Pulley: tail calls (#9251)

* Document host return address and extract to const

Copyright (c) 2024, Arm Limited.

Signed-off-by: Karl Meakin <[email protected]>

* Replace `Continutation` with `ControlFlow`

Copyright (c) 2024, Arm Limited.

Signed-off-by: Karl Meakin <[email protected]>

* Tail recursive interpreter loop

Copyright (c) 2024, Arm Limited.

Signed-off-by: Karl Meakin <[email protected]>

* Delete visitor based interpreter implementation

Copyright (c) 2024, Arm Limited.

Signed-off-by: Karl Meakin <[email protected]>

* Move pulley tail call CI step to a job that uses nightly rust

* Record the PC at which a trap occurred

Not the initial PC passed into `run`

---------

Signed-off-by: Karl Meakin <[email protected]>
Co-authored-by: Nick Fitzgerald <[email protected]>

.github/workflows/main.yml

@@ -552,6 +552,13 @@ jobs:
       with:
         toolchain: wasmtime-ci-pinned-nightly
 
+    # Check that `pulley-interpreter` compiles with tail calls enabled.  Don't
+    # actually run the tests with tail calls enabled, because they are not yet
+    # implemented in rustc and cause an ICE.
+    - run: cargo check -p pulley-interpreter
+      env:
+        RUSTFLAGS: "--cfg pulley_tail_calls"
+
     # Ensure that fuzzers still build.
     #
     # Install the OCaml packages necessary for fuzz targets that use the

Cargo.toml

@@ -176,6 +176,9 @@ unused_import_braces = 'warn'
 unused-lifetimes = 'warn'
 unused-macro-rules = 'warn'
 
+# Don't warn about unknown cfg condition in `#[cfg(pulley_tail_calls)]`
+unexpected_cfgs = { level = "warn", check-cfg = ['cfg(pulley_tail_calls)'] }
+
 [workspace.lints.clippy]
 # The default set of lints in Clippy is viewed as "too noisy" right now so
 # they're all turned off by default. Selective lints are then enabled below as

pulley/Cargo.toml

@@ -27,7 +27,7 @@ arbitrary = ["dep:arbitrary", "arbitrary/derive", "std", "cranelift-bitset/arbit
 encode = []
 decode = []
 disas = ["decode"]
-interp = ["decode"]
+interp = ["decode", "encode"]
 
 [package.metadata.docs.rs]
 all-features = true

pulley/src/decode.rs

@@ -260,7 +260,8 @@ impl BytecodeStream for UnsafeBytecodeStream {
 
 /// Anything that can be decoded from a bytecode stream, e.g. opcodes,
 /// immediates, registers, etc...
-trait Decode: Sized {
+pub trait Decode: Sized {
+    /// Decode this type from the given bytecode stream.
     fn decode<T>(bytecode: &mut T) -> Result<Self, T::Error>
     where
         T: BytecodeStream;
@@ -377,6 +378,32 @@ impl Decode for PcRelOffset {
     }
 }
 
+impl Decode for Opcode {
+    fn decode<T>(bytecode: &mut T) -> Result<Self, T::Error>
+    where
+        T: BytecodeStream,
+    {
+        let byte = u8::decode(bytecode)?;
+        match Opcode::new(byte) {
+            Some(v) => Ok(v),
+            None => Err(bytecode.invalid_opcode(byte)),
+        }
+    }
+}
+
+impl Decode for ExtendedOpcode {
+    fn decode<T>(bytecode: &mut T) -> Result<Self, T::Error>
+    where
+        T: BytecodeStream,
+    {
+        let word = u16::decode(bytecode)?;
+        match ExtendedOpcode::new(word) {
+            Some(v) => Ok(v),
+            None => Err(bytecode.invalid_extended_opcode(word)),
+        }
+    }
+}
+
 impl<R: Reg> Decode for BinaryOperands<R> {
     fn decode<T>(bytecode: &mut T) -> Result<Self, T::Error>
     where
@@ -655,3 +682,53 @@ macro_rules! define_extended_decoder {
     };
 }
 for_each_extended_op!(define_extended_decoder);
+
+/// Unwrap a `Result<T, Uninhabited>`.
+/// Always succeeds, since `Uninhabited` is uninhabited.
+pub fn unwrap_uninhabited<T>(res: Result<T, Uninhabited>) -> T {
+    match res {
+        Ok(ok) => ok,
+
+        // Nightly rust warns that this pattern is unreachable, but stable rust
+        // doesn't.
+        #[allow(unreachable_patterns)]
+        Err(err) => match err {},
+    }
+}
+
+/// Functions for decoding the operands of an instruction, assuming the opcode
+/// has already been decoded.
+pub mod operands {
+    use super::*;
+
+    macro_rules! define_operands_decoder {
+        (
+            $(
+                $( #[$attr:meta] )*
+                    $snake_name:ident = $name:ident $( {
+                    $(
+                        $( #[$field_attr:meta] )*
+                        $field:ident : $field_ty:ty
+                    ),*
+                } )? ;
+            )*
+        ) => {
+            $(
+                #[allow(unused_variables)]
+                #[allow(missing_docs)]
+                pub fn $snake_name<T: BytecodeStream>(pc: &mut T) -> Result<($($($field_ty,)*)?), T::Error> {
+                    Ok((($($((<$field_ty>::decode(pc))?,)*)?)))
+                }
+            )*
+        };
+    }
+
+    for_each_op!(define_operands_decoder);
+
+    #[allow(missing_docs)]
+    pub fn extended<T: BytecodeStream>(pc: &mut T) -> Result<(ExtendedOpcode,), T::Error> {
+        Ok((ExtendedOpcode::decode(pc)?,))
+    }
+
+    for_each_extended_op!(define_operands_decoder);
+}