Bitcasting at control flow exits (#1272)

* Bitcast vectors immediately before a return

* Bitcast vectors immediately before a block end

* Use helper function for bitcasting arguments

* Add FuncTranslationState::peekn_mut; allows mutating of peeked values

* Bitcast values in place, avoiding an allocation

Also, retrieves the correct EBB header types for bitcasting on Operator::End.

* Bitcast values of a function with no explicit Wasm return instruction

* Add Signature::return_types method

This eliminates some duplicate code and avoids extra `use`s of `Vec`.

* Add Signature::param_types method; only collect normal parameters in both this and Signature::return_types

* Move normal_args to Signature::num_normal_params method

This matches the organization of the other Signature::num_*_params methods.

* Bitcast values of Operator::Call and Operator::CallIndirect

* Add DataFlowGraph::ebb_param_types

* Bitcast values of Operator::Br and Operator::BrIf

* Bitcast values of Operator::BrTable

cranelift/wasm/src/func_translator.rs

@@ -4,7 +4,7 @@
 //! function to Cranelift IR guided by a `FuncEnvironment` which provides information about the
 //! WebAssembly module and the runtime environment.
 
-use crate::code_translator::translate_operator;
+use crate::code_translator::{bitcast_arguments, translate_operator};
 use crate::environ::{FuncEnvironment, ReturnMode, WasmResult};
 use crate::state::{FuncTranslationState, ModuleTranslationState};
 use crate::translation_utils::get_vmctx_value_label;
@@ -240,7 +240,11 @@ fn parse_function_body<FE: FuncEnvironment + ?Sized>(
         debug_assert!(builder.is_pristine());
         if !builder.is_unreachable() {
             match environ.return_mode() {
-                ReturnMode::NormalReturns => builder.ins().return_(&state.stack),
+                ReturnMode::NormalReturns => {
+                    let return_types = &builder.func.signature.return_types();
+                    bitcast_arguments(&mut state.stack, &return_types, builder);
+                    builder.ins().return_(&state.stack)
+                }
                 ReturnMode::FallthroughReturn => builder.ins().fallthrough_return(&state.stack),
             };
         }