Walk blocks in CFG order

compiler/src/regify.rs

@@ -109,10 +109,18 @@ impl RegFn {
         let pcs = flat_fn.code.iter().map(|i| i.pc).collect::<Vec<usize>>();
         let (starts, mut blocks) = find_basic_blocks(&flat_fn.code, &pcs);
 
-        let mut output = Vec::with_capacity(flat_fn.code.len());
         let mut next_val = flat_fn.var_count; // Reserve values for locals
 
-        for b in 0..blocks.len() {
+        // Emit per block, so blocks can be WALKED in the order control reaches
+        // them and still be READ in listing order. Listing order alone is not
+        // enough: a for-of puts its body before the test, and the body is
+        // reached only by the back edge, so walking it first finds no stack.
+        let mut emitted = vec![Vec::new(); blocks.len()];
+        let mut seen = vec![false; blocks.len()];
+        let mut todo = vec![0];
+        seen[0] = true;
+
+        while let Some(b) = todo.pop() {
             // A block starts with the stack it is ENTERED with, not with
             // whatever the block above it in the listing left behind. This one
             // line is what stops the walk running off a `return` into a
@@ -212,18 +220,25 @@ impl RegFn {
                 }
 
                 reg_instr.block_start = std::mem::take(&mut first);
-                output.push(reg_instr);
+                emitted[b].push(reg_instr);
             }
 
             // Every edge out of here arrives at its target with this stack.
-            // One pass in listing order reaches every block, because a block is
-            // always entered from earlier in the listing before any back edge
-            // returns to it.
+            // Walking a block only after something has arrived at it is what
+            // makes the entry stack right, and it is why the order is the CFG's
+            // and not the listing's.
             for i in 0..blocks[b].succ.len() {
                 let to = blocks[b].succ[i];
                 blocks[to].arrive(&stack);
+
+                if !seen[to] {
+                    seen[to] = true;
+                    todo.push(to);
                 }
             }
+        }
+
+        let output = emitted.into_iter().flatten().collect::<Vec<RegInstr>>();
 
         Self {
             name: flat_fn.name,