How is sin() implemented in eglibc-2.13?

Question

I need to track down how exactly is double sin(double x) implemented in eglibc-2.13. I downloaded the source code and the only part that made sense was __sin function, that was platform-specific. Is it the heart of what I have in /usr/lib/i386-linux-gnu/libm.a?

How to track down the macrodefinitions that lead from sin() to __sin()? What I really need is the exact code (filename and the line is enough) and a way in which the build process deduces which implementation to use. The architecture's i386.

Answer 1

The (e)glibc build process is black, black magic. You do not want to try to comprehend it. However, glibc adheres to a one-file-per-public-function coding style, so in general, if you have the source tree and you want to find the implementation(s) of some function, the easiest thing to do is

$ find * -name '*function*' -print

from the top level, replacing function with the name of the function, of course.

Talking specifically about sin: the generic implementations of the math functions are in the math directory: however, it appears that there is no generic definition of sin. So the next place to look is sysdeps. Everything that isn't generic is in sysdeps, and in particular, sysdeps/ieee754 is where all the math functions that have some dependence on the IEEE 754 floating point specification, but no other system dependencies, live. This directory is organized by type: sysdeps/ieee754/dbl-64 contains all the math functions for IEEE double. And here you will find sysdeps/ieee754/dbl-64/s_sin.c, which is the code you are looking for. (The e_, s_, k_, etc prefixes on all these files used to mean something but AFAIK no longer do.)

If there were an implementation of sin in assembly language for a particular processor, it would be in a file named sin.S (or possibly s_sin.S) somewhere else in sysdeps. It does not appear that there is one, though.

Answer 2

Not an answer, but just a bit of a background:

When you use sin() or cos() in your C code, it is almost certainly the compiler that provides the implementation, rather than your C library. As an example, look at the list of builtins GCC provides. The linked page also describes the cases where the built-ins are used rather than the versions the C library provides.