Use CursorBase utility functions to reduce repositioning clutter.

lib/cretonne/src/ir/layout.rs

@@ -788,10 +788,27 @@ pub trait CursorBase {
         self.set_position(CursorPosition::At(inst));
     }
 
+    /// Go to the position for inserting instructions at the beginning of `ebb`,
+    /// which unlike `goto_first_inst` doesn't assume that any instructions have
+    /// been inserted into `ebb` yet.
+    fn goto_first_insertion_point(&mut self, ebb: Ebb) {
+        if let Some(inst) = self.layout().ebbs[ebb].first_inst.expand() {
+            self.goto_inst(inst);
+        } else {
+            self.goto_bottom(ebb);
+        }
+    }
+
     /// Go to the first instruction in `ebb`.
     fn goto_first_inst(&mut self, ebb: Ebb) {
         let inst = self.layout().ebbs[ebb].first_inst.expect("Empty EBB");
-        self.set_position(CursorPosition::At(inst));
+        self.goto_inst(inst);
+    }
+
+    /// Go to the last instruction in `ebb`.
+    fn goto_last_inst(&mut self, ebb: Ebb) {
+        let inst = self.layout().ebbs[ebb].last_inst.expect("Empty EBB");
+        self.goto_inst(inst);
     }
 
     /// Go to the top of `ebb` which must be inserted into the layout.

lib/cretonne/src/legalizer/boundary.rs

@@ -63,8 +63,7 @@ fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
     // We want to insert instructions before the first instruction in the entry block.
     // If the entry block is empty, append instructions to it instead.
     let mut pos = Cursor::new(&mut func.layout);
-    pos.goto_top(entry);
-    pos.next_inst();
+    pos.goto_first_inst(entry);
 
     // Keep track of the argument types in the ABI-legalized signature.
     let abi_types = &func.signature.argument_types;

lib/cretonne/src/legalizer/split.rs

@@ -235,8 +235,7 @@ fn split_value(
                 //
                 // Note that it is safe to move `pos` here since `reuse` was set above, so we don't
                 // need to insert a split instruction before returning.
-                pos.goto_top(ebb);
-                pos.next_inst();
+                pos.goto_first_inst(ebb);
                 dfg.ins(pos).with_result(value).Binary(
                     concat,
                     split_type,

lib/cretonne/src/licm.rs

@@ -34,8 +34,7 @@ pub fn do_licm(
                     let pre_header =
                         create_pre_header(loop_analysis.loop_header(lp), func, cfg, domtree);
                     pos = Cursor::new(&mut func.layout);
-                    pos.goto_bottom(pre_header);
-                    pos.prev_inst();
+                    pos.goto_last_inst(pre_header);
                 }
                 // If there is a natural pre-header we insert new instructions just before the
                 // related jumping instruction (which is not necessarily at the end).

lib/frontend/src/ssa.rs

@@ -445,8 +445,7 @@ where
                     layout.append_ebb(dest_ebb)
                 };
                 let mut cur = Cursor::new(layout);
-                cur.goto_top(dest_ebb);
-                cur.next_inst();
+                cur.goto_first_insertion_point(dest_ebb);
                 let ty = dfg.value_type(temp_arg_val);
                 let val = if ty.is_int() {
                     dfg.ins(&mut cur).iconst(ty, 0)