I am calling the exp function in a tight loop, a little over 238 million times. As expected, it takes a while, but it seemed to take longer than necessary, so I was looking into this. What I found was that if I call exp as I am with a single precision floating point value: exp( myFloat ); it takes a long time, but if I cast my single precision value to a double: exp( double(myFloat) ); it takes about 1/3 of the time to execute. This is fantastic, but I need to know why this works so I can fix my code to use this as well as updating any other code that might benefit from this change. Here are the relevent build and link flags that I am using: CLINTFLAGS=-Wstrict-prototypes -Wmissing-prototypes -Wbad-function-cast -Wnested-externs CXXLINTFLAGS=-W -Wpointer-arith -Wreturn-type -Wcast-qual -Wwrite-strings -Wswitch -Wcast-align -Wuninitialized -Wchar-subscripts -Wno-unused-parameter -Wno-comment CFLAGS=-march=k8 -Winvalid-pch -Wall -fPIC -fopenmp -pipe $(LOCAL_CFLAGS) $(INCLUDES) $(DEFINES) C_OPT_FLAGS=-O3 -funroll-all-loops -fpeel-loops -fno-schedule-insns2 CXXFLAGS=$(CFLAGS) LD=$(CXX) $(CXXFLAGS) LIBS=$(LIB_DIRS) $(LOCAL_LIBS) -lm -lstdc++ I did use gprof to profile this code. In both cases it claims that I am calling std::exp(float). I doubt this can be correct. I have also tried the -ffast-math flag to no effect. I am in the process of using readelf and nm to try to find the exp function that I am actually linking to, but that is going slowly, so any pointers would be great. Along these lines, I believe that the fast exp may be from a non-standard library, but I have not been able to find which one. readelf says that that I am getting exp from GLIBC_2.2.5. It mentions exp, expf, ldexp and frexp. I don't have any really good idea on what to do with this information just yet. GCC and kernel version: gcc version 4.1.1 20070105 (Red Hat 4.1.1-53) Red Hat Enterprise Linux 4 (2.6.9-55.0.2.ELsmp) AMD Opteron Any help would be greatly appriciated. Thanks, David Blankenship KLA-Tencor
