diff --git a/appveyor.yml b/appveyor.yml
index 46ff9a252a0..744ef2b2b24 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -144,10 +144,10 @@ on_failure:
   - cat %CD%/sccache.log
 
 cache:
-  - "build/i686-pc-windows-msvc/llvm -> src/rustllvm/llvm-auto-clean-trigger"
-  - "build/x86_64-pc-windows-msvc/llvm -> src/rustllvm/llvm-auto-clean-trigger"
-  - "i686-pc-windows-msvc/llvm -> src/rustllvm/llvm-auto-clean-trigger"
-  - "x86_64-pc-windows-msvc/llvm -> src/rustllvm/llvm-auto-clean-trigger"
+  - "build/i686-pc-windows-msvc/llvm -> src/rustllvm/llvm-rebuild-trigger"
+  - "build/x86_64-pc-windows-msvc/llvm -> src/rustllvm/llvm-rebuild-trigger"
+  - "i686-pc-windows-msvc/llvm -> src/rustllvm/llvm-rebuild-trigger"
+  - "x86_64-pc-windows-msvc/llvm -> src/rustllvm/llvm-rebuild-trigger"
 
 branches:
   only:
diff --git a/src/bootstrap/config.rs b/src/bootstrap/config.rs
index 431d4a333d3..4b79e4beb4b 100644
--- a/src/bootstrap/config.rs
+++ b/src/bootstrap/config.rs
@@ -60,6 +60,7 @@ pub struct Config {
     pub llvm_link_shared: bool,
     pub llvm_targets: Option<String>,
     pub llvm_link_jobs: Option<u32>,
+    pub llvm_clean_rebuild: bool,
 
     // rust codegen options
     pub rust_optimize: bool,
@@ -181,6 +182,7 @@ struct Llvm {
     static_libstdcpp: Option<bool>,
     targets: Option<String>,
     link_jobs: Option<u32>,
+    clean_rebuild: Option<bool>,
 }
 
 #[derive(RustcDecodable, Default, Clone)]
@@ -241,6 +243,7 @@ impl Config {
     pub fn parse(build: &str, file: Option<PathBuf>) -> Config {
         let mut config = Config::default();
         config.llvm_optimize = true;
+        config.llvm_clean_rebuild = true;
         config.use_jemalloc = true;
         config.backtrace = true;
         config.rust_optimize = true;
@@ -334,6 +337,7 @@ impl Config {
             set(&mut config.llvm_release_debuginfo, llvm.release_debuginfo);
             set(&mut config.llvm_version_check, llvm.version_check);
             set(&mut config.llvm_static_stdcpp, llvm.static_libstdcpp);
+            set(&mut config.llvm_clean_rebuild, llvm.clean_rebuild);
             config.llvm_targets = llvm.targets.clone();
             config.llvm_link_jobs = llvm.link_jobs;
         }
diff --git a/src/bootstrap/config.toml.example b/src/bootstrap/config.toml.example
index 776bd729119..4e4b11fdc11 100644
--- a/src/bootstrap/config.toml.example
+++ b/src/bootstrap/config.toml.example
@@ -61,6 +61,13 @@
 # controlled by rustbuild's -j parameter.
 #link-jobs = 0
 
+# Delete LLVM build directory on LLVM rebuild.
+# This option's default (`true`) is optimized for CI needs, and CI wants to
+# perform clean full builds only (possibly accelerated by (s)ccache).
+# You may want to override this option for local builds to enable partial LLVM
+# rebuilds.
+#clean-rebuild = true
+
 # =============================================================================
 # General build configuration options
 # =============================================================================
diff --git a/src/bootstrap/native.rs b/src/bootstrap/native.rs
index c13235b9c76..a7754f2669e 100644
--- a/src/bootstrap/native.rs
+++ b/src/bootstrap/native.rs
@@ -41,9 +41,9 @@ pub fn llvm(build: &Build, target: &str) {
         }
     }
 
-    let clean_trigger = build.src.join("src/rustllvm/llvm-auto-clean-trigger");
-    let mut clean_trigger_contents = String::new();
-    t!(t!(File::open(&clean_trigger)).read_to_string(&mut clean_trigger_contents));
+    let rebuild_trigger = build.src.join("src/rustllvm/llvm-rebuild-trigger");
+    let mut rebuild_trigger_contents = String::new();
+    t!(t!(File::open(&rebuild_trigger)).read_to_string(&mut rebuild_trigger_contents));
 
     let out_dir = build.llvm_out(target);
     let done_stamp = out_dir.join("llvm-finished-building");
@@ -51,18 +51,15 @@ pub fn llvm(build: &Build, target: &str) {
         let mut done_contents = String::new();
         t!(t!(File::open(&done_stamp)).read_to_string(&mut done_contents));
 
-        // LLVM was already built previously.
-        // We don't track changes in LLVM sources, so we need to choose between reusing
-        // what was built previously, or cleaning the directory and doing a fresh build.
-        // The choice depends on contents of the clean-trigger file.
-        // If the contents are the same as during the previous build, then no action is required.
-        // If the contents differ from the previous build, then cleaning is triggered.
-        if done_contents == clean_trigger_contents {
+        // If LLVM was already built previously and contents of the rebuild-trigger file
+        // didn't change from the previous build, then no action is required.
+        if done_contents == rebuild_trigger_contents {
             return
-        } else {
-            t!(fs::remove_dir_all(&out_dir));
         }
     }
+    if build.config.llvm_clean_rebuild {
+        t!(fs::remove_dir_all(&out_dir));
+    }
 
     println!("Building LLVM for {}", target);
     let _time = util::timeit();
@@ -154,7 +151,7 @@ pub fn llvm(build: &Build, target: &str) {
     //        tools and libs on all platforms.
     cfg.build();
 
-    t!(t!(File::create(&done_stamp)).write_all(clean_trigger_contents.as_bytes()));
+    t!(t!(File::create(&done_stamp)).write_all(rebuild_trigger_contents.as_bytes()));
 }
 
 fn check_llvm_version(build: &Build, llvm_config: &Path) {
diff --git a/src/rustllvm/llvm-auto-clean-trigger b/src/rustllvm/llvm-rebuild-trigger
similarity index 66%
rename from src/rustllvm/llvm-auto-clean-trigger
rename to src/rustllvm/llvm-rebuild-trigger
index e30ad63d52f..aeabf4a1dd3 100644
--- a/src/rustllvm/llvm-auto-clean-trigger
+++ b/src/rustllvm/llvm-rebuild-trigger
@@ -1,4 +1,4 @@
-# If this file is modified, then llvm will be forcibly cleaned and then rebuilt.
+# If this file is modified, then llvm will be (optionally) cleaned and then rebuilt.
 # The actual contents of this file do not matter, but to trigger a change on the
 # build bots then the contents should be changed so git updates the mtime.
 2017-03-04