diff --git a/src/librustc_data_structures/bitvec.rs b/src/librustc_data_structures/bitvec.rs
index f26307fd8c58e..70f50b4c042b1 100644
--- a/src/librustc_data_structures/bitvec.rs
+++ b/src/librustc_data_structures/bitvec.rs
@@ -24,12 +24,14 @@ impl BitVector {
         (self.data[word] & mask) != 0
     }
 
+    /// Returns true if the bit has changed.
     pub fn insert(&mut self, bit: usize) -> bool {
         let (word, mask) = word_mask(bit);
         let data = &mut self.data[word];
         let value = *data;
-        *data = value | mask;
-        (value | mask) != value
+        let new_value = value | mask;
+        *data = new_value;
+        new_value != value
     }
 
     pub fn insert_all(&mut self, all: &BitVector) -> bool {
diff --git a/src/librustc_trans/trans/mir/analyze.rs b/src/librustc_trans/trans/mir/analyze.rs
index 9d4c7663cb0d3..23cca55e4d480 100644
--- a/src/librustc_trans/trans/mir/analyze.rs
+++ b/src/librustc_trans/trans/mir/analyze.rs
@@ -11,7 +11,7 @@
 //! An analysis to determine which temporaries require allocas and
 //! which do not.
 
-use rustc_data_structures::fnv::FnvHashSet;
+use rustc_data_structures::bitvec::BitVector;
 use rustc::mir::repr as mir;
 use rustc::mir::visit::{Visitor, LvalueContext};
 use trans::common::{self, Block};
@@ -19,8 +19,8 @@ use super::rvalue;
 
 pub fn lvalue_temps<'bcx,'tcx>(bcx: Block<'bcx,'tcx>,
                                mir: &mir::Mir<'tcx>)
-                               -> FnvHashSet<usize> {
-    let mut analyzer = TempAnalyzer::new();
+                               -> BitVector {
+    let mut analyzer = TempAnalyzer::new(mir.temp_decls.len());
 
     analyzer.visit_mir(mir);
 
@@ -51,18 +51,28 @@ pub fn lvalue_temps<'bcx,'tcx>(bcx: Block<'bcx,'tcx>,
 }
 
 struct TempAnalyzer {
-    lvalue_temps: FnvHashSet<usize>,
+    lvalue_temps: BitVector,
+    seen_assigned: BitVector
 }
 
 impl TempAnalyzer {
-    fn new() -> TempAnalyzer {
-        TempAnalyzer { lvalue_temps: FnvHashSet() }
+    fn new(temp_count: usize) -> TempAnalyzer {
+        TempAnalyzer {
+            lvalue_temps: BitVector::new(temp_count),
+            seen_assigned: BitVector::new(temp_count)
+        }
     }
 
     fn mark_as_lvalue(&mut self, temp: usize) {
         debug!("marking temp {} as lvalue", temp);
         self.lvalue_temps.insert(temp);
     }
+
+    fn mark_assigned(&mut self, temp: usize) {
+        if !self.seen_assigned.insert(temp) {
+            self.mark_as_lvalue(temp);
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for TempAnalyzer {
@@ -74,6 +84,7 @@ impl<'tcx> Visitor<'tcx> for TempAnalyzer {
 
         match *lvalue {
             mir::Lvalue::Temp(index) => {
+                self.mark_assigned(index as usize);
                 if !rvalue::rvalue_creates_operand(rvalue) {
                     self.mark_as_lvalue(index as usize);
                 }
diff --git a/src/librustc_trans/trans/mir/mod.rs b/src/librustc_trans/trans/mir/mod.rs
index 75ce33da2c9b9..cdde4cbb28604 100644
--- a/src/librustc_trans/trans/mir/mod.rs
+++ b/src/librustc_trans/trans/mir/mod.rs
@@ -97,7 +97,7 @@ pub fn trans_mir<'bcx, 'tcx>(bcx: Block<'bcx, 'tcx>) {
     let temps = mir.temp_decls.iter()
                               .map(|decl| bcx.monomorphize(&decl.ty))
                               .enumerate()
-                              .map(|(i, mty)| if lvalue_temps.contains(&i) {
+                              .map(|(i, mty)| if lvalue_temps.contains(i) {
                                   TempRef::Lvalue(LvalueRef::alloca(bcx,
                                                                     mty,
                                                                     &format!("temp{:?}", i)))
diff --git a/src/test/run-pass/mir_temp_promotions.rs b/src/test/run-pass/mir_temp_promotions.rs
new file mode 100644
index 0000000000000..de83c1f5ee0cd
--- /dev/null
+++ b/src/test/run-pass/mir_temp_promotions.rs
@@ -0,0 +1,22 @@
+// Copyright 2016 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.
+
+#![feature(rustc_attrs)]
+
+#[rustc_mir]
+fn test1(f: f32) -> bool {
+    // test that we properly promote temporaries to allocas when a temporary is assigned to
+    // multiple times (assignment is still happening once ∀ possible dataflows).
+    !(f.is_nan() || f.is_infinite())
+}
+
+fn main() {
+    assert_eq!(test1(0.0), true);
+}