Elide unnecessary ret slot allocas

When there is only a single store to the ret slot that dominates the
load that gets the value for the "ret" instruction, we can elide the
ret slot and directly return the operand of the dominating store
instruction. This is the same thing that clang does, except for a
special case that doesn't seem to affect us.

Fixes #8238

2 files changed, 28 insertions, 0 deletions

diff --git a/src/test/codegen/single-return-value.cc b/src/test/codegen/single-return-value.cc
new file mode 100644
index 00000000000..97d80d3950f
--- /dev/null
+++ b/src/test/codegen/single-return-value.cc
@@ -0,0 +1,14 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+extern "C"
+int test() {
+  return 5;
+}
diff --git a/src/test/codegen/single-return-value.rs b/src/test/codegen/single-return-value.rs
new file mode 100644
index 00000000000..e6eb9a2be72
--- /dev/null
+++ b/src/test/codegen/single-return-value.rs
@@ -0,0 +1,14 @@
+// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[no_mangle]
+fn test() -> int {
+    5
+}