From e46bed73f190484984c42efcb6fae0ccc42da4ee Mon Sep 17 00:00:00 2001
From: Angelo Perez <102934797+Angelopgit@users.noreply.github.com>
Date: Tue, 13 Oct 2026 09:16:00 -0400
Subject: [PATCH] fix(managed): decode the CIL no. prefix (#2241)

The CIL opcode table held five of the six ECMA-335 prefix opcodes and
omitted no. (FE 19 <unsigned int8>, III.2.2). A method body using it was
reported as malformed with "Unsupported CIL opcode 0xfe19", losing its
instruction counts and normalized IL digest.

Decode it with its one-byte check mask and treat it as a prefix, so it
shares an instruction boundary with the opcode it modifies.


Claude-Session: https://claude.ai/code/session_012pRGskbPGjT444CgJfuhhj

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>
---
 src/dotnet/ManagedCilDecoding.test.ts         | 25 +++++++++++++++++++
 src/dotnet/ManagedMemberInstructionDecoder.ts |  2 ++
 2 files changed, 27 insertions(+)

diff --git a/src/dotnet/ManagedCilDecoding.test.ts b/src/dotnet/ManagedCilDecoding.test.ts
index 55a008b72..4ba33f4c9 100644
--- a/src/dotnet/ManagedCilDecoding.test.ts
+++ b/src/dotnet/ManagedCilDecoding.test.ts
@@ -345,6 +345,31 @@ describe("managed exception region prefix boundaries", () => {
     });
   });
 
+  it("decodes the no. prefix and keeps it with its opcode", () => {
+    // ldarg.0; no. nullcheck; ldfld 0x04000001; pop; ret
+    const il = Buffer.from([2, 0xfe, 0x19, 4, 0x7b, 1, 0, 0, 4, 0x26, 0x2a]);
+    expect(
+      managedBodyWithRegion({ il, tryLength: 10, handlerOffset: 10 }),
+    ).toMatchObject({
+      status: "present",
+      issue: null,
+      decoded_instruction_count: 5,
+      opcode_counts: { "ldarg.0": 1, "no.": 1, ldfld: 1, pop: 1, ret: 1 },
+    });
+    expect(
+      managedBodyWithRegion({
+        il,
+        tryOffset: 4,
+        tryLength: 5,
+        handlerOffset: 10,
+      }),
+    ).toMatchObject({
+      status: "malformed",
+      issue:
+        "Exception clause try range does not align with CIL instruction boundaries",
+    });
+  });
+
   it.each([
     ["start", { tryOffset: 5, tryLength: 1, handlerOffset: 6 }],
     [
diff --git a/src/dotnet/ManagedMemberInstructionDecoder.ts b/src/dotnet/ManagedMemberInstructionDecoder.ts
index b05dd083f..01c2b7814 100644
--- a/src/dotnet/ManagedMemberInstructionDecoder.ts
+++ b/src/dotnet/ManagedMemberInstructionDecoder.ts
@@ -12,6 +12,7 @@ const PREFIX_OPCODES = new Set([
   "tail.",
   "constrained.",
   "readonly.",
+  "no.",
 ]);
 
 const OPCODES = new Map<
@@ -232,6 +233,7 @@ const OPCODES = new Map<
   [0xfe16, { name: "constrained.", operand: "type" }],
   [0xfe17, { name: "cpblk", operand: "none" }],
   [0xfe18, { name: "initblk", operand: "none" }],
+  [0xfe19, { name: "no.", operand: "short-i" }],
   [0xfe1a, { name: "rethrow", operand: "none" }],
   [0xfe1c, { name: "sizeof", operand: "type" }],
   [0xfe1d, { name: "refanytype", operand: "none" }],