diff --git a/cranelift/codegen/src/isle_prelude.rs b/cranelift/codegen/src/isle_prelude.rs
index 26788399c7..0e3e293c1f 100644
--- a/cranelift/codegen/src/isle_prelude.rs
+++ b/cranelift/codegen/src/isle_prelude.rs
@@ -140,33 +140,33 @@ macro_rules! isle_common_prelude_methods {
         }
 
         #[inline]
-        fn u64_sextend_u32(&mut self, x: u64) -> u64 {
-            x as u32 as i32 as i64 as u64
+        fn i64_sextend_imm64(&mut self, ty: Type, mut x: Imm64) -> i64 {
+            x.sign_extend_from_width(ty.bits());
+            x.bits()
         }
 
         #[inline]
-        fn u64_uextend_u32(&mut self, x: u64) -> u64 {
-            x & 0xffff_ffff
+        fn u64_uextend_imm64(&mut self, ty: Type, x: Imm64) -> u64 {
+            (x.bits() as u64) & self.ty_mask(ty)
         }
 
         #[inline]
         fn imm64_icmp(&mut self, ty: Type, cc: &IntCC, x: Imm64, y: Imm64) -> Imm64 {
-            let shift = u32::checked_sub(64, ty.bits()).unwrap_or(0);
-            let mask = u64::MAX >> shift;
-            let x = (x.bits() as u64) & mask;
-            let y = (y.bits() as u64) & mask;
-            let sext = |v| ((v << shift) as i64) >> shift;
+            let ux = self.u64_uextend_imm64(ty, x);
+            let uy = self.u64_uextend_imm64(ty, y);
+            let sx = self.i64_sextend_imm64(ty, x);
+            let sy = self.i64_sextend_imm64(ty, y);
             let result = match cc {
-                IntCC::Equal => x == y,
-                IntCC::NotEqual => x != y,
-                IntCC::UnsignedGreaterThanOrEqual => x >= y,
-                IntCC::UnsignedGreaterThan => x > y,
-                IntCC::UnsignedLessThanOrEqual => x <= y,
-                IntCC::UnsignedLessThan => x < y,
-                IntCC::SignedGreaterThanOrEqual => sext(x) >= sext(y),
-                IntCC::SignedGreaterThan => sext(x) > sext(y),
-                IntCC::SignedLessThanOrEqual => sext(x) <= sext(y),
-                IntCC::SignedLessThan => sext(x) < sext(y),
+                IntCC::Equal => ux == uy,
+                IntCC::NotEqual => ux != uy,
+                IntCC::UnsignedGreaterThanOrEqual => ux >= uy,
+                IntCC::UnsignedGreaterThan => ux > uy,
+                IntCC::UnsignedLessThanOrEqual => ux <= uy,
+                IntCC::UnsignedLessThan => ux < uy,
+                IntCC::SignedGreaterThanOrEqual => sx >= sy,
+                IntCC::SignedGreaterThan => sx > sy,
+                IntCC::SignedLessThanOrEqual => sx <= sy,
+                IntCC::SignedLessThan => sx < sy,
             };
             Imm64::new(result.into())
         }
@@ -586,12 +586,7 @@ macro_rules! isle_common_prelude_methods {
 
         #[inline]
         fn imm64_masked(&mut self, ty: Type, x: u64) -> Imm64 {
-            debug_assert!(ty.bits() <= 64);
-            // Careful: we can't do `(1 << bits) - 1` because that
-            // would overflow for `bits == 64`. Instead,
-            // right-shift an all-ones mask.
-            let mask = u64::MAX >> (64 - ty.bits());
-            Imm64::new((x & mask) as i64)
+            Imm64::new((x & self.ty_mask(ty)) as i64)
         }
 
         #[inline]
diff --git a/cranelift/codegen/src/opts/algebraic.isle b/cranelift/codegen/src/opts/algebraic.isle
index 3b6b067a9a..d889fd8faa 100644
--- a/cranelift/codegen/src/opts/algebraic.isle
+++ b/cranelift/codegen/src/opts/algebraic.isle
@@ -6,12 +6,6 @@
 ;; than a piece of the input or a new node that we construct; but we
 ;; can freely rewrite e.g. `x+y-y` to `x`.
 
-;; uextend/sextend of a constant.
-(rule (simplify (uextend $I64 (iconst $I32 (u64_from_imm64 imm))))
-      (iconst $I64 (imm64 (u64_uextend_u32 imm))))
-(rule (simplify (sextend $I64 (iconst $I32 (u64_from_imm64 imm))))
-      (iconst $I64 (imm64 (u64_sextend_u32 imm))))
-
 ;; x+0 == 0+x == x.
 (rule (simplify (iadd ty
                       x
diff --git a/cranelift/codegen/src/opts/cprop.isle b/cranelift/codegen/src/opts/cprop.isle
index ee843087e2..7c58ee6ad6 100644
--- a/cranelift/codegen/src/opts/cprop.isle
+++ b/cranelift/codegen/src/opts/cprop.isle
@@ -70,6 +70,12 @@
                       (iconst _ k2)))
       (subsume (iconst ty (imm64_sshr ty k1 k2))))
 
+(rule (simplify (uextend wide (iconst narrow imm)))
+      (subsume (iconst wide (imm64 (u64_uextend_imm64 narrow imm)))))
+
+(rule (simplify (sextend wide (iconst narrow imm)))
+      (subsume (iconst wide (imm64_masked wide (i64_as_u64 (i64_sextend_imm64 narrow imm))))))
+
 (rule (simplify
        (icmp result_ty
             cc
diff --git a/cranelift/codegen/src/prelude.isle b/cranelift/codegen/src/prelude.isle
index 6f603ff63b..701f414a92 100644
--- a/cranelift/codegen/src/prelude.isle
+++ b/cranelift/codegen/src/prelude.isle
@@ -144,11 +144,11 @@
 (decl pure u64_eq (u64 u64) bool)
 (extern constructor u64_eq u64_eq)
 
-(decl pure u64_sextend_u32 (u64) u64)
-(extern constructor u64_sextend_u32 u64_sextend_u32)
+(decl pure i64_sextend_imm64 (Type Imm64) i64)
+(extern constructor i64_sextend_imm64 i64_sextend_imm64)
 
-(decl pure u64_uextend_u32 (u64) u64)
-(extern constructor u64_uextend_u32 u64_uextend_u32)
+(decl pure u64_uextend_imm64 (Type Imm64) u64)
+(extern constructor u64_uextend_imm64 u64_uextend_imm64)
 
 (decl pure imm64_icmp (Type IntCC Imm64 Imm64) Imm64)
 (extern constructor imm64_icmp imm64_icmp)