare the qualitative differences of object code vectorization is statistically problematic as the survey group is larger than the qualified population.
Help stamp out iliturcy.
Mod parent up +1 insightful.
Unless you suspect and are trying to debug a code generator error (one of the least pleasant/most difficult debugging experiences I've had), the base assertion that you should understand your compiler's code generation is at best unrealistic, and probably just dumb. Code generation is extremely complex, requiring deep knowledge of both this specific compiler's design and this specific computer's instruction set architecture, how the caches work, pre-fetching approaches, timing dependencies in instruction pipelines, etc, etc. If you do suspect a code generator error, you're best off hiring a compiler expert at least as a consultant, and be prepared for a long hard slog.
Maybe 30 years ago, for a PDP-8, you could assert that the C code you wrote had some semblance to the generated machine code. That hasn't been true for a very long time, and C++ is most definitely not C in this regard.
Yeah, on Intel processors. What about AMD and other x86 processors? Don't ever forget that ICC was once caught red-handed disabling important features when the CPUID did not return GenuineIntel...