diff --git a/cranelift/codegen/meta/src/cdsl/type_inference.rs b/cranelift/codegen/meta/src/cdsl/type_inference.rs
index 9852dd1e7b..0313345175 100644
--- a/cranelift/codegen/meta/src/cdsl/type_inference.rs
+++ b/cranelift/codegen/meta/src/cdsl/type_inference.rs
@@ -161,8 +161,16 @@ impl TypeEnvironment {
 
         // Check extra conditions for InTypeset constraints.
         if let Constraint::InTypeset(tv, _) = &constraint {
-            assert!(tv.base.is_none());
-            assert!(tv.name.starts_with("typeof_"));
+            assert!(
+                tv.base.is_none(),
+                "type variable is {:?}, while expecting none",
+                tv
+            );
+            assert!(
+                tv.name.starts_with("typeof_"),
+                "Name \"{}\" should start with \"typeof_\"",
+                tv.name
+            );
         }
 
         self.constraints.push(constraint);
diff --git a/cranelift/codegen/meta/src/shared/legalize.rs b/cranelift/codegen/meta/src/shared/legalize.rs
index 2f20e5a613..7c7b121ab4 100644
--- a/cranelift/codegen/meta/src/shared/legalize.rs
+++ b/cranelift/codegen/meta/src/shared/legalize.rs
@@ -207,6 +207,28 @@ pub(crate) fn define(insts: &InstructionGroup, imm: &Immediates) -> TransformGro
     // embedded as part of arguments), so use a custom legalization for now.
     narrow.custom_legalize(iconst, "narrow_iconst");
 
+    {
+        let inst = uextend.bind(I128).bind(I64);
+        narrow.legalize(
+            def!(a = inst(x)),
+            vec![
+                def!(ah = iconst(Literal::constant(&imm.imm64, 0))),
+                def!(a = iconcat(x, ah)),
+            ],
+        );
+    }
+
+    {
+        let inst = sextend.bind(I128).bind(I64);
+        narrow.legalize(
+            def!(a = inst(x)),
+            vec![
+                def!(ah = sshr_imm(x, Literal::constant(&imm.imm64, 63))), // splat sign bit to whole number
+                def!(a = iconcat(x, ah)),
+            ],
+        );
+    }
+
     for &bin_op in &[band, bor, bxor, band_not, bor_not, bxor_not] {
         narrow.legalize(
             def!(a = bin_op(x, y)),
diff --git a/cranelift/filetests/filetests/isa/x86/extend-i128-run.clif b/cranelift/filetests/filetests/isa/x86/extend-i128-run.clif
new file mode 100644
index 0000000000..c2550b8f4e
--- /dev/null
+++ b/cranelift/filetests/filetests/isa/x86/extend-i128-run.clif
@@ -0,0 +1,26 @@
+test run
+target x86_64
+
+function u0:0() -> b1 {
+ebb0:
+    v0 = iconst.i64 0xffff_ffff_eeee_0000
+    v1 = uextend.i128 v0
+    v2, v3 = isplit v1
+    v4 = icmp_imm eq v2, 0xffff_ffff_eeee_0000
+    v5 = icmp_imm eq v3, 0
+    v6 = band v4, v5
+    return v6
+}
+; run
+
+function u0:1() -> b1 {
+ebb0:
+    v0 = iconst.i64 0xffff_ffff_eeee_0000
+    v1 = sextend.i128 v0
+    v2, v3 = isplit v1
+    v4 = icmp_imm eq v2, 0xffff_ffff_eeee_0000
+    v5 = icmp_imm eq v3, 0xffff_ffff_ffff_ffff
+    v6 = band v4, v5
+    return v6
+}
+; run
diff --git a/cranelift/filetests/filetests/isa/x86/extend-i128.clif b/cranelift/filetests/filetests/isa/x86/extend-i128.clif
new file mode 100644
index 0000000000..0d9e4c8aa9
--- /dev/null
+++ b/cranelift/filetests/filetests/isa/x86/extend-i128.clif
@@ -0,0 +1,37 @@
+test compile
+target x86_64
+
+function u0:0() -> b1 {
+ebb0:
+    v0 = iconst.i64 0xffff_ffff_eeee_0000
+    ; check: v0 = iconst.i64 0xffff_ffff_eeee_0000
+    ; nextln: v2 -> v0
+    v1 = uextend.i128 v0
+    ; nextln: v7 = iconst.i64 0
+    ; nextln: v3 -> v7
+    ; nextln: v1 = iconcat v0, v7
+
+    v2, v3 = isplit v1
+    v4 = icmp_imm eq v2, 0xffff_ffff_eeee_0000
+    v5 = icmp_imm eq v3, 0
+
+    v6 = band v4, v5
+    return v6
+}
+
+function u0:1() -> b1 {
+ebb0:
+    v0 = iconst.i64 0xffff_ffff_eeee_0000
+    ; check: v0 = iconst.i64 0xffff_ffff_eeee_0000
+    ; nextln: v2 -> v0
+    v1 = sextend.i128 v0
+    ; nextln: v8 = copy v0
+    ; nextln: v7 = sshr_imm v8, 63
+    ; nextln: v3 -> v7
+
+    v2, v3 = isplit v1
+    v4 = icmp_imm eq v2, 0xffff_ffff_eeee_0000
+    v5 = icmp_imm eq v3, 0xffff_ffff_ffff_ffff
+    v6 = band v4, v5
+    return v6
+}
diff --git a/cranelift/filetests/filetests/legalizer/popcnt-i128.clif b/cranelift/filetests/filetests/legalizer/popcnt-i128.clif
index b241c640a9..0ecf7f74c5 100644
--- a/cranelift/filetests/filetests/legalizer/popcnt-i128.clif
+++ b/cranelift/filetests/filetests/legalizer/popcnt-i128.clif
@@ -24,8 +24,8 @@ ebb0:
 ; check: v16, v17 = iadd_ifcout v12, v14
 ; check: v18 = iadd_ifcin v13, v15, v17
 ; check: v11 = iconcat v16, v18
-; check: v4 = uextend.i128 v11
-; check: v19, v20 = isplit v4
-; check: v21, v22 = isplit v19
-; check: v23, v24 = isplit v20
-; check: return v21, v22, v23, v24
+; check: v20 = iconst.i32 0
+; check: v21 = iconst.i32 0
+; check: v19 = iconcat v20, v21
+; check: v4 = iconcat v11, v19
+; check: return v16, v18, v20, v21