diff --git a/bindings/rust/build.rs b/bindings/rust/build.rs
index ad1338f2..0b6e6a6d 100644
--- a/bindings/rust/build.rs
+++ b/bindings/rust/build.rs
@@ -116,7 +116,11 @@ fn main() {
         .flag_if_supported("-Wno-unused-function")
         .flag_if_supported("-Wno-unused-command-line-argument");
     if !cfg!(debug_assertions) {
-        cc.opt_level(2);
+        if target_arch.eq("s390x") {
+            cc.opt_level(3);
+        } else {
+            cc.opt_level(2);
+        }
     }
     cc.files(&file_vec).compile("libblst.a");
 
diff --git a/src/no_asm.h b/src/no_asm.h
index 4f12f537..59e49263 100644
--- a/src/no_asm.h
+++ b/src/no_asm.h
@@ -6,6 +6,8 @@
 
 #if LIMB_T_BITS==32
 typedef unsigned long long llimb_t;
+#else
+typedef unsigned __int128 llimb_t;
 #endif
 
 #if defined(__clang__)
diff --git a/src/vect.h b/src/vect.h
index 11b5836a..1788cfcd 100644
--- a/src/vect.h
+++ b/src/vect.h
@@ -18,7 +18,7 @@ typedef unsigned long long limb_t;
 typedef unsigned __int64 limb_t;
 # define LIMB_T_BITS    64
 
-#elif defined(__BLST_NO_ASM__) || defined(__wasm64__)
+#elif defined(__wasm64__)
 typedef unsigned int limb_t;
 # define LIMB_T_BITS    32
 # ifndef __BLST_NO_ASM__
@@ -31,8 +31,10 @@ typedef unsigned long limb_t;
 #   define LIMB_T_BITS   64
 #  else
 #   define LIMB_T_BITS   32
-#   define __BLST_NO_ASM__
 #  endif
+# ifndef __BLST_NO_ASM__
+#  define __BLST_NO_ASM__
+# endif
 #endif
 
 /*