Faster makecontext

[tarqd]

makecontext and friends have to make system calls every time they are used to store the current signal mask. Instead of using this function directly it'd be more performant to store the callee-saved registers yourself (depends on your target systems ABI).

You'd need to store any preserved registers in the table here. You can also probably skip out of preserving float registers as they are also not guaranteed to be preserved accross function calls.

You could store all registers like makecontext does but it's not necessary.

References:

• Excellent blog post by rethinkdb
• SystemV x86_64 calling convention
• SystemV 32-bit calling convention
• Microsoft 64-bit calling convention

[tarqd]

Also a good reference (with a decent license): android's bionic libc.

Code for storing registers for each arch except windows is in libc/[arch]/bionic/setjmp.S