cranelift-assembler-x64: add Intel XED as a fuzzer disassembler oracle

.github/workflows/main.yml

@@ -717,6 +717,12 @@ jobs:
       env:
         RUSTFLAGS: "--cfg arc_try_new"
 
+    # Check the x64 assembler against the Intel XED disassembler, which knows
+    # about newer encodings that the bundled Capstone cannot decode. This is
+    # opt-in via `fuzz-xed` because XED is built from source; the C compiler and
+    # Python that requires are already present on this runner.
+    - run: cargo test -p cranelift-assembler-x64 --features fuzz-xed
+
     # Smoke test the coverage script.
     - run: rustup target add wasm32-unknown-unknown
     - run: rustup component add llvm-tools
@@ -735,7 +741,7 @@ jobs:
     - uses: ./.github/actions/apt-get-install
       with:
         packages: ocaml-nox ocamlbuild ocaml-findlib libzarith-ocaml-dev
-    - run: cargo fuzz check --dev
+    - run: cargo fuzz check --dev --features fuzz-xed
     - run: cargo fuzz check --dev --fuzz-dir ./cranelift/isle/fuzz
     - run: cargo fuzz check --dev --fuzz-dir ./crates/environ/fuzz --features component-model
     - run: cargo fuzz check --dev --fuzz-dir ./cranelift/assembler-x64/fuzz

Cargo.toml

@@ -410,6 +410,7 @@ clap = { version = "4.5.48", default-features = false, features = ["std", "deriv
 clap_complete = "4.5.58"
 hashbrown = { version = "0.17", default-features = false }
 capstone = { version = "0.14.0", default-features = false, features = ['full', 'arch_x86', 'arch_riscv', 'arch_arm64', 'arch_sysz'] }
+xed-sys = { version = "0.6" }
 smallvec = { version = "1.15.1", features = ["union"] }
 tracing = { version = "0.1.41", default-features = false }
 bitflags = "2.9.4"

cranelift/assembler-x64/Cargo.toml

@@ -10,6 +10,15 @@ rust-version.workspace = true
 arbitrary = { workspace = true, features = ["derive"], optional = true }
 capstone = { workspace = true, optional = true }
 
+# XED is built from C source by its own Python build system, which is picky
+# about its environment: it only recognizes x86 host CPUs, and it invokes LLVM
+# toolchain binaries (e.g. `llvm-ar`) that are not present on every runner.
+# Restrict it to x86_64 Linux -- the only configuration where the oracle
+# actually runs, i.e. the `fuzz-xed` CI job and OSS-Fuzz -- so that
+# `--all-features` builds keep working elsewhere (see `ci/run-tests.py`).
+[target.'cfg(all(target_arch = "x86_64", target_os = "linux"))'.dependencies]
+xed-sys = { workspace = true, optional = true }
+
 [dev-dependencies]
 arbitrary = { workspace = true, features = ["derive"] }
 arbtest = { workspace = true }
@@ -23,3 +32,8 @@ workspace = true
 
 [features]
 fuzz = ['dep:arbitrary', 'dep:capstone']
+# Adds Intel XED as a second, optional disassembler oracle for the roundtrip
+# fuzzer. This is additive on top of `fuzz` (Capstone remains the default
+# oracle); XED is only built when this feature is explicitly enabled, and only
+# on x86_64 Linux. Note that building XED requires Python 3.9+ and a C compiler.
+fuzz-xed = ['fuzz', 'dep:xed-sys']