diff --git a/fuzz/Cargo.toml b/fuzz/Cargo.toml
index 3705e031e644..eb6f929dccf4 100644
--- a/fuzz/Cargo.toml
+++ b/fuzz/Cargo.toml
@@ -9,7 +9,7 @@ publish = false
 cargo-fuzz = true
 
 [dependencies]
-cranelift-codegen = { path = "../cranelift/codegen" }
+cranelift-codegen = { path = "../cranelift/codegen", features = ["incremental-cache"] }
 cranelift-reader = { path = "../cranelift/reader" }
 cranelift-wasm = { path = "../cranelift/wasm" }
 cranelift-filetests = { path = "../cranelift/filetests" }
diff --git a/fuzz/fuzz_targets/cranelift-icache.rs b/fuzz/fuzz_targets/cranelift-icache.rs
index 95d6b80003a8..3debeff58199 100644
--- a/fuzz/fuzz_targets/cranelift-icache.rs
+++ b/fuzz/fuzz_targets/cranelift-icache.rs
@@ -1,13 +1,18 @@
 #![no_main]
 
-use cranelift_codegen::{incremental_cache as icache, isa, settings, Context};
+use cranelift_codegen::{
+    cursor::{Cursor, FuncCursor},
+    incremental_cache as icache,
+    ir::{self, immediates::Imm64, ExternalName},
+    isa, settings, Context,
+};
 use libfuzzer_sys::fuzz_target;
 
 use cranelift_fuzzgen::*;
 use target_lexicon::Triple;
 
 fuzz_target!(|func: SingleFunction| {
-    let func = func.0;
+    let mut func = func.0;
 
     let flags = settings::Flags::new(settings::builder());
 
@@ -29,30 +34,107 @@ fuzz_target!(|func: SingleFunction| {
         Ok(info) => info,
         Err(_) => return,
     };
+    let prev_result = context.mach_compile_result.unwrap();
 
-    let serialized = icache::serialize_compiled(
-        cache_key.clone(),
-        &func,
-        context.mach_compile_result.as_ref().unwrap(),
-    )
-    .expect("serialization failure");
-
-    let mut prev_assembly = vec![0; prev_info.total_size as usize];
-    unsafe {
-        context.emit_to_memory(prev_assembly.as_mut_ptr());
-    }
+    let serialized = icache::serialize_compiled(cache_key.clone(), &func, &prev_result)
+        .expect("serialization failure");
 
     let new_result = icache::try_finish_recompile(&cache_key, &func, &serialized)
         .expect("recompilation should always work for identity");
-    let new_info = new_result.code_info();
 
+    assert_eq!(new_result, prev_result, "MachCompileResult:s don't match");
+
+    let new_info = new_result.code_info();
     assert_eq!(new_info, prev_info, "CodeInfo:s don't match");
 
-    context.mach_compile_result = Some(new_result);
-    let mut new_assembly = vec![0; new_info.total_size as usize];
-    unsafe {
-        context.emit_to_memory(new_assembly.as_mut_ptr());
+    // If the func has at least one user-defined func ref, change it to match a
+    // different external function.
+    let expect_cache_hit = if let Some((func_ref, namespace, index)) =
+        func.dfg.ext_funcs.iter().find_map(|(func_ref, data)| {
+            if let ExternalName::User { namespace, index } = &data.name {
+                Some((func_ref, *namespace, *index))
+            } else {
+                None
+            }
+        }) {
+        func.dfg.ext_funcs[func_ref].name = ExternalName::User {
+            namespace: namespace.checked_add(1).unwrap_or(namespace - 1),
+            index: index.checked_add(1).unwrap_or(index - 1),
+        };
+        true
+    } else {
+        // otherwise just randomly change one instruction in the middle and see what happens.
+        let mut changed = false;
+        let mut cursor = FuncCursor::new(&mut func);
+        'out: while let Some(_block) = cursor.next_block() {
+            while let Some(inst) = cursor.next_inst() {
+                // It's impractical to do any replacement at this point. Try to find any
+                // instruction that returns one int value, and replace it with an iconst.
+                if cursor.func.dfg.inst_results(inst).len() != 1 {
+                    continue;
+                }
+                let out_ty = cursor
+                    .func
+                    .dfg
+                    .value_type(cursor.func.dfg.first_result(inst));
+                match out_ty {
+                    ir::types::I32 | ir::types::I64 => {}
+                    _ => continue,
+                }
+
+                if let ir::InstructionData::UnaryImm {
+                    opcode: ir::Opcode::Iconst,
+                    imm,
+                } = cursor.func.dfg[inst]
+                {
+                    let imm = imm.bits();
+                    cursor.func.dfg[inst] = ir::InstructionData::UnaryImm {
+                        opcode: ir::Opcode::Iconst,
+                        imm: Imm64::new(imm.checked_add(1).unwrap_or(imm - 1)),
+                    };
+                } else {
+                    cursor.func.dfg[inst] = ir::InstructionData::UnaryImm {
+                        opcode: ir::Opcode::Iconst,
+                        imm: Imm64::new(42),
+                    };
+                }
+
+                changed = true;
+                break 'out;
+            }
+        }
+
+        if !changed {
+            return;
+        }
+
+        // We made it so that there shouldn't be a cache hit.
+        false
+    };
+
+    let (new_cache_key, _cache_key_hash) = icache::compute_cache_key(&*isa, &func);
+
+    if expect_cache_hit {
+        assert!(cache_key == new_cache_key);
+    } else {
+        assert!(cache_key != new_cache_key);
     }
 
-    assert_eq!(prev_assembly, new_assembly, "assembly buffers don't match");
+    context = Context::for_function(func.clone());
+
+    let after_mutation_info = match context.compile(&*isa) {
+        Ok(info) => info,
+        Err(_) => return,
+    };
+    let after_mutation_result = context.mach_compile_result.unwrap();
+
+    if expect_cache_hit {
+        let after_mutation_result_from_cache =
+            icache::try_finish_recompile(&new_cache_key, &func, &serialized)
+                .expect("recompilation should always work for identity");
+        assert_eq!(after_mutation_result, after_mutation_result_from_cache);
+
+        let new_info = after_mutation_result_from_cache.code_info();
+        assert_eq!(new_info, after_mutation_info, "CodeInfo:s don't match");
+    }
 });