Run DCE in per-crate optimize, too (before saving SPIR-V for .rlibs/linking/etc.).

Author: eddyb

crates/rustc_codegen_spirv/src/lib.rs

@@ -309,10 +309,17 @@ impl ThinBufferMethods for SpirvModuleBuffer {
 impl SpirvCodegenBackend {
     fn optimize_common(
         _cgcx: &CodegenContext<Self>,
-        _module: &mut ModuleCodegen<<Self as WriteBackendMethods>::Module>,
+        module: &mut ModuleCodegen<<Self as WriteBackendMethods>::Module>,
     ) -> Result<(), FatalError> {
-        // FIXME(eddyb) actually run as many optimization passes as possible,
-        // before ever serializing `.spv` files that will later get linked.
+        // Apply DCE ("dead code elimination") to modules before ever serializing
+        // them as `.spv` files (technically, `.rcgu.o` files inside `.rlib`s),
+        // that will later get linked (potentially many times, esp. if this is
+        // some big upstream library, e.g. `core` itself), and will therefore
+        // benefit from not having to clean up all sorts of unreachable helpers.
+        linker::dce::dce(&mut module.module_llvm);
+
+        // FIXME(eddyb) run as many optimization passes as possible, not just DCE.
+
         Ok(())
     }
 }

crates/rustc_codegen_spirv/src/linker/mod.rs

@@ -1,7 +1,7 @@
 #[cfg(test)]
 mod test;
 
-mod dce;
+pub(crate) mod dce;
 mod destructure_composites;
 mod duplicates;
 mod entry_interface;
@@ -277,13 +277,6 @@ pub fn link(
         output
     };
 
-    // FIXME(eddyb) do this before ever saving the original `.spv`s to disk.
-    {
-        let _timer = sess.timer("link_dce-post-merge");
-        dce::dce(&mut output);
-    }
-
-    // HACK(eddyb) this has to be after DCE, to not break SPIR-T w/ dead decorations.
     if let Some(dir) = &opts.dump_post_merge {
         dump_spv_and_spirt(&output, dir.join(disambiguated_crate_name_for_dumps));
     }