Putting -lm Before User Objects/Libraries on Link Line Can Impact Performance

Reference Number : DPD200121218

Version : 11.0

Operating System : Linux*

Problem Description : Starting with 11.0.081, a fix was made to the compiler driver to link the Intel Math Library libimf statically by default (the intended and documented behavior) instead of dynamically.  If -lm is used with the compiler driver (icc/icpc/ifort), the driver automatically inserts libimf before libm in the link line.  Linking in libimf.a when this is done causes a problem if -lm precedes any user-created objects or libraries.  For example:

icc -lm user1.o user2.o -luser3
Gets converted by the driver to:

ld ... -Bstatic -limf -Bdynamic -lm ... user1.o user2.o -luser3 ...
On Linux, static libraries must come after the object/library files that use them in the link line in order for the symbols to resolve.  Since libimf.a comes before the objects/libraries that use standard math functions, these math functions won't resolve to the static Intel math library.  However, the dynamic libm doesn't have an order dependency because it is a dynamic library, so the math functions will resolve to the GNU math library.  This can have a significant performance impact.

Resolution Status : Starting with 11.1.056, the compiler now emits a warning if -lm precedes other user objects or libraries on the linker command line.  If you run into a performance regression between compilers prior to 11.0.081 and compilers from 11.0.081 on, please verify that you don't have -lm being used prior to your objects/libraries in your link lines if you use icc/icpc/ifort to link. Using the above example, make the following change:

icc user1.o user2.o -luser3 -lm
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