diff --git a/src/backend.rs b/src/backend.rs
index aae9aa7..97466df 100644
--- a/src/backend.rs
+++ b/src/backend.rs
@@ -7,31 +7,166 @@ pub enum Shape {
     Block { head: BlockId, children: Vec<Shape> },
     Loop { head: BlockId, children: Vec<Shape> },
     Leaf { block: BlockId, succs: Vec<BlockId> },
+    None,
 }
 
+/// Index in RPO.
+type OrderedBlockId = usize;
+
+#[derive(Clone, Copy, Debug)]
 enum Region {
     /// Forward-branch region. Extends from end (just prior to
     /// terminator) of first block to just before second block. Can be
     /// extended earlier, prior to the beginning, if needed.
-    Forward(BlockId, BlockId),
+    Forward(OrderedBlockId, OrderedBlockId),
 
     /// Backward-branch region. Extends from start of first block to
     /// end (after terminator) of second block. Can be extended past
-    /// the end if needed. TODO: actually record all jump-points.
-    Backward(BlockId, BlockId),
+    /// the end if needed.
+    Backward(OrderedBlockId, OrderedBlockId),
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+struct RegionEndpoint {
+    block: OrderedBlockId,
+    pt: BlockPoint,
+}
+
+impl RegionEndpoint {
+    fn start(block: OrderedBlockId) -> Self {
+        RegionEndpoint {
+            block,
+            pt: BlockPoint::Start,
+        }
+    }
+    fn end(block: OrderedBlockId) -> Self {
+        RegionEndpoint {
+            block,
+            pt: BlockPoint::Start,
+        }
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
+enum BlockPoint {
+    Start,
+    End,
+}
+
+impl Region {
+    fn start(&self) -> RegionEndpoint {
+        match self {
+            &Region::Forward(a, _) => RegionEndpoint::end(a),
+            &Region::Backward(a, _) => RegionEndpoint::start(a),
+        }
+    }
+
+    fn end(&self) -> RegionEndpoint {
+        match self {
+            &Region::Forward(_, b) => RegionEndpoint::start(b),
+            &Region::Backward(_, b) => RegionEndpoint::end(b),
+        }
+    }
+
+    fn contains(&self, other: &Region) -> bool {
+        self.start() <= other.start() && self.end() >= other.end()
+    }
+
+    fn overlaps(&self, other: &Region) -> bool {
+        self.end() >= other.start() && other.end() >= self.start()
+    }
 }
 
 impl Shape {
-    pub fn compute(f: &FunctionBody, cfg: &CFGInfo) -> Self {
+    pub fn compute(_f: &FunctionBody, cfg: &CFGInfo) -> Self {
         // Process all non-contiguous edges in RPO block order. For
         // forward and backward edges, emit Regions.
+        let order = cfg.rpo();
+        assert_eq!(order[0], 0); // Entry block should come first.
+        let mut regions = vec![];
+        for (block_pos, &block) in order.iter().enumerate() {
+            for &succ in cfg.succs(block) {
+                let succ_pos = cfg
+                    .rpo_pos(succ)
+                    .expect("if block is reachable then succ should be too");
+                if succ_pos < block_pos {
+                    regions.push(Region::Backward(succ_pos, block_pos));
+                } else if succ_pos > block_pos + 1 {
+                    regions.push(Region::Forward(block_pos, succ_pos));
+                }
+            }
+        }
 
-        // Sort regions by start. Then examine adjacent regions to
-        // resolve nesting. If out-of-order, we can extend a Forward
-        // region's start backward, or a Backward region's end
-        // forward. If still out-of-order, drop any conflicting
-        // Backward; we'll handle by duplication.
+        // Initially sort regions by start pos to get them in rough
+        // order; we'll resolve exact ordering below by extending
+        // regions where necessary and duplicating where we find
+        // irreducible control flow.
+        regions.sort_by_key(|r| r.start());
 
-        todo!()
+        // Examine each region in the sequence, determining whether it
+        // is properly nested with respect to all overlapping regions.
+        let mut i = 0;
+        while i + 1 < regions.len() {
+            i += 1;
+            let prev = regions[i - 1];
+            let this = regions[i];
+
+            if !prev.overlaps(&this) {
+                continue;
+            }
+
+            match (prev, this) {
+                (Region::Backward(a, b), Region::Backward(c, d)) if a == c => {
+                    // Merge by extending end.
+                    regions[i - 1] = Region::Backward(a, std::cmp::max(b, d));
+                    regions.remove(i);
+                }
+                (Region::Backward(a, b), Region::Backward(c, _d)) if a < c && c <= b => {
+                    panic!("Irreducible CFG");
+                }
+                (Region::Backward(a, b), Region::Forward(c, d)) if a <= c && c <= b => {
+                    // Put the Forward before the Backward (extend its
+                    // start) to ensure proper nesting.
+                    regions[i - 1] = Region::Forward(a, d);
+                    regions[i] = Region::Backward(a, b);
+                }
+                (Region::Forward(_a, b), Region::Backward(c, d)) if b > c && b <= d => {
+                    panic!("Irreducible CFG");
+                }
+                (Region::Forward(a, b), Region::Forward(c, d)) if b == d => {
+                    // Merge.
+                    regions[i - 1] = Region::Forward(std::cmp::min(a, c), b);
+                    regions.remove(i);
+                }
+                (Region::Forward(a, b), Region::Forward(c, d)) if a <= c && b < d => {
+                    regions[i - 1] = Region::Forward(a, d);
+                    regions[i] = Region::Forward(a, b);
+                }
+                _ => {}
+            }
+        }
+
+        // Ensure the regions properly nest.
+        let mut stack: Vec<Region> = vec![];
+        for region in &regions {
+            while let Some(top) = stack.last() {
+                if top.contains(region) {
+                    stack.push(region.clone());
+                    break;
+                } else if region.contains(top) {
+                    stack.pop();
+                } else if region.overlaps(top) {
+                    panic!(
+                        "Non-nested region: {:?} in nest: {:?} (overall: {:?})",
+                        region, stack, regions
+                    );
+                }
+            }
+            if stack.is_empty() {
+                stack.push(region.clone());
+            }
+        }
+
+        Shape::None
     }
 }
diff --git a/src/cfg/mod.rs b/src/cfg/mod.rs
index 898faa8..f0359a0 100644
--- a/src/cfg/mod.rs
+++ b/src/cfg/mod.rs
@@ -88,4 +88,12 @@ impl CFGInfo {
         let is_return = self.return_blocks.binary_search(&block).is_ok();
         self.succs(block).len() + if is_return { 1 } else { 0 }
     }
+
+    pub fn rpo(&self) -> Vec<BlockId> {
+        self.postorder.iter().cloned().rev().collect()
+    }
+
+    pub fn rpo_pos(&self, block: BlockId) -> Option<usize> {
+        self.postorder_pos[block].map(|fwd_pos| self.postorder.len() - 1 - fwd_pos)
+    }
 }
diff --git a/src/ir.rs b/src/ir.rs
index 41275d5..ca64f3e 100644
--- a/src/ir.rs
+++ b/src/ir.rs
@@ -1,6 +1,6 @@
 //! Intermediate representation for Wasm.
 
-use crate::{backend, frontend};
+use crate::{backend::Shape, cfg::CFGInfo, frontend};
 use anyhow::Result;
 use wasmparser::{FuncType, Operator, Type};
 
@@ -210,7 +210,16 @@ impl<'a> Module<'a> {
         frontend::wasm_to_ir(bytes)
     }
 
-    pub fn to_wasm_bytes(mut self) -> Vec<u8> {
+    pub fn to_wasm_bytes(self) -> Vec<u8> {
+        for func in &self.funcs {
+            match func {
+                &FuncDecl::Body(_, ref body) => {
+                    let cfg = CFGInfo::new(body);
+                    let _shape = Shape::compute(body, &cfg);
+                }
+                _ => {}
+            }
+        }
         // TODO
         self.orig_bytes.to_vec()
     }