diff --git a/.github/workflows/release.yml b/.github/workflows/release.yml
index f0b1212986d..d5242926eb4 100644
--- a/.github/workflows/release.yml
+++ b/.github/workflows/release.yml
@@ -53,7 +53,7 @@ jobs:
     - name: Build
       run: |
         ./y.sh prepare --only-libcore
-        ./y.sh build --sysroot --release --release-sysroot
+        EMBED_LTO_BITCODE=1 ./y.sh build --sysroot --release --release-sysroot
         cargo test
         ./y.sh clean all
 
@@ -70,4 +70,4 @@ jobs:
       run: |
         # FIXME(antoyo): we cannot enable LTO for stdarch tests currently because of some failing LTO tests using proc-macros.
         echo -n 'lto = "fat"' >> build_system/build_sysroot/Cargo.toml
-        ./y.sh test --release --clean --release-sysroot --build-sysroot ${{ matrix.commands }}
+        EMBED_LTO_BITCODE=1 ./y.sh test --release --clean --release-sysroot --build-sysroot ${{ matrix.commands }}
diff --git a/Readme.md b/Readme.md
index d0b57d2c729..bd552b84f92 100644
--- a/Readme.md
+++ b/Readme.md
@@ -119,6 +119,17 @@ $ CHANNEL="release" $CG_GCCJIT_DIR/y.sh cargo run
 
 If you compiled cg_gccjit in debug mode (aka you didn't pass `--release` to `./y.sh test`) you should use `CHANNEL="debug"` instead or omit `CHANNEL="release"` completely.
 
+### LTO
+
+To use LTO, you need to set the variable `FAT_LTO=1` and `EMBED_LTO_BITCODE=1` in addition to setting `lto = "fat"` in the `Cargo.toml`.
+Don't set `FAT_LTO` when compiling the sysroot, though: only set `EMBED_LTO_BITCODE=1`.
+
+Failing to set `EMBED_LTO_BITCODE` will give you the following error:
+
+```
+error: failed to copy bitcode to object file: No such file or directory (os error 2)
+```
+
 ### Rustc
 
 If you want to run `rustc` directly, you can do so with:
diff --git a/build_system/src/config.rs b/build_system/src/config.rs
index 965aedd8be8..30321939f64 100644
--- a/build_system/src/config.rs
+++ b/build_system/src/config.rs
@@ -387,6 +387,12 @@ pub fn setup(
             rustflags.push("-Csymbol-mangling-version=v0".to_string());
         }
 
+        // Since we don't support ThinLTO, disable LTO completely when not trying to do LTO.
+        // TODO(antoyo): remove when we can handle ThinLTO.
+        // TODO: remove:
+        /*if !env.contains_key(&"FAT_LTO".to_string()) {
+            rustflags.push("-Clto=off".to_string());
+        }*/
         // FIXME(antoyo): remove once the atomic shim is gone
         if os_name == "Darwin" {
             rustflags.extend_from_slice(&[
diff --git a/src/back/lto.rs b/src/back/lto.rs
index 88eff198739..19573628b1b 100644
--- a/src/back/lto.rs
+++ b/src/back/lto.rs
@@ -307,7 +307,6 @@ fn fat_lto(
             match bc_decoded {
                 SerializedModule::Local(ref module_buffer) => {
                     module.module_llvm.should_combine_object_files = true;
-                    module.module_llvm.fat_lto = true;
                     module
                         .module_llvm
                         .context
@@ -490,6 +489,7 @@ fn thin_lto(
                 //let path = module_buffer.0.to_str().expect("path");
                 //let my_path = PathBuf::from(path);
                 //let exists = my_path.exists();
+                //println!("Path: {:?}: {}", path, exists);
                 /*module.module_llvm.should_combine_object_files = true;
                 module
                 .module_llvm
@@ -626,6 +626,11 @@ pub unsafe fn optimize_thin_module(
             match *module {
                 SerializedModule::Local(ref module_buffer) => {
                     let path = module_buffer.0.to_str().expect("path");
+
+                    //let my_path = PathBuf::from(path);
+                    //let exists = my_path.exists();
+                    //println!("Path2: {:?}: {}", path, exists);
+
                     context.add_driver_option(path);
                     should_combine_object_files = true;
                     /*module.module_llvm.should_combine_object_files = true;
@@ -643,12 +648,7 @@ pub unsafe fn optimize_thin_module(
         }
     };
     let module = ModuleCodegen {
-        module_llvm: GccContext {
-            context,
-            should_combine_object_files,
-            fat_lto: false,
-            temp_dir: None,
-        },
+        module_llvm: GccContext { context, should_combine_object_files, temp_dir: None },
         name: thin_module.name().to_string(),
         kind: ModuleKind::Regular,
     };
diff --git a/src/back/write.rs b/src/back/write.rs
index 8922e7e742b..b9bb62a0351 100644
--- a/src/back/write.rs
+++ b/src/back/write.rs
@@ -32,12 +32,12 @@ pub(crate) unsafe fn codegen(
         // NOTE: Only generate object files with GIMPLE when this environment variable is set for
         // now because this requires a particular setup (same gcc/lto1/lto-wrapper commit as libgccjit).
         // TODO: remove this environment variable.
-        let fat_lto = module.module_llvm.fat_lto;
+        let fat_lto = env::var("EMBED_LTO_BITCODE").as_deref() == Ok("1");
 
         let bc_out = cgcx.output_filenames.temp_path(OutputType::Bitcode, module_name);
         let obj_out = cgcx.output_filenames.temp_path(OutputType::Object, module_name);
 
-        if config.bitcode_needed() {
+        if config.bitcode_needed() && fat_lto {
             let _timer = cgcx
                 .prof
                 .generic_activity_with_arg("GCC_module_codegen_make_bitcode", &*module.name);
@@ -57,8 +57,6 @@ pub(crate) unsafe fn codegen(
                     .generic_activity_with_arg("GCC_module_codegen_emit_bitcode", &*module.name);
                 context.add_command_line_option("-flto=auto");
                 context.add_command_line_option("-flto-partition=one");
-                // TODO: remove since we don't want fat objects when it is for Bitcode only.
-                context.add_command_line_option("-ffat-lto-objects");
                 context
                     .compile_to_file(OutputKind::ObjectFile, bc_out.to_str().expect("path to str"));
             }
@@ -120,11 +118,11 @@ pub(crate) unsafe fn codegen(
                     if fat_lto {
                         context.add_command_line_option("-flto=auto");
                         context.add_command_line_option("-flto-partition=one");
-                    }
 
-                    // NOTE: without -fuse-linker-plugin, we get the following error:
-                    // lto1: internal compiler error: decompressed stream: Destination buffer is too small
-                    //context.add_driver_option("-fuse-linker-plugin");
+                        // NOTE: without -fuse-linker-plugin, we get the following error:
+                        // lto1: internal compiler error: decompressed stream: Destination buffer is too small
+                        context.add_driver_option("-fuse-linker-plugin");
+                    }
 
                     context.add_driver_option("-Wl,-r");
                     // NOTE: we need -nostdlib, otherwise, we get the following error:
@@ -137,6 +135,7 @@ pub(crate) unsafe fn codegen(
                         obj_out.to_str().expect("path to str"),
                     );
                 } else {
+                    //println!("Combining to object file");
                     context.compile_to_file(
                         OutputKind::ObjectFile,
                         obj_out.to_str().expect("path to str"),
diff --git a/src/base.rs b/src/base.rs
index 0287c73569a..be149ffe5a1 100644
--- a/src/base.rs
+++ b/src/base.rs
@@ -225,7 +225,6 @@ fn module_codegen(
             name: cgu_name.to_string(),
             module_llvm: GccContext {
                 context: Arc::new(SyncContext::new(context)),
-                fat_lto: false,
                 should_combine_object_files: false,
                 temp_dir: None,
             },
diff --git a/src/lib.rs b/src/lib.rs
index 123cebf5137..1132b0cd2f5 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -304,7 +304,6 @@ fn codegen_allocator(
     ) -> Self::Module {
         let mut mods = GccContext {
             context: Arc::new(SyncContext::new(new_context(tcx))),
-            fat_lto: false,
             should_combine_object_files: false,
             temp_dir: None,
         };
@@ -337,7 +336,6 @@ fn target_machine_factory(
 pub struct GccContext {
     context: Arc<SyncContext>,
     should_combine_object_files: bool,
-    fat_lto: bool,
     // Temporary directory used by LTO. We keep it here so that it's not removed before linking.
     temp_dir: Option<TempDir>,
 }
diff --git a/tests/failing-ui-tests.txt b/tests/failing-ui-tests.txt
index 5a55bdb156e..ce867be7390 100644
--- a/tests/failing-ui-tests.txt
+++ b/tests/failing-ui-tests.txt
@@ -94,4 +94,3 @@ tests/ui/consts/const-eval/parse_ints.rs
 tests/ui/simd/intrinsic/generic-arithmetic-pass.rs
 tests/ui/backtrace/backtrace.rs
 tests/ui/lifetimes/tail-expr-lock-poisoning.rs
-tests/ui/runtime/rt-explody-panic-payloads.rs