diff --git a/bindings/rust/build.rs b/bindings/rust/build.rs
index 44bf2644..511bae38 100644
--- a/bindings/rust/build.rs
+++ b/bindings/rust/build.rs
@@ -231,7 +231,11 @@ fn main() {
         cc.define("SCRATCH_LIMIT", "(45 * 1024)");
     }
     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("blst");
 
diff --git a/src/no_asm.h b/src/no_asm.h
index be7bf47e..802b78f2 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(__STDC_VERSION__) || __STDC_VERSION__<199901 || defined(__STDC_NO_VLA__)
@@ -1155,7 +1157,7 @@ limb_t div_3_limbs(const limb_t div_top[2], limb_t d_lo, limb_t d_hi)
 static limb_t quot_rem_n(limb_t *div_rem, const limb_t *divisor,
                                           limb_t quotient, size_t n)
 {
-    __builtin_assume(n != 0 && n%2 == 0);
+    __builtin_assume(n != 0);
     llimb_t limbx;
     limb_t tmp[n+1], carry, mask, borrow;
     size_t i;
diff --git a/src/vect.h b/src/vect.h
index 19640b11..938a5ffd 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
 
 /*