From c8b83babf358cd3094fb05a65889a6d017a81d96 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Tomasz=20Mi=C4=85sko?= <tomasz.miasko@gmail.com>
Date: Mon, 6 Apr 2020 00:00:00 +0000
Subject: [PATCH] Keep codegen units unmerged when building compiler builtins

---
 src/librustc_mir/monomorphize/partitioning.rs |  9 ++++-
 .../partitioning/compiler-builtins.rs         | 40 +++++++++++++++++++
 2 files changed, 48 insertions(+), 1 deletion(-)
 create mode 100644 src/test/codegen-units/partitioning/compiler-builtins.rs

diff --git a/src/librustc_mir/monomorphize/partitioning.rs b/src/librustc_mir/monomorphize/partitioning.rs
index 9068c0541a428..44aa9a710b653 100644
--- a/src/librustc_mir/monomorphize/partitioning.rs
+++ b/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
diff --git a/src/test/codegen-units/partitioning/compiler-builtins.rs b/src/test/codegen-units/partitioning/compiler-builtins.rs
new file mode 100644
index 0000000000000..25195743b0400
--- /dev/null
+++ b/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() {}
+}