diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e58a1596bff..3bccb5ccdd7 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -36,7 +36,7 @@ jobs:
           "--test-successful-rustc --nb-parts 2 --current-part 0",
           "--test-successful-rustc --nb-parts 2 --current-part 1",
           "--projects",
-          "--gcc-asm-tests",
+          "--gcc-asm-tests --test-release-libcore",
         ]
 
     steps:
diff --git a/build_system/src/test.rs b/build_system/src/test.rs
index 6cc2282c802..d8333e15785 100644
--- a/build_system/src/test.rs
+++ b/build_system/src/test.rs
@@ -31,6 +31,7 @@ fn get_runners() -> Runners {
     runners.insert("--run-ui-tests", ("Run specified rustc UI tests", run_ui_tests));
     runners.insert("--projects", ("Run the tests of popular crates", test_projects));
     runners.insert("--test-libcore", ("Run libcore tests", test_libcore));
+    runners.insert("--test-release-libcore", ("Run libcore tests", test_release_libcore));
     runners.insert("--alloc-tests", ("Run alloc tests", test_alloc));
     runners.insert("--clean", ("Empty cargo target directory", clean));
     runners.insert("--build-sysroot", ("Build sysroot", build_sysroot));
@@ -766,12 +767,23 @@ fn test_projects(env: &Env, args: &TestArg) -> Result<(), String> {
 }
 
 fn test_libcore(env: &Env, args: &TestArg) -> Result<(), String> {
+    test_libcore_inner(env, args, false)
+}
+
+fn test_release_libcore(env: &Env, args: &TestArg) -> Result<(), String> {
+    test_libcore_inner(env, args, true)
+}
+
+fn test_libcore_inner(env: &Env, args: &TestArg, release: bool) -> Result<(), String> {
     // FIXME: create a function "display_if_not_quiet" or something along the line.
     println!("[TEST] libcore");
     let path = get_sysroot_dir().join("sysroot_src/library/coretests");
     let _ = remove_dir_all(path.join("target"));
-    // FIXME(antoyo): run in release mode when we fix the failures.
-    run_cargo_command(&[&"test"], Some(&path), env, args)?;
+    let mut command: Vec<&dyn AsRef<OsStr>> = vec![&"test"];
+    if release {
+        command.push(&"--release");
+    }
+    run_cargo_command(&command, Some(&path), env, args)?;
     Ok(())
 }
 
diff --git a/src/int.rs b/src/int.rs
index 0c9a7556945..9633539a16b 100644
--- a/src/int.rs
+++ b/src/int.rs
@@ -178,6 +178,9 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
         } else {
             debug_assert!(a_type.dyncast_array().is_some());
             debug_assert!(b_type.dyncast_array().is_some());
+            if a_type != b_type {
+                b = self.gcc_int_cast(b, a_type);
+            }
             let signed = a_type.is_compatible_with(self.i128_type);
             let func_name = match (operation, signed) {
                 (BinaryOp::Plus, true) => "__rust_i128_add",
@@ -187,7 +190,7 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
                 _ => unreachable!("unexpected additive operation {:?}", operation),
             };
             let param_a = self.context.new_parameter(self.location, a_type, "a");
-            let param_b = self.context.new_parameter(self.location, b_type, "b");
+            let param_b = self.context.new_parameter(self.location, a_type, "b");
             let func = self.context.new_function(
                 self.location,
                 FunctionType::Extern,
@@ -238,10 +241,13 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
         } else {
             debug_assert!(a_type.dyncast_array().is_some());
             debug_assert!(b_type.dyncast_array().is_some());
+            if a_type != b_type {
+                b = self.gcc_int_cast(b, a_type);
+            }
             let sign = if signed { "" } else { "u" };
             let func_name = format!("__{}{}ti3", sign, operation_name);
             let param_a = self.context.new_parameter(self.location, a_type, "a");
-            let param_b = self.context.new_parameter(self.location, b_type, "b");
+            let param_b = self.context.new_parameter(self.location, a_type, "b");
             let func = self.context.new_function(
                 self.location,
                 FunctionType::Extern,
@@ -470,7 +476,10 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
                     lhs_high = self.context.new_cast(self.location, lhs_high, signed_type);
                     rhs_high = self.context.new_cast(self.location, rhs_high, signed_type);
                 }
-                IntPredicate::IntEQ | IntPredicate::IntNE => (),
+                IntPredicate::IntEQ | IntPredicate::IntNE => {
+                    lhs_high = self.context.new_cast(self.location, lhs_high, unsigned_type);
+                    rhs_high = self.context.new_cast(self.location, rhs_high, unsigned_type);
+                }
             }
 
             let condition = self.context.new_comparison(
@@ -637,6 +646,9 @@ impl<'a, 'gcc, 'tcx> Builder<'a, 'gcc, 'tcx> {
             }
             a ^ b
         } else {
+            if a_type != b_type {
+                b = self.gcc_int_cast(b, a_type);
+            }
             self.concat_low_high_rvalues(
                 a_type,
                 self.low(a) ^ self.low(b),
@@ -846,6 +858,9 @@ impl<'gcc, 'tcx> CodegenCx<'gcc, 'tcx> {
                 !a_native && !b_native,
                 "both types should either be native or non-native for or operation"
             );
+            if a_type != b_type {
+                b = self.gcc_int_cast(b, a_type);
+            }
             let native_int_type = a_type.dyncast_array().expect("get element type");
             self.concat_low_high_rvalues(
                 a_type,