Add a FuncEnvironment::make_indirect_sig() callback.

The function environment is now expected to keep track of the function
signatures in the module, and it is asked to generate Cretonne
signatures to be used for indirect calls.

The combination of make_indirect_sig() and translate_call_indirect()
callbacks allow the runtime to insert additional function arguments for
indirect calls such as vmctx pointers and CFI-style signature identifiers.

lib/wasm/src/runtime/dummy.rs

@@ -1,6 +1,6 @@
 use runtime::{FuncEnvironment, GlobalValue, WasmRuntime};
-use translation_utils::{Local, Global, Memory, Table, GlobalIndex, TableIndex, FunctionIndex,
-                        MemoryIndex};
+use translation_utils::{Local, Global, Memory, Table, GlobalIndex, TableIndex, SignatureIndex,
+                        FunctionIndex, MemoryIndex};
 use cton_frontend::FunctionBuilder;
 use cretonne::ir::{self, Value, InstBuilder, SigRef};
 use cretonne::ir::types::*;
@@ -9,13 +9,18 @@ use cretonne::ir::types::*;
 /// placeholders when forced to. Don't try to execute code translated with this runtime, it is
 /// essentially here for translation debug purposes.
 pub struct DummyRuntime {
+    // Unprocessed signatures exactly as provided by `declare_signature()`.
+    signatures: Vec<ir::Signature>,
     globals: Vec<Global>,
 }
 
 impl DummyRuntime {
     /// Allocates the runtime data structures.
     pub fn new() -> Self {
-        Self { globals: Vec::new() }
+        Self {
+            signatures: Vec::new(),
+            globals: Vec::new(),
+        }
     }
 }
 
@@ -42,6 +47,12 @@ impl FuncEnvironment for DummyRuntime {
             style: ir::HeapStyle::Static { bound: 0x1_0000_0000.into() },
         })
     }
+
+    fn make_indirect_sig(&self, func: &mut ir::Function, index: SignatureIndex) -> ir::SigRef {
+        // A real implementation would probably change the calling convention and add `vmctx` and
+        // signature index arguments.
+        func.dfg.signatures.push(self.signatures[index].clone())
+    }
 }
 
 impl WasmRuntime for DummyRuntime {
@@ -61,6 +72,11 @@ impl WasmRuntime for DummyRuntime {
         let call_inst = builder.ins().call_indirect(sig_ref, index_val, call_args);
         builder.inst_results(call_inst)
     }
+
+    fn declare_signature(&mut self, sig: &ir::Signature) {
+        self.signatures.push(sig.clone());
+    }
+
     fn declare_global(&mut self, global: Global) {
         self.globals.push(global);
     }