diff --git a/docs/testing.rst b/docs/testing.rst
index 9a59f2c7be..f58a413033 100644
--- a/docs/testing.rst
+++ b/docs/testing.rst
@@ -336,3 +336,11 @@ that instruction is compared to the directive::
         [R#200c,%x8]        v11 = isub v1, v2       ; bin: 40628433
         return
     }
+
+If any instructions are unencoded (indicated with a `[-]` encoding field), they
+will be encoded using the same mechanism as the legalizer uses. However,
+illegal instructions for the ISA won't be expanded into other instruction
+sequences. Instead the test will fail.
+
+Value locations must be present if they are required to compute the binary
+bits. Missing value locations will cause the test to crash.
diff --git a/filetests/isa/riscv/binary32.cton b/filetests/isa/riscv/binary32.cton
index 0d2805681a..c2f117f5da 100644
--- a/filetests/isa/riscv/binary32.cton
+++ b/filetests/isa/riscv/binary32.cton
@@ -6,7 +6,7 @@ function int32() {
 ebb0:
     [-,%x5]             v1 = iconst.i32 1
     [-,%x6]             v2 = iconst.i32 2
-    [R#0c,%x7]          v10 = iadd v1, v2       ; bin: 006283b3
+    [-,%x7]             v10 = iadd v1, v2       ; bin: 006283b3
     [R#200c,%x8]        v11 = isub v1, v2       ; bin: 40628433
     [R#10c]             v12 = imul v1, v2
     return
diff --git a/src/filetest/binemit.rs b/src/filetest/binemit.rs
index 2c6a725431..16c2613b20 100644
--- a/src/filetest/binemit.rs
+++ b/src/filetest/binemit.rs
@@ -3,7 +3,7 @@
 //! The `binemit` test command generates binary machine code for every instruction in the input
 //! functions and compares the results to the expected output.
 
-use std::borrow::{Borrow, Cow};
+use std::borrow::Cow;
 use std::fmt::Write;
 use cretonne::binemit;
 use cretonne::ir;
@@ -60,7 +60,7 @@ impl SubTest for TestBinEmit {
     }
 
     fn is_mutating(&self) -> bool {
-        false
+        true
     }
 
     fn needs_isa(&self) -> bool {
@@ -71,7 +71,7 @@ impl SubTest for TestBinEmit {
         let isa = context.isa.expect("binemit needs an ISA");
         // TODO: Run a verifier pass over the code first to detect any bad encodings or missing/bad
         // value locations. The current error reporting is just crashing...
-        let func = func.borrow();
+        let mut func = func.into_owned();
 
         let mut sink = TextSink { text: String::new() };
 
@@ -85,6 +85,22 @@ impl SubTest for TestBinEmit {
                                            comment.text))
                     }
                 };
+
+                // Compute an encoding for `inst` if one wasn't provided.
+                if !func.encodings
+                        .get(inst)
+                        .map(|e| e.is_legal())
+                        .unwrap_or(false) {
+                    match isa.encode(&func.dfg, &func.dfg[inst]) {
+                        Ok(enc) => *func.encodings.ensure(inst) = enc,
+                        Err(_) => {
+                            return Err(format!("{} can't be encoded: {}",
+                                               inst,
+                                               func.dfg.display_inst(inst)))
+                        }
+                    }
+                }
+
                 sink.text.clear();
                 isa.emit_inst(&func, inst, &mut sink);
                 let have = sink.text.trim();