link builtins in a way that lets unused functions get omitted from final library/binary

[andrewrk]

when linking without libc, build memcpy.o and memset.o. we can cache these objects with a hash of (release/debug, conditional compilation values, target triple/data layout).

generate a compile unit without memcpy/memset exported. when linking the final output, use ar rcs foo.a foo.o memcpy.o memset.o. Then ld -o foo foo.a (or equivalent).

This accomplishes several things:

• omit memcpy/memset when the functions are unused
• avoid compiling memcpy/memset with every compiler invocation
• allow linking zig-created .o files with libc .o files, so that a single project can have both c code and zig code and everything can be linked together. in this case zig generated code will depend on the libc memset/memcpy implementations.

[thejoshwolfe]

Why have separate memcpy.o and memset.o instead of prebuilding a mem.a and linking with ld -o foo foo.o mem.a? Doesn't that still provide the selective linking feature we want?

[andrewrk]

That should work too.