Add a minimal SimpleJIT example program.

This minimally demonstrates usage of the API, and serves as a very small
testcase to test that the basic JIT mechanisms are working.

lib/simplejit/examples/simplejit-minimal.rs

@@ -0,0 +1,58 @@
+extern crate cranelift;
+extern crate cranelift_module;
+extern crate cranelift_simplejit;
+
+use cranelift::prelude::*;
+use cranelift_module::{Linkage, Module};
+use cranelift_simplejit::{SimpleJITBackend, SimpleJITBuilder};
+use std::mem;
+
+fn main() {
+    let mut module: Module<SimpleJITBackend> = Module::new(SimpleJITBuilder::new());
+    let mut ctx = module.make_context();
+    let mut func_ctx = FunctionBuilderContext::new();
+    let sig = module.make_signature();
+
+    let func_a = module.declare_function("a", Linkage::Local, &sig).unwrap();
+    let func_b = module.declare_function("b", Linkage::Local, &sig).unwrap();
+
+    ctx.func.name = ExternalName::user(0, func_a.index() as u32);
+    {
+        let mut bcx: FunctionBuilder = FunctionBuilder::new(&mut ctx.func, &mut func_ctx);
+        let ebb = bcx.create_ebb();
+
+        bcx.switch_to_block(ebb);
+        bcx.ins().return_(&[]);
+        bcx.seal_all_blocks();
+        bcx.finalize();
+    }
+    module.define_function(func_a, &mut ctx).unwrap();
+    module.clear_context(&mut ctx);
+
+    ctx.func.name = ExternalName::user(0, func_b.index() as u32);
+    {
+        let mut bcx: FunctionBuilder = FunctionBuilder::new(&mut ctx.func, &mut func_ctx);
+        let ebb = bcx.create_ebb();
+
+        bcx.switch_to_block(ebb);
+        let local_func = module.declare_func_in_func(func_a, &mut bcx.func);
+        bcx.ins().call(local_func, &[]);
+        bcx.ins().return_(&[]);
+        bcx.seal_all_blocks();
+        bcx.finalize();
+    }
+    module.define_function(func_b, &mut ctx).unwrap();
+    module.clear_context(&mut ctx);
+
+    // Perform linking.
+    module.finalize_all();
+
+    // Get a raw pointer to the generated code.
+    let code_b = module.get_finalized_function(func_b);
+
+    // Cast it to a rust function pointer type.
+    let ptr_b = unsafe { mem::transmute::<_, fn()>(code_b) };
+
+    // Call it!
+    ptr_b();
+}