Compiling Eigen library for iPhone with vectorisation

Question

I am struggling with the compilation of Eigen library for iPhone 4 which has an ARM processor with armv7 instruction set. Everything works fine so far when I specify the preprocessor define EIGEN_DONT_VECTORIZE. But due to some performance issues I would like to use armv7 optimised code.

Regardless which compiler I use LLVM-GCC 4.2 or LLVM CLang 2.0, I always run into compilation errors. I figured out (or better think so), that LLVM-GCC 4.2 is the only way to get access to these ARM-NEON specific instructions.

When I do not set EIGEN_DONT_VECTORIZE (and provide -mfloat-abi=softfp -mfpu=neon to gcc) I get the following gcc compiler error:

src/m3CoreLib/Eigen/src/Core/arch/NEON/PacketMath.h:89: error: expected unqualified-id before '__ extension__'

I have read about issues using the "old" gcc 4.2 and the recommendation to use a newer version of gcc. I am not sure but I believe this is not an option because of app store approval. Is there anything else I can do to get it compiled for iPhone.? Anybody out there who solved this?

Thanks, Kay

I don't know enough to properly answer your question, but I don't believe Apple care about what you compile with as long as it passes the static analysis. Certainly before iOS 4 came along there were apps that were modifying the compiler to obtain blocks support, for example. As long as you are producing a signed .ipa as your output you can really compile with whatever (to my knowledge: prepared to be proved wrong on this one). — lxt
@lxt: Thanks, maybe I should give using a newer gcc version a try. Then I get an idea about the possible amount of performance improvement. — Kay

Kay Kay · Accepted Answer · 2011-06-07T17:40:50

After fiddling around with different compiler settings hours and hours I found myself a satisfying solution and came to following conclusion.

There is a surprisingly huge difference between debug and release settings regarding Eigen's template library approach: Release settings with usual optimisation flags enabled let the application run 20 to 40 times faster than debug. I have never seen such a difference before in any language, from my experience it is usually 1.5 - 3.

Although I still cannot force vectorisation i.e. code compiles only with EIGEN_DONT_VECTORIZE defined, the resulting performance fits my needs now.

Compiling Eigen library for iPhone with vectorisation

1 Answers