How to build math libraries of glibc package?

Question

I already know how to build the new version of Glibc (e.g. 2.3) beside the older version (e.g. 2.19). I followed the instructions in https://www.tldp.org/HOWTO/html_single/Glibc-Install-HOWTO/

But rather compiling whole libraries in Glibc, I only want to build math libraries (libm.so).

How can I only recompile libm.so in ubuntu?

The second question is that possible to recompile that specific part (maths) in glibc using the clang compiler? I found that I cannot recompile the whole Glibc using clang because of standards compliance and etc.

Answer 1

libm depends on internals in the dynamic loader and libc, for example for IFUNC resolvers. You cannot simply take libm from a newer version and use install it on a system with an older glibc version. A while back, a few patches were proposed to optionally enables this:

• Introduce --enable-math-noprivate
• math: Use local strto_*nan function in nan* for CONFIG_MATH_NOPRIVATE
• math: Merge strtod_nam_*.h into math-type-macros-*.h (This one was actually merged.)

Even with those patches, it was still necessary to build glibc with --disable-multi-arch, incurring a significant performance loss for some math functions.