diff --git a/artiq/firmware/Cargo.lock b/artiq/firmware/Cargo.lock
index 683195487..61bab6c85 100644
--- a/artiq/firmware/Cargo.lock
+++ b/artiq/firmware/Cargo.lock
@@ -38,6 +38,15 @@ dependencies = [
  "rustc-cfg 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
+[[package]]
+name = "compiler_builtins"
+version = "0.1.0"
+source = "git+https://github.com/rust-lang-nursery/compiler-builtins?rev=631b568#631b5687b24af413fdbffa4c2644484e60660b00"
+dependencies = [
+ "gcc 0.3.42 (registry+https://github.com/rust-lang/crates.io-index)",
+ "rustc-cfg 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
 [[package]]
 name = "cslice"
 version = "0.3.0"
@@ -114,7 +123,7 @@ dependencies = [
  "board 0.0.0",
  "build_artiq 0.0.0",
  "byteorder 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
- "compiler_builtins 0.1.0 (git+https://github.com/rust-lang-nursery/compiler-builtins)",
+ "compiler_builtins 0.1.0 (git+https://github.com/rust-lang-nursery/compiler-builtins?rev=631b568)",
  "cslice 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "fringe 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -173,6 +182,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 [metadata]
 "checksum byteorder 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "c40977b0ee6b9885c9013cd41d9feffdd22deb3bb4dc3a71d901cc7a77de18c8"
 "checksum compiler_builtins 0.1.0 (git+https://github.com/rust-lang-nursery/compiler-builtins)" = "<none>"
+"checksum compiler_builtins 0.1.0 (git+https://github.com/rust-lang-nursery/compiler-builtins?rev=631b568)" = "<none>"
 "checksum cslice 0.3.0 (registry+https://github.com/rust-lang/crates.io-index)" = "0f8cb7306107e4b10e64994de6d3274bd08996a7c1322a27b86482392f96be0a"
 "checksum fringe 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "987689dcfad85eee8d76b477865641ec483e63fb86d52966bfc350c4a647d78a"
 "checksum gcc 0.3.42 (registry+https://github.com/rust-lang/crates.io-index)" = "291055c78f59ca3d84c99026c9501c469413d386bb46be1e1cf1d285cd1db3b0"