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...d versus unoptimized code, which of course is allowable, but probably to be avoided when possible. However, there's a de facto standard of producing NaN for these cases. The gcc and xlc compilers will optimize the call into a constant NaN, given the right options (-O2 -ffast-math for gcc, -O2 -qignerrno for xlc). All of gcc, icc, and xlc produce NaN via the library call with unoptimized code. Choosing 0.0 for Clang/LLVM introduces an unnecessary incompatibility. This is important in practice because of the xalanc benchmark in SPEC CPU2006. This code initializes a variable to a NaN value by ass...