feat(xlang-opt): Start cfg builder

xlang/xlang_opt/src/cfg.rs

@@ -1 +1,53 @@
-pub struct ControlFlow {}
+use core::option::Option::Some;
+use xlang::abi::slice::SplitOut as _;
+use xlang::ir::{meta::block::ReachableFrom, FunctionBody, Terminator};
+use xlang::ir::{BranchCondition, Switch};
+use xlang::prelude::v1::*;
+
+pub struct ControlFlow<'a> {
+    body: &'a mut FunctionBody,
+}
+
+impl<'a> ControlFlow<'a> {
+    fn can_branch_to(t: &Terminator, target: u32) -> bool {
+        match t {
+            Terminator::BranchIndirect => false, // Handle this otherwise
+            Terminator::Asm(a) => a.targets.iter().any(|targ| targ.target == target),
+            Terminator::Branch(BranchCondition::Always, a, _) => a.target == target,
+            Terminator::Branch(BranchCondition::Never, _, b) => b.target == target,
+            Terminator::Branch(_, a, b) => a.target == target || b.target == target,
+            Terminator::Jump(targ) => targ.target == target,
+            Terminator::Switch(switch) => {
+                switch.default().target == target
+                    || switch.case_targets().any(|targ| targ.target == target)
+            }
+            Terminator::Call(_, _, next) => next.target == target,
+            Terminator::Empty
+            | Terminator::Unreachable
+            | Terminator::Exit(_)
+            | Terminator::Tailcall(_, _) => false,
+        }
+    }
+    pub fn get_or_build_reachability_for(&mut self, id: u32) -> &ReachableFrom {
+        if let Ok(idx) = self.body.blocks.binary_search_by_key(&id, |blk| blk.target) {
+            let (start, blk, end) = self.body.blocks.split_out_mut(idx);
+            if let Some(meta) = blk.meta.get() {
+                // SAFETY:
+                // nll again
+                unsafe { &*(meta as *const _) }
+            } else {
+                let mut meta = ReachableFrom::new();
+                for remote_block in start.iter().chain(&*end) {
+                    if Self::can_branch_to(&remote_block.term, id) {
+                        meta.reachable.push(remote_block.target);
+                    }
+                }
+                blk.meta.push(meta);
+
+                blk.meta.get().unwrap()
+            }
+        } else {
+            panic!("Invalid block {id}")
+        }
+    }
+}