Codegen the return block last

Author: saethlin

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -260,6 +260,8 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // Apply debuginfo to the newly allocated locals.
     fx.debug_introduce_locals(&mut start_bx);
 
+    // If we have multiple return terminators, create a special return block. If this block is
+    // present, return terminator codegen will be a jump to it instead of a return.
     if mir
         .basic_blocks
         .iter()
@@ -268,15 +270,21 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
         > 1
     {
         let llbb = Bx::append_block(fx.cx, fx.llfn, "return");
-        let mut bx = Bx::build(fx.cx, llbb);
-        fx.codegen_return_terminator(&mut bx);
         fx.return_block = Some(llbb);
     }
 
     // Codegen the body of each block using reverse postorder
     for (bb, _) in traversal::reverse_postorder(&mir) {
         fx.codegen_block(bb);
     }
+
+    // Actually codegen the return block. We need to delay until here because our return place
+    // might not be generated yet, and will only be generated by handling codegen for the
+    // assignment to it.
+    if let Some(llbb) = fx.return_block {
+        let mut bx = Bx::build(fx.cx, llbb);
+        fx.codegen_return_terminator(&mut bx);
+    }
 }
 
 /// Produces, for each argument, a `Value` pointing at the