build: avoid compiling self-hosted twice

build.zig: add a 'compile' step to compile the self-hosted compiler
without installing it.
Compilation: set cache mode to whole when using the LLVM backend and
--enable-cache is passed.

This makes `zig build` act the same as it does with stage1. Upside is
that a second invocation of `zig build` on an unmodified source tree
will avoid redoing the compilation again. Downside is that it will
proliferate more garbage in the project-local cache (same as stage1).

This can eventually be fixed when Zig's incremental compilation is more
robust; we can go back to having LLVM use CacheMode.incremental and rely
on it detecting no changes and avoiding doing the flush() step.

2 files changed, 9 insertions, 6 deletions

diff --git a/src/Compilation.zig b/src/Compilation.zig
index 0ff9481875..659cfda9bd 100644
--- a/src/Compilation.zig
+++ b/src/Compilation.zig
@@ -1109,11 +1109,6 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
 
         const use_stage1 = options.use_stage1 orelse false;
 
-        const cache_mode = if (use_stage1 and !options.disable_lld_caching)
-            CacheMode.whole
-        else
-            options.cache_mode;
-
         // Make a decision on whether to use LLVM or our own backend.
         const use_llvm = build_options.have_llvm and blk: {
             if (options.use_llvm) |explicit|
@@ -1154,6 +1149,14 @@ pub fn create(gpa: Allocator, options: InitOptions) !*Compilation {
             }
         }
 
+        // TODO: once we support incremental compilation for the LLVM backend via
+        // saving the LLVM module into a bitcode file and restoring it, along with
+        // compiler state, the second clause here can be removed so that incremental
+        // cache mode is used for LLVM backend too. We need some fuzz testing before
+        // that can be enabled.
+        const cache_mode = if ((use_stage1 and !options.disable_lld_caching) or
+            (use_llvm and !options.disable_lld_caching)) CacheMode.whole else options.cache_mode;
+
         const tsan = options.want_tsan orelse false;
         // TSAN is implemented in C++ so it requires linking libc++.
         const link_libcpp = options.link_libcpp or tsan;
diff --git a/src/link/Elf.zig b/src/link/Elf.zig
index 4e67c095c0..1a722c1dde 100644
--- a/src/link/Elf.zig
+++ b/src/link/Elf.zig
@@ -1282,7 +1282,7 @@ fn linkWithLLD(self: *Elf, comp: *Compilation, prog_node: *std.Progress.Node) !v
     // linked are in the hash that namespaces the directory we are outputting to. Therefore,
     // we must hash those now, and the resulting digest will form the "id" of the linking
     // job we are about to perform.
-    // After a successful link, we store the id in the metadata of a symlink named "id.txt" in
+    // After a successful link, we store the id in the metadata of a symlink named "lld.id" in
     // the artifact directory. So, now, we check if this symlink exists, and if it matches
     // our digest. If so, we can skip linking. Otherwise, we proceed with invoking LLD.
     const id_symlink_basename = "lld.id";