diff --git a/artiq/firmware/Cargo.lock b/artiq/firmware/Cargo.lock
index 62c1989fd..f1e035733 100644
--- a/artiq/firmware/Cargo.lock
+++ b/artiq/firmware/Cargo.lock
@@ -36,15 +36,6 @@ name = "byteorder"
 version = "1.0.0"
 source = "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#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 = "compiler_builtins"
 version = "0.1.0"
@@ -182,7 +173,7 @@ dependencies = [
  "alloc_artiq 0.0.0",
  "board 0.0.0",
  "build_artiq 0.0.0",
- "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)",
  "drtioaux 0.0.0",
  "log 0.3.6 (registry+https://github.com/rust-lang/crates.io-index)",
  "logger_artiq 0.0.0",
@@ -220,7 +211,6 @@ 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"
diff --git a/artiq/firmware/ksupport/glue.c b/artiq/firmware/ksupport/glue.c
index 807ad563e..475e22a84 100644
--- a/artiq/firmware/ksupport/glue.c
+++ b/artiq/firmware/ksupport/glue.c
@@ -41,6 +41,7 @@ int fprintf(FILE *stream, const char *fmt, ...)
 /* called by libunwind */
 int fflush(FILE *stream)
 {
+    return 0;
 }
 
 /* called by libunwind */