make -fcompiler_rt default to enabled when building static libraries

[markfirmware]

I also need this for armv6, and another test case was missing memcpy.
I see something similar in #254 which is closed.

debian9/x64
Linux localhost 4.9.0-8-amd64 #1 SMP Debian 4.9.130-2 (2018-10-27) x86_64 GNU/Linux

$ zig version
0.3.0+db31c252

$ cat file.zig
const std = @import("std");

export fn zigmain() void {
    var warnBuf: [1024]u8 = undefined;
    _ = std.fmt.bufPrint(&warnBuf, "");
}


$ zig build-obj --target-arch armv7 --target-os freestanding --target-environ gnueabihf file.zig
$ nm file.o | grep U
        U __aeabi_uidivmod
        U memset

[andrewrk]

These functions are provided automatically by Zig when you use build-exe, but not build-obj.

One thing it may make sense to do is to provide these symbols but with weak linkage, for build-lib --static. So then instead of zig build-obj --target-arch armv7 --target-os freestanding --target-environ gnueabihf file.zig you would do zig build-lib --static --target-arch armv7 --target-os freestanding --target-environ gnueabihf file.zig. Does that sound good?

[markfirmware]

That sounds great. This is not holding me up, I just noticed that I was linking in an extra dependency that I don't want. Mark.

[andrewrk]

I want to note - LLVM emits libcalls to these functions; its generated code depends on them being available. Furthermore, when the optimizer runs it may collapse a for loop into a call to memset or memcpy, or other functions such as these. Zig provides all these functions in compiler_rt and builtin.

So you cannot escape them as dependencies, but you can rely on the dependencies always being fulfilled.

[markfirmware]

I see. The dependency is supplied in zig's stdlib.

[andrewrk]

That's correct, they are supplied by zig's stdlib, in std/special/builtin.zig and std/special/compiler_rt/*. However it's common for these symbols to be supplied by external libs such as libc, libgcc, compiler_rt, and they have well-defined ABIs. So Zig supplies these dependencies in a way that will not take up double-space if they are ultimately fulfilled externally. For example, if you link libc, then Zig's implementation of memcpy and memset are garbage-collected by the linker thus waste no space.

[markfirmware]

Thanks for the attention. Great service!

[andrewrk]

Zig has --bundle-compiler-rt and corresponding build.zig API, so this is a low priority issue.

[markfirmware]

@andrewrk Thanks I will look at --bundle-compiler-rt. This issue arose when trying to integrate zig with https://ultibo.org which is taking lower priority to zig-only solutions.