Type checking and Dominator Tree integrity checks in Verifier (#66)

* Verify that a recomputed dominator tree is identical to the existing one.
* The verifier now typechecks instruction results and arguments.
* The verifier now typechecks instruction results and arguments.
* The verifier now typechecks instruction results and arguments.
* Added `inst_{fixed,variable}_args` accessor functions.
* Improved error messages in verifier.
* Type check return statements against the function signature.

lib/cretonne/src/ir/dfg.rs

@@ -344,16 +344,40 @@ impl DataFlowGraph {
         DisplayInst(self, inst)
     }
 
-    /// Get the value arguments on `inst` as a slice.
+    /// Get all value arguments on `inst` as a slice.
     pub fn inst_args(&self, inst: Inst) -> &[Value] {
         self.insts[inst].arguments(&self.value_lists)
     }
 
-    /// Get the value arguments on `inst` as a mutable slice.
+    /// Get all value arguments on `inst` as a mutable slice.
     pub fn inst_args_mut(&mut self, inst: Inst) -> &mut [Value] {
         self.insts[inst].arguments_mut(&mut self.value_lists)
     }
 
+    /// Get the fixed value arguments on `inst` as a slice.
+    pub fn inst_fixed_args(&self, inst: Inst) -> &[Value] {
+        let fixed_args = self[inst].opcode().constraints().fixed_value_arguments();
+        &self.inst_args(inst)[..fixed_args]
+    }
+
+    /// Get the fixed value arguments on `inst` as a mutable slice.
+    pub fn inst_fixed_args_mut(&mut self, inst: Inst) -> &mut [Value] {
+        let fixed_args = self[inst].opcode().constraints().fixed_value_arguments();
+        &mut self.inst_args_mut(inst)[..fixed_args]
+    }
+
+    /// Get the variable value arguments on `inst` as a slice.
+    pub fn inst_variable_args(&self, inst: Inst) -> &[Value] {
+        let fixed_args = self[inst].opcode().constraints().fixed_value_arguments();
+        &self.inst_args(inst)[fixed_args..]
+    }
+
+    /// Get the variable value arguments on `inst` as a mutable slice.
+    pub fn inst_variable_args_mut(&mut self, inst: Inst) -> &mut [Value] {
+        let fixed_args = self[inst].opcode().constraints().fixed_value_arguments();
+        &mut self.inst_args_mut(inst)[fixed_args..]
+    }
+
     /// Create result values for an instruction that produces multiple results.
     ///
     /// Instructions that produce 0 or 1 result values only need to be created with `make_inst`. If

lib/cretonne/src/ir/instructions.rs

@@ -398,7 +398,6 @@ pub struct OpcodeConstraints {
     /// Offset into `OPERAND_CONSTRAINT` table of the descriptors for this opcode. The first
     /// `fixed_results()` entries describe the result constraints, then follows constraints for the
     /// fixed `Value` input operands. (`fixed_value_arguments()` of them).
-    /// format.
     constraint_offset: u16,
 }