Use SipHasher rather than SHA-512 for ISLE manifest.

cranelift/codegen/Cargo.toml

@@ -37,7 +37,6 @@ criterion = "0.3"
 cranelift-codegen-meta = { path = "meta", version = "0.79.0" }
 cranelift-isle = { path = "../isle/isle", version = "=0.79.0", optional = true }
 miette = { version = "3", features = ["fancy"], optional = true }
-sha2 = "0.9.8"
 
 [features]
 default = ["std", "unwind"]

cranelift/codegen/build.rs

@@ -16,7 +16,6 @@
 
 use cranelift_codegen_meta as meta;
 
-use sha2::{Digest, Sha512};
 use std::env;
 use std::io::Read;
 use std::process;
@@ -163,7 +162,28 @@ impl IsleCompilation {
     /// `<generated_filename>.manifest` and use it to verify that a
     /// rebuild was done if necessary.
     fn compute_manifest(&self) -> Result<String, Box<dyn std::error::Error + 'static>> {
+        // We use the deprecated SipHasher from std::hash in order to verify
+        // that ISLE sources haven't changed since the generated source was
+        // last regenerated.
+        //
+        // We use this instead of a stronger and more usual content hash, like
+        // SHA-{160,256,512}, because it's built into the standard library and
+        // we don't want to pull in a separate crate. We try to keep Cranelift
+        // crate dependencies as intentionally small as possible. In fact, we
+        // used to use the `sha2` crate for SHA-512 and this turns out to be
+        // undesirable for downstream consumers (see #3609).
+        //
+        // Why not the recommended replacement
+        // `std::collections::hash_map::DefaultHasher`? Because we need the
+        // hash to be deterministic, both between runs (i.e., not seeded with
+        // random state) and across Rust versions.
+        //
+        // If `SipHasher` is ever actually removed from `std`, we'll need to
+        // find a new option, either a very small crate or something else
+        // that's built-in.
+        #![allow(deprecated)]
         use std::fmt::Write;
+        use std::hash::{Hasher, SipHasher};
 
         let mut manifest = String::new();
 
@@ -176,11 +196,11 @@ impl IsleCompilation {
             // to `\r\n`; canonicalize the source that we hash to
             // Unix-style (`\n`) so hashes will match.
             let content = content.replace("\r\n", "\n");
-            // One line in the manifest: <filename> <sha-512 hash>.
-            let mut hasher = Sha512::default();
-            hasher.update(content.as_bytes());
+            // One line in the manifest: <filename> <siphash>.
+            let mut hasher = SipHasher::new_with_keys(0, 0); // fixed keys for determinism
+            hasher.write(content.as_bytes());
             let filename = format!("{}", filename.display()).replace("\\", "/");
-            writeln!(&mut manifest, "{} {:x}", filename, hasher.finalize())?;
+            writeln!(&mut manifest, "{} {:x}", filename, hasher.finish())?;
         }
 
         Ok(manifest)

cranelift/codegen/src/isa/aarch64/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
-src/clif.isle be1359b4b6b153f378517c1dd95cd80f4a6bed0c7b86eaba11c088fd71b7bfe80a3c868ace245b2da0bfbbd6ded262ea9576c8e0eeacbf89d03c34a17a709602
-src/prelude.isle 15c8dd937171bd0f619179e219422d43af0eb0ef9a6e88f23b2aa55776712e27342309dd3a4441876b2dfec7f16ce7fe13b3a926ace89b25cfc9577e7b1d1578
-src/isa/aarch64/inst.isle a6921329a85a252b8059657056ee0c4477304dff461bd58c39133a2870666b23a34c911be55e25e7887074cb54b640ff09998730af09b3c1ba792f434309af24
-src/isa/aarch64/lower.isle 3cc84f8e3907818da7e0cbb4afe7f269da7090f3d504c74ecf02ef57463b281f4a320e52656d9397720ec8e4af98c1ecea05c4ffbe3f0c4126af4ed95d2734cc
+src/clif.isle f176ef3bba99365
+src/prelude.isle babc931e5dc5b4cf
+src/isa/aarch64/inst.isle abbd648f4a0b479a
+src/isa/aarch64/lower.isle c46d0692df0a9c0d

cranelift/codegen/src/isa/x64/lower/isle/generated_code.manifest

@@ -1,4 +1,4 @@
-src/clif.isle be1359b4b6b153f378517c1dd95cd80f4a6bed0c7b86eaba11c088fd71b7bfe80a3c868ace245b2da0bfbbd6ded262ea9576c8e0eeacbf89d03c34a17a709602
-src/prelude.isle 15c8dd937171bd0f619179e219422d43af0eb0ef9a6e88f23b2aa55776712e27342309dd3a4441876b2dfec7f16ce7fe13b3a926ace89b25cfc9577e7b1d1578
-src/isa/x64/inst.isle 1a44ccc0c2cad90447762848461fcae714216ef058d42bdba89330a6008061526e92bbf1c17055c465b20fc75d98d1faa34feda8b22fa7ae0504a0f808798b41
-src/isa/x64/lower.isle c7943201b32e9eb9726466e8cc417f7e84c4c4052de31e05ab6e0ad7502a587cf1d7d9835703c4ff5a506390f7a0668741e7f3feaa1edda6396571a425949fc9
+src/clif.isle f176ef3bba99365
+src/prelude.isle babc931e5dc5b4cf
+src/isa/x64/inst.isle fb5d3ac8e68c46d2
+src/isa/x64/lower.isle d39e01add89178d5