Implement the machinery to create a TargetIsa.

Add an isa::lookup() function which serves as a target registry for creating
Box<TargetIsa> trait objects.

An isa::Builder makes it possible to confugure the trait object before it is
created.

cranelift/src/libcretonne/isa/mod.rs

@@ -7,7 +7,7 @@
 //!
 //! # Constructing a `TargetIsa` instance
 //!
-//! The target ISA is build from the following information:
+//! The target ISA is built from the following information:
 //!
 //! - The name of the target ISA as a string. Cretonne is a cross-compiler, so the ISA to target
 //!   can be selected dynamically. Individual ISAs can be left out when Cretonne is compiled, so a
@@ -19,18 +19,72 @@
 //! The `isa::lookup()` function is the main entry point which returns an `isa::Builder`
 //! appropriate for the requested ISA:
 //!
-//! ```ignore
-//! let isa_builder = isa::lookup("riscv").unwrap();
-//! adjust_isa_settings(&mut isa_builder.settings);
-//! let isa = builder.finish(shared_settings());
+//! ```
+//! use cretonne::settings::{self, Configurable};
+//! use cretonne::isa;
+//!
+//! let shared_builder = settings::builder();
+//! let shared_flags = settings::Flags::new(shared_builder);
+//!
+//! match isa::lookup("riscv") {
+//!     None => {
+//!         // The RISC-V target ISA is not available.
+//!     }
+//!     Some(mut isa_builder) => {
+//!         isa_builder.set("supports_m", "on");
+//!         let isa = isa_builder.finish(shared_flags);
+//!     }
+//! }
 //! ```
 //!
+//! The configured target ISA trait object is a `Box<TargetIsa>` which can be used for multiple
+//! concurrent function compilations.
 
 pub mod riscv;
 
+use settings;
 use ir::dfg::DataFlowGraph;
 use ir::entities::Inst;
 
+/// Look for a supported ISA with the given `name`.
+/// Return a builder that can create a corresponding `TargetIsa`.
+pub fn lookup(name: &str) -> Option<Builder> {
+    match name {
+        "riscv" => riscv_builder(),
+        _ => None,
+    }
+}
+
+// Make a builder for RISC-V.
+fn riscv_builder() -> Option<Builder> {
+    Some(riscv::isa_builder())
+}
+
+/// Builder for a `TargetIsa`.
+/// Modify the ISA-specific settings before creating the `TargetIsa` trait object with `finish`.
+pub struct Builder {
+    setup: settings::Builder,
+    constructor: fn(settings::Flags, settings::Builder) -> Box<TargetIsa>,
+}
+
+impl Builder {
+    /// Combine the ISA-specific settings with the provided ISA-independent settings and allocate a
+    /// fully configured `TargetIsa` trait object.
+    pub fn finish(self, shared_flags: settings::Flags) -> Box<TargetIsa> {
+        (self.constructor)(shared_flags, self.setup)
+    }
+}
+
+impl settings::Configurable for Builder {
+    fn set(&mut self, name: &str, value: &str) -> settings::Result<()> {
+        self.setup.set(name, value)
+    }
+
+    fn set_bool(&mut self, name: &str, value: bool) -> settings::Result<()> {
+        self.setup.set_bool(name, value)
+    }
+}
+
 pub trait TargetIsa {
     /// Encode an instruction after determining it is legal.
     ///

cranelift/src/libcretonne/isa/riscv/mod.rs

@@ -1,3 +1,37 @@
 //! RISC-V Instruction Set Architecture.
 
 pub mod settings;
+
+use super::super::settings as shared_settings;
+use super::Builder as IsaBuilder;
+use super::{TargetIsa, Encoding};
+use ir::dfg::DataFlowGraph;
+use ir::entities::Inst;
+
+#[allow(dead_code)]
+struct Isa {
+    shared_flags: shared_settings::Flags,
+    isa_flags: settings::Flags,
+}
+
+pub fn isa_builder() -> IsaBuilder {
+    IsaBuilder {
+        setup: settings::builder(),
+        constructor: isa_constructor,
+    }
+}
+
+fn isa_constructor(shared_flags: shared_settings::Flags,
+                   builder: shared_settings::Builder)
+                   -> Box<TargetIsa> {
+    Box::new(Isa {
+        shared_flags: shared_flags,
+        isa_flags: settings::Flags::new(builder),
+    })
+}
+
+impl TargetIsa for Isa {
+    fn encode(&self, _: &DataFlowGraph, _: &Inst) -> Option<Encoding> {
+        unimplemented!()
+    }
+}

meta/gen_settings.py

@@ -211,6 +211,7 @@ def gen_group(sgrp, fmt):
     """
     byte_size = layout_group(sgrp)
 
+    fmt.line('#[derive(Clone)]')
     fmt.doc_comment('Flags group `{}`.'.format(sgrp.name))
     with fmt.indented('pub struct Flags {', '}'):
         fmt.line('bytes: [u8; {}],'.format(byte_size))