From 55419bdf697801d0dda8a0ba60a6c5c9ffef76ea Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Bj=C3=B6rn=20Steinbrink?= <bsteinbr@gmail.com>
Date: Tue, 7 Oct 2014 22:38:23 +0200
Subject: [PATCH] Generate better code for "large" vec![] macro invocations

Even when optimized, something like vec![1i,2,3,4,5,6,7,8,9,10] will
allocate three times, because LLVM can only drop the call based on the
observed size of the previous allocation. Using a simple macro to count
the number of elements, we can initialize the vector with the correct
size and avoid reallocations.

Since LLVM already does constant folding for us, the new count!() macro
results in a single integer constant even if building without
optimizations, so we don't end up with an awful lot of additions in the
generated code.
---
 src/libstd/macros.rs | 27 +++++++++++++++++++++++++++
 1 file changed, 27 insertions(+)

diff --git a/src/libstd/macros.rs b/src/libstd/macros.rs
index fa356432a6731..3ca46f8f3e138 100644
--- a/src/libstd/macros.rs
+++ b/src/libstd/macros.rs
@@ -321,7 +321,22 @@ macro_rules! try(
     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
 )
 
+/// A macro to count the number of expressions passed to it
+#[macro_export]
+macro_rules! count(
+    ($x:expr, $($xs:expr),+) => {
+        1 + count!($($xs),+)
+    };
+    ($x:expr) => {
+        1u
+    };
+    ($($x:expr),*) => {
+        0u
+    };
+)
+
 /// Create a `std::vec::Vec` containing the arguments.
+#[cfg(stage0)]
 #[macro_export]
 macro_rules! vec(
     ($($e:expr),*) => ({
@@ -333,6 +348,18 @@ macro_rules! vec(
     ($($e:expr),+,) => (vec!($($e),+))
 )
 
+#[not(cfg(stage0))]
+#[macro_export]
+macro_rules! vec(
+    ($($e:expr),*) => ({
+        // leading _ to allow empty construction without a warning.
+        let mut _temp = ::std::vec::Vec::with_capacity(count!($($e),*));
+        $(_temp.push($e);)*
+        _temp
+    });
+    ($($e:expr),+,) => (vec!($($e),+))
+)
+
 
 /// A macro to select an event from a number of receivers.
 ///