Rollup merge of rust-lang#70846 - tmiasko:compiler-builtins-codegen-units, r=alexcrichton

Keep codegen units unmerged when building compiler builtins

Make it possible to control how mono items are partitioned into code generation
units, when compiling the compiler builtins, by retaining the original partitioning.

Helps with rust-lang#48625, rust-lang#61063, rust-lang#67960, rust-lang#70489.

r? @alexcrichton

Author: Dylan-DPC

src/librustc_mir/monomorphize/partitioning.rs

@@ -455,11 +455,18 @@ fn default_visibility(tcx: TyCtxt<'_>, id: DefId, is_generic: bool) -> Visibilit
 fn merge_codegen_units<'tcx>(
     tcx: TyCtxt<'tcx>,
     initial_partitioning: &mut PreInliningPartitioning<'tcx>,
-    target_cgu_count: usize,
+    mut target_cgu_count: usize,
 ) {
     assert!(target_cgu_count >= 1);
     let codegen_units = &mut initial_partitioning.codegen_units;
 
+    if tcx.is_compiler_builtins(LOCAL_CRATE) {
+        // Compiler builtins require some degree of control over how mono items
+        // are partitioned into compilation units. Provide it by keeping the
+        // original partitioning when compiling the compiler builtins crate.
+        target_cgu_count = codegen_units.len();
+    }
+
     // Note that at this point in time the `codegen_units` here may not be in a
     // deterministic order (but we know they're deterministically the same set).
     // We want this merging to produce a deterministic ordering of codegen units

src/test/codegen-units/partitioning/compiler-builtins.rs

@@ -0,0 +1,40 @@
+// Verifies that during compiler_builtins compilation the codegen units are kept
+// unmerged. Even when only a single codegen unit is requested with -Ccodegen-units=1.
+//
+// compile-flags: -Zprint-mono-items=eager -Ccodegen-units=1
+
+#![compiler_builtins]
+#![crate_type="lib"]
+#![feature(compiler_builtins)]
+
+mod atomics {
+    //~ MONO_ITEM fn compiler_builtins::atomics[0]::sync_1[0] @@ compiler_builtins-cgu.0[External]
+    #[no_mangle]
+    pub extern "C" fn sync_1() {}
+
+    //~ MONO_ITEM fn compiler_builtins::atomics[0]::sync_2[0] @@ compiler_builtins-cgu.0[External]
+    #[no_mangle]
+    pub extern "C" fn sync_2() {}
+
+    //~ MONO_ITEM fn compiler_builtins::atomics[0]::sync_3[0] @@ compiler_builtins-cgu.0[External]
+    #[no_mangle]
+    pub extern "C" fn sync_3() {}
+}
+
+mod x {
+    //~ MONO_ITEM fn compiler_builtins::x[0]::x[0] @@ compiler_builtins-cgu.1[External]
+    #[no_mangle]
+    pub extern "C" fn x() {}
+}
+
+mod y {
+    //~ MONO_ITEM fn compiler_builtins::y[0]::y[0] @@ compiler_builtins-cgu.2[External]
+    #[no_mangle]
+    pub extern "C" fn y() {}
+}
+
+mod z {
+    //~ MONO_ITEM fn compiler_builtins::z[0]::z[0] @@ compiler_builtins-cgu.3[External]
+    #[no_mangle]
+    pub extern "C" fn z() {}
+}