add -femit-llvm-bc CLI option and implement it

 * Added doc comments for `std.Target.ObjectFormat` enum
 * `std.Target.oFileExt` is removed because it is incorrect for Plan-9
   targets. Instead, use `std.Target.ObjectFormat.fileExt` and pass a
   CPU architecture.
 * Added `Compilation.Directory.joinZ` for when a null byte is desired.
 * Improvements to `Compilation.create` logic for computing `use_llvm`
   and reporting errors in contradictory flags. `-femit-llvm-ir` and
   `-femit-llvm-bc` will now imply `-fLLVM`.
 * Fix compilation when passing `.bc` files on the command line.
 * Improvements to the stage2 LLVM backend:
   - cleaned up error messages and error reporting. Properly bubble up
     some errors rather than dumping to stderr; others turn into panics.
   - properly call ZigLLVMCreateTargetMachine and
     ZigLLVMTargetMachineEmitToFile and implement calculation of the
     respective parameters (cpu features, code model, abi name, lto,
     tsan, etc).
   - LLVM module verification only runs in debug builds of the compiler
   - use LLVMDumpModule rather than printToString because in the case
     that we incorrectly pass a null pointer to LLVM it may crash during
     dumping the module and having it partially printed is helpful in
     this case.
   - support -femit-asm, -fno-emit-bin, -femit-llvm-ir, -femit-llvm-bc
   - Support LLVM backend when used with Mach-O and WASM linkers.

1 files changed, 7 insertions, 3 deletions

diff --git a/src/link.zig b/src/link.zig
index 562896d14c..85ff2ca603 100644
--- a/src/link.zig
+++ b/src/link.zig
@@ -206,7 +206,8 @@ pub const File = struct {
         const use_lld = build_options.have_llvm and options.use_lld; // comptime known false when !have_llvm
         const sub_path = if (use_lld) blk: {
             if (options.module == null) {
-                // No point in opening a file, we would not write anything to it. Initialize with empty.
+                // No point in opening a file, we would not write anything to it.
+                // Initialize with empty.
                 return switch (options.object_format) {
                     .coff => &(try Coff.createEmpty(allocator, options)).base,
                     .elf => &(try Elf.createEmpty(allocator, options)).base,
@@ -219,8 +220,11 @@ pub const File = struct {
                     .raw => return error.RawObjectFormatUnimplemented,
                 };
             }
-            // Open a temporary object file, not the final output file because we want to link with LLD.
-            break :blk try std.fmt.allocPrint(allocator, "{s}{s}", .{ emit.sub_path, options.target.oFileExt() });
+            // Open a temporary object file, not the final output file because we
+            // want to link with LLD.
+            break :blk try std.fmt.allocPrint(allocator, "{s}{s}", .{
+                emit.sub_path, options.object_format.fileExt(options.target.cpu.arch),
+            });
         } else emit.sub_path;
         errdefer if (use_lld) allocator.free(sub_path);