diff --git a/cranelift/isle/isle/build.rs b/cranelift/isle/isle/build.rs
index d3081c8536..6209dcc216 100644
--- a/cranelift/isle/isle/build.rs
+++ b/cranelift/isle/isle/build.rs
@@ -12,6 +12,7 @@ fn main() {
     emit_tests(&mut out, "isle_examples/pass", "run_pass");
     emit_tests(&mut out, "isle_examples/fail", "run_fail");
     emit_tests(&mut out, "isle_examples/link", "run_link");
+    emit_tests(&mut out, "isle_examples/run", "run_run");
 
     let output = out_dir.join("isle_tests.rs");
     std::fs::write(output, out).unwrap();
diff --git a/cranelift/isle/isle/isle_examples/run/iconst.isle b/cranelift/isle/isle/isle_examples/run/iconst.isle
new file mode 100644
index 0000000000..ba407b9883
--- /dev/null
+++ b/cranelift/isle/isle/isle_examples/run/iconst.isle
@@ -0,0 +1,17 @@
+(type i64 (primitive i64))
+
+(decl X (i64) i64)
+(rule (X -1) -2)
+(rule (X -2) -3)
+(rule (X 0x7fff_ffff_ffff_ffff) 0x8000_0000_0000_0000)
+(rule (X 0xffff_ffff_ffff_fff0) 1)
+
+(type i128 (primitive i128))
+
+(decl Y (i128) i128)
+
+(rule (Y 0x1000_0000_0000_0000_1234_5678_9abc_def0) -1)
+(rule (Y 0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff) 3)
+
+(rule (Y -0x1000_0000_0000_0000_1234_5678_9abc_def0) 1)
+(rule (Y -0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff) -3)
diff --git a/cranelift/isle/isle/isle_examples/run/iconst_main.rs b/cranelift/isle/isle/isle_examples/run/iconst_main.rs
new file mode 100644
index 0000000000..abce597a33
--- /dev/null
+++ b/cranelift/isle/isle/isle_examples/run/iconst_main.rs
@@ -0,0 +1,18 @@
+mod iconst;
+
+struct Context;
+impl iconst::Context for Context {}
+
+fn main() {
+    let mut ctx = Context;
+
+    assert_eq!(iconst::constructor_X(&mut ctx, -1), Some(-2));
+    assert_eq!(iconst::constructor_X(&mut ctx, -2), Some(-3));
+    assert_eq!(iconst::constructor_X(&mut ctx, 0x7fff_ffff_ffff_ffff), Some(0x8000_0000_0000_0000u64 as i64));
+    assert_eq!(iconst::constructor_X(&mut ctx, 0xffff_ffff_ffff_fff0_u64 as i64), Some(1));
+
+    assert_eq!(iconst::constructor_Y(&mut ctx, 0x1000_0000_0000_0000_1234_5678_9abc_def0), Some(-1));
+    assert_eq!(iconst::constructor_Y(&mut ctx, 0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffffu128 as i128), Some(3));
+    assert_eq!(iconst::constructor_Y(&mut ctx, -0x1000_0000_0000_0000_1234_5678_9abc_def0), Some(1));
+    assert_eq!(iconst::constructor_Y(&mut ctx, -(0xffff_ffff_ffff_ffff_ffff_ffff_ffff_ffffu128 as i128)), Some(-3));
+}
diff --git a/cranelift/isle/isle/src/ast.rs b/cranelift/isle/isle/src/ast.rs
index 51ebdf23c7..5882f21331 100644
--- a/cranelift/isle/isle/src/ast.rs
+++ b/cranelift/isle/isle/src/ast.rs
@@ -121,7 +121,7 @@ pub enum Pattern {
         pos: Pos,
     },
     /// An operator that matches a constant integer value.
-    ConstInt { val: i64, pos: Pos },
+    ConstInt { val: i128, pos: Pos },
     /// An operator that matches an external constant value.
     ConstPrim { val: Ident, pos: Pos },
     /// An application of a type variant or term.
@@ -306,7 +306,7 @@ pub enum Expr {
     /// A variable use.
     Var { name: Ident, pos: Pos },
     /// A constant integer.
-    ConstInt { val: i64, pos: Pos },
+    ConstInt { val: i128, pos: Pos },
     /// A constant of some other primitive type.
     ConstPrim { val: Ident, pos: Pos },
     /// The `(let ((var ty val)*) body)` form.
diff --git a/cranelift/isle/isle/src/codegen.rs b/cranelift/isle/isle/src/codegen.rs
index c94c0e97cd..63219efe42 100644
--- a/cranelift/isle/isle/src/codegen.rs
+++ b/cranelift/isle/isle/src/codegen.rs
@@ -261,7 +261,7 @@ impl<'a> Codegen<'a> {
         ctx.values.insert(value.clone(), (is_ref, ty));
     }
 
-    fn const_int(&self, val: i64, ty: TypeId) -> String {
+    fn const_int(&self, val: i128, ty: TypeId) -> String {
         let is_bool = match &self.typeenv.types[ty.index()] {
             &Type::Primitive(_, name, _) => &self.typeenv.syms[name.index()] == "bool",
             _ => unreachable!(),
@@ -269,7 +269,12 @@ impl<'a> Codegen<'a> {
         if is_bool {
             format!("{}", val != 0)
         } else {
-            format!("{}", val)
+            let ty_name = self.type_name(ty, /* by_ref = */ false);
+            if ty_name == "i128" {
+                format!("{}i128", val)
+            } else {
+                format!("{}i128 as {}", val, ty_name)
+            }
         }
     }
 
@@ -523,10 +528,14 @@ impl<'a> Codegen<'a> {
                 false
             }
             &PatternInst::MatchInt {
-                ref input, int_val, ..
+                ref input,
+                int_val,
+                ty,
+                ..
             } => {
+                let int_val = self.const_int(int_val, ty);
                 let input = self.value_by_val(input, ctx);
-                writeln!(code, "{}if {} == {} {{", indent, input, int_val).unwrap();
+                writeln!(code, "{}if {} == {}  {{", indent, input, int_val).unwrap();
                 false
             }
             &PatternInst::MatchPrim { ref input, val, .. } => {
diff --git a/cranelift/isle/isle/src/ir.rs b/cranelift/isle/isle/src/ir.rs
index 2cf50d63fc..cb58cc1749 100644
--- a/cranelift/isle/isle/src/ir.rs
+++ b/cranelift/isle/isle/src/ir.rs
@@ -49,7 +49,7 @@ pub enum PatternInst {
         /// The value's type.
         ty: TypeId,
         /// The integer to match against the value.
-        int_val: i64,
+        int_val: i128,
     },
 
     /// Try matching the given value as the given constant. Produces no values.
@@ -128,7 +128,7 @@ pub enum ExprInst {
         /// This integer type.
         ty: TypeId,
         /// The integer value. Must fit within the type.
-        val: i64,
+        val: i128,
     },
 
     /// Produce a constant extern value.
@@ -222,7 +222,7 @@ impl ExprSequence {
     /// Is this expression sequence producing a constant integer?
     ///
     /// If so, return the integer type and the constant.
-    pub fn is_const_int(&self) -> Option<(TypeId, i64)> {
+    pub fn is_const_int(&self) -> Option<(TypeId, i128)> {
         if self.insts.len() == 2 && matches!(&self.insts[1], &ExprInst::Return { .. }) {
             match &self.insts[0] {
                 &ExprInst::ConstInt { ty, val } => Some((ty, val)),
@@ -266,7 +266,7 @@ impl PatternSequence {
         self.add_inst(PatternInst::MatchEqual { a, b, ty });
     }
 
-    fn add_match_int(&mut self, input: Value, ty: TypeId, int_val: i64) {
+    fn add_match_int(&mut self, input: Value, ty: TypeId, int_val: i128) {
         self.add_inst(PatternInst::MatchInt { input, ty, int_val });
     }
 
@@ -486,7 +486,7 @@ impl ExprSequence {
         id
     }
 
-    fn add_const_int(&mut self, ty: TypeId, val: i64) -> Value {
+    fn add_const_int(&mut self, ty: TypeId, val: i128) -> Value {
         let inst = InstId(self.insts.len());
         self.add_inst(ExprInst::ConstInt { ty, val });
         Value::Expr { inst, output: 0 }
diff --git a/cranelift/isle/isle/src/lexer.rs b/cranelift/isle/isle/src/lexer.rs
index 9a8a07e880..058ddf7765 100644
--- a/cranelift/isle/isle/src/lexer.rs
+++ b/cranelift/isle/isle/src/lexer.rs
@@ -68,7 +68,7 @@ pub enum Token {
     /// A symbol, e.g. `Foo`.
     Symbol(String),
     /// An integer.
-    Int(i64),
+    Int(i128),
     /// `@`
     At,
 }
@@ -252,7 +252,8 @@ impl<'a> Lexer<'a> {
 
                 // Check for hex literals.
                 if self.buf.get(self.pos.offset).copied() == Some(b'0')
-                    && self.buf.get(self.pos.offset + 1).copied() == Some(b'x')
+                    && (self.buf.get(self.pos.offset + 1).copied() == Some(b'x')
+                        || self.buf.get(self.pos.offset + 1).copied() == Some(b'X'))
                 {
                     self.advance_pos();
                     self.advance_pos();
@@ -262,7 +263,7 @@ impl<'a> Lexer<'a> {
                 // Find the range in the buffer for this integer literal. We'll
                 // pass this range to `i64::from_str_radix` to do the actual
                 // string-to-integer conversion.
-                let start_offset = self.pos.offset;
+                let mut s = vec![];
                 while self.pos.offset < self.buf.len()
                     && ((radix == 10
                         && self.buf[self.pos.offset] >= b'0'
@@ -273,21 +274,25 @@ impl<'a> Lexer<'a> {
                                 || (self.buf[self.pos.offset] >= b'a'
                                     && self.buf[self.pos.offset] <= b'f')
                                 || (self.buf[self.pos.offset] >= b'A'
-                                    && self.buf[self.pos.offset] <= b'F'))))
+                                    && self.buf[self.pos.offset] <= b'F')))
+                        || self.buf[self.pos.offset] == b'_')
                 {
+                    if self.buf[self.pos.offset] != b'_' {
+                        s.push(self.buf[self.pos.offset]);
+                    }
                     self.advance_pos();
                 }
-                let end_offset = self.pos.offset;
+                let s_utf8 = std::str::from_utf8(&s[..]).unwrap();
 
-                let num = i64::from_str_radix(
-                    std::str::from_utf8(&self.buf[start_offset..end_offset]).unwrap(),
-                    radix,
-                )
-                .map_err(|e| self.error(start_pos, e.to_string()))?;
+                // Support either signed range (-2^127..2^127) or
+                // unsigned range (0..2^128).
+                let num = i128::from_str_radix(s_utf8, radix)
+                    .or_else(|_| u128::from_str_radix(s_utf8, radix).map(|val| val as i128))
+                    .map_err(|e| self.error(start_pos, e.to_string()))?;
 
                 let tok = if neg {
                     Token::Int(num.checked_neg().ok_or_else(|| {
-                        self.error(start_pos, "integer literal cannot fit in i64")
+                        self.error(start_pos, "integer literal cannot fit in i128")
                     })?)
                 } else {
                     Token::Int(num)
diff --git a/cranelift/isle/isle/src/parser.rs b/cranelift/isle/isle/src/parser.rs
index c6b518f00a..495366391b 100644
--- a/cranelift/isle/isle/src/parser.rs
+++ b/cranelift/isle/isle/src/parser.rs
@@ -114,7 +114,7 @@ impl<'a> Parser<'a> {
         }
     }
 
-    fn int(&mut self) -> Result<i64> {
+    fn int(&mut self) -> Result<i128> {
         match self.take(|tok| tok.is_int())? {
             Token::Int(i) => Ok(i),
             _ => unreachable!(),
@@ -391,7 +391,7 @@ impl<'a> Parser<'a> {
                         iflets,
                         expr,
                         pos,
-                        prio,
+                        prio: prio.map(|prio| i64::try_from(prio).unwrap()),
                     });
                 }
             }
diff --git a/cranelift/isle/isle/src/sema.rs b/cranelift/isle/isle/src/sema.rs
index db075d25f3..9d599de873 100644
--- a/cranelift/isle/isle/src/sema.rs
+++ b/cranelift/isle/isle/src/sema.rs
@@ -449,7 +449,7 @@ pub enum Pattern {
 
     /// Match the current value against a constant integer of the given integer
     /// type.
-    ConstInt(TypeId, i64),
+    ConstInt(TypeId, i128),
 
     /// Match the current value against a constant primitive value of the given
     /// primitive type.
@@ -474,7 +474,7 @@ pub enum Expr {
     /// Get the value of a variable that was bound in the left-hand side.
     Var(TypeId, VarId),
     /// Get a constant integer.
-    ConstInt(TypeId, i64),
+    ConstInt(TypeId, i128),
     /// Get a constant primitive.
     ConstPrim(TypeId, Sym),
     /// Evaluate the nested expressions and bind their results to the given
diff --git a/cranelift/isle/isle/tests/run_tests.rs b/cranelift/isle/isle/tests/run_tests.rs
index 2447b45117..bbed96f372 100644
--- a/cranelift/isle/isle/tests/run_tests.rs
+++ b/cranelift/isle/isle/tests/run_tests.rs
@@ -18,7 +18,7 @@ pub fn run_fail(filename: &str) {
     assert!(build(filename).is_err());
 }
 
-pub fn run_link(isle_filename: &str) {
+fn build_and_link_isle(isle_filename: &str) -> (tempfile::TempDir, std::path::PathBuf) {
     let tempdir = tempfile::tempdir().unwrap();
     let code = build(isle_filename).unwrap();
 
@@ -45,10 +45,27 @@ pub fn run_link(isle_filename: &str) {
     let mut rustc = std::process::Command::new("rustc")
         .arg(&rust_driver)
         .arg("-o")
-        .arg(output)
+        .arg(output.clone())
         .spawn()
         .unwrap();
     assert!(rustc.wait().unwrap().success());
+
+    (tempdir, output)
+}
+
+pub fn run_link(isle_filename: &str) {
+    build_and_link_isle(isle_filename);
+}
+
+pub fn run_run(isle_filename: &str) {
+    let (_tempdir, exe) = build_and_link_isle(isle_filename);
+
+    assert!(std::process::Command::new(exe)
+        .spawn()
+        .unwrap()
+        .wait()
+        .unwrap()
+        .success());
 }
 
 // Generated by build.rs.