figure out why unused functions from builtin.a are not getting omitted from final binary

[andrewrk]

I'm pretty sure when I solved #54 it was working but now if I do this:

$ ./zig build-exe ../examples/hello_world/hello.zig --release-small
$ objdump -d hello
...
0000000000201a40 <sqrt>:
...
0000000000201bf0 <sqrtf>:
...

That's not supposed to be in there. sqrt (among others) isn't called from this application.

On the other hand functions from compiler_rt.a are correctly getting omitted.

Maybe we should do -ffunction-sections?

[justinbalexander]

It seems to happen only in release-safe and release-fast modes for me when compiling for
armv7em-freestanding-eabihf.

In debug and release small modes, there are no calls to any compiler-rt functions that I can see.

[andrewrk]

Note for webassembly what @shritesh found: #2248 (comment)

[justinbalexander]

I just pulled the latest zig and my freestanding arm project won't build anymore:

lld: error: undefined symbol: __zig_probe_stack
>>> referenced by debug.zig:1822 (/home/alexander/Code/zig/build/lib/zig/std/debug.zig:1822)
>>>               /home/alexander/Code/msp432-tiva-zig/zig-cache/o/siMKlSLAjLobrJZ7TlKJUPyWP9vPyHsp5b0NmB2ajmfniXlyxalD_J1uFQo1EQ7b/build.o:(std.debug.getLineNumberInfoDwarf)

I think it is related to this issue.

Also, unlike my previous comment, this error happens in all build modes.

EDIT: nevermind, build issue... sorry for the noise

[FireFox317]

I'm pretty sure that the problem is that all the different compiler_rt functions all end up in one object file (compiler_rt.o). In that case the linker has no option to link the complete object and thus end up with all the unnecessary code in the final executable.

Instead to solve this problem all the different zig files in the compiler_rt should end up in a separate object file inside the libcompiler_rt.a This way the linker can choose the correct object file.

I found this by doing ar -t on the libcompiler_rt.a in the .local/share/zig cache folder and by doing the same command on the libclang_rt.a file.

Edit: I'm gonna try to make a PR for this, if I get a confirmation that this is the actual problem. I'm quite sure tho.

[FireFox317]

@andrewrk I would like to solve this, because it's pretty annoying when build stuff for small embedded microcontrollers. However, I'm not sure if I should implement -ffunction-sections or go with the method described above?

[andrewrk]

I would try -ffunction-sections first since in theory it should just be an option provided to LLVM. Getting compiler_rt to be more than 1 object file would be a lot more work and probably make compilation slower.

[andrewrk]

Landed in 372b615. Thanks @FireFox317! Hello World on Linux is now 6 KB after being stripped of debug info:

[nix-shell:~/downloads/zig/build]$ ./zig build-exe ../example/hello_world/hello.zig --release-small --strip --single-threaded

[nix-shell:~/downloads/zig/build]$ ls -ahl hello
-rwxr-xr-x 1 andy users 9.8K Jul  3 15:49 hello

[nix-shell:~/downloads/zig/build]$ strip hello # zig's --strip isn't fully complete yet

[nix-shell:~/downloads/zig/build]$ ls -ahl hello
-rwxr-xr-x 1 andy users 6.0K Jul  3 15:49 hello

[nix-shell:~/downloads/zig/build]$ ./hello
Hello, world!