From dc262d9aa742adadb92558a588aeaa57267fdee4 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Steinbrink?= <bsteinbr@gmail.com>
Date: Wed, 19 Jun 2013 20:28:42 +0200
Subject: [PATCH] Avoid unnecessary scratch datums for by-copy function
 arguments

Currently, by-copy function arguments are always stored into a scratch
datum, which serves two purposes.  First, it is required to be able to
have a temporary cleanup, in case that the call fails before the callee
actually takes ownership of the value. Second, if the argument is to be
passed by reference, the copy is required, so that the function doesn't
get a reference to the original value.

But in case that the datum does not need a drop glue call and it is
passed by value, there's no need to perform the extra copy.
---
 src/librustc/middle/trans/callee.rs | 39 ++++++++++++++++++-----------
 1 file changed, 24 insertions(+), 15 deletions(-)

diff --git a/src/librustc/middle/trans/callee.rs b/src/librustc/middle/trans/callee.rs
index bfbe078c4f5..47e5f9bf35f 100644
--- a/src/librustc/middle/trans/callee.rs
+++ b/src/librustc/middle/trans/callee.rs
@@ -826,24 +826,33 @@ pub fn trans_arg_expr(bcx: block,
                         val = arg_datum.to_ref_llval(bcx);
                     }
                     ty::ByCopy => {
-                        debug!("by copy arg with type %s, storing to scratch",
-                               bcx.ty_to_str(arg_datum.ty));
-                        let scratch = scratch_datum(bcx, arg_datum.ty, false);
+                        if ty::type_needs_drop(bcx.tcx(), arg_datum.ty) ||
+                                arg_datum.appropriate_mode().is_by_ref() {
+                            debug!("by copy arg with type %s, storing to scratch",
+                                   bcx.ty_to_str(arg_datum.ty));
+                            let scratch = scratch_datum(bcx, arg_datum.ty, false);
 
-                        arg_datum.store_to_datum(bcx,
-                                                 arg_expr.id,
-                                                 INIT,
-                                                 scratch);
+                            arg_datum.store_to_datum(bcx,
+                                                     arg_expr.id,
+                                                     INIT,
+                                                     scratch);
 
-                        // Technically, ownership of val passes to the callee.
-                        // However, we must cleanup should we fail before the
-                        // callee is actually invoked.
-                        scratch.add_clean(bcx);
-                        temp_cleanups.push(scratch.val);
+                            // Technically, ownership of val passes to the callee.
+                            // However, we must cleanup should we fail before the
+                            // callee is actually invoked.
+                            scratch.add_clean(bcx);
+                            temp_cleanups.push(scratch.val);
 
-                        match arg_datum.appropriate_mode() {
-                            ByValue => val = Load(bcx, scratch.val),
-                            ByRef(_) => val = scratch.val,
+                            match scratch.appropriate_mode() {
+                                ByValue => val = Load(bcx, scratch.val),
+                                ByRef(_) => val = scratch.val,
+                            }
+                        } else {
+                            debug!("by copy arg with type %s");
+                            match arg_datum.mode {
+                                ByRef(_) => val = Load(bcx, arg_datum.val),
+                                ByValue => val = arg_datum.val,
+                            }
                         }
                     }
                 }