ability to use `zig build-obj -fcompiler-rt` or `zig build-lib -fcompiler-rt` to produce a compiler-rt build artifact

[andrewrk]

This critical build artifact is already built automatically when it is needed by other build artifacts, but it can often be useful alone, especially when integrating with a third party build system.

I started trying to implement it, thinking it would be a quick fix, but I think it will be a little bit more involved because we probably want to de-duplicate the code from the buildOutputFromZig function.

Here's what I tried so far:

diff --git a/src/Compilation.zig b/src/Compilation.zig
index b7660718d..9d681f1b5 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -432,6 +432,10 @@ pub fn create(gpa: *Allocator, options: InitOptions) !*Compilation {
 
         // Make a decision on whether to use LLVM or our own backend.
         const use_llvm = if (options.use_llvm) |explicit| explicit else blk: {
+            // Self-hosted is not yet capable of building compiler-rt.
+            if (options.is_compiler_rt_or_libc)
+                break :blk true;
+
             // If we have no zig code to compile, no need for LLVM.
             if (options.root_pkg == null)
                 break :blk false;
diff --git a/src/main.zig b/src/main.zig
index 5e406152d..396c713a1 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -1314,6 +1314,8 @@ fn buildOutputType(
         fatal("`zig test` expects a zig source file argument", .{});
     }
 
+    var is_compiler_rt = false;
+
     const root_name = if (provided_name) |n| n else blk: {
         if (arg_mode == .zig_test) {
             break :blk "test";
@@ -1336,6 +1338,10 @@ fn buildOutputType(
             // TODO once the attempt to unwrap error: LinkingWithoutZigSourceUnimplemented
             // is solved, remove the above fatal() and uncomment the `break` below.
             //break :blk "run";
+        } else if (want_compiler_rt == true) {
+            is_compiler_rt = true;
+            function_sections = true;
+            break :blk "compiler_rt";
         } else {
             fatal("expected a positional argument, -femit-bin=[path], --show-builtin, or --name [name]", .{});
         }
@@ -1630,7 +1636,7 @@ fn buildOutputType(
                 .path = local_cache_dir_path,
             };
         }
-        if (arg_mode == .run) {
+        if (arg_mode == .run or is_compiler_rt) {
             break :l global_cache_directory;
         }
         const cache_dir_path = blk: {
@@ -1741,6 +1747,7 @@ fn buildOutputType(
         .test_name_prefix = test_name_prefix,
         .disable_lld_caching = !have_enable_cache,
         .subsystem = subsystem,
+        .is_compiler_rt_or_libc = is_compiler_rt,
     }) catch |err| {
         fatal("unable to create compilation: {}", .{@errorName(err)});
     };

[alloveras]

Hey @andrewrk,
I am not super familiar with Zig's on-demand build process of the libcompiler_rt.a and libcompiler_rt.a.o files. I am currently trying to do the exact same thing this ticket aims to provide (a way to build these files in a standalone way).

From my own experimentation, I think I managed to get the files built independently but I wonder if there are any flaws with the approach that I may have missed due to my ignorance of the topic itself and Zig's internals:

$ zig version
0.10.0

$ zig build-lib -target x86_64-linux-gnu "$(zig env | jq -r .lib_dir)/compiler_rt.zig"

$ ls lib*
libcompiler_rt.a   libcompiler_rt.a.o