I am developing a FORTRAN code where the exponent function (exp) takes substantial time.
Its profile shows that __libm_error_support takes almost the same time as exp.
Can I specify some option to avoid this subroutine? What does it do exactly?
I tried to specify -fno-math-errno on the critical modules but it did not help.
Also, what is exp.L and is there a faster version of exp? Small numbers
near zero (underflow) etc are no problem for my application, if they are computed
exactly or set to 0.
time seconds seconds calls Ts/call Ts/call name
8.75 21.82 21.82 exp.L
8.29 42.50 20.68 mkl_lapack_dlaed4
7.72 61.74 19.24 LN8_M4_Kgas_1
6.30 77.45 15.71 __libm_error_support
1.00 164.35 2.50 exp
I am using ifort 14.0 (cannot upgrade as free compiler is no longer provided).
Continuing the soliloquy, I have wormed into this problem with the following results.
1. The offending __libm_error_support is found in Intel's libimf library.
2. I found the source code of libm_error_support.c on the Internet and got some
idea of what it is about.
3. libm_error_support seems to be called when some error or error-like situation
arises. In my case, it was an underflow, like exp(-x) for a large x. libm_error_support
is called for each such call to exp, whereas when x is not a problem (for example,
when it is 1.0), libm_error_support is not called.
4. I was not able to find a compiler option to prevent libm_error_support from being called.
It is a waste to call it internally as it does nothing useful (for me).
5. I was able to improve the efficiency of my code by doing an "if",
like if(x>100) res=0; else res=exp(-x). In my case it worked marvelously, because
I do not have something like exp(-1000)*exp(1000), which would be ruined by the if.
In my code, exp is used like s=s+exp(-x), so neglecting small values is fine.
Summarizing I solved my problem and obtained a substantial boost of efficiency.
I find it very strange that I could not find any human-readable description on libm_error_support,
what it does, when it is called and how to prevent it being called.