From 3bde79a15d3e0a9a59b348630f1da806470e4293 Mon Sep 17 00:00:00 2001
From: Chris Fallin <chris@cfallin.org>
Date: Mon, 28 Nov 2022 22:08:08 -0800
Subject: [PATCH] WIP.

---
 .gitignore              |  2 ++
 src/backend/mod.rs      |  2 +-
 src/backend/stackify.rs | 15 +++++++++------
 wasm_tests/test.wat     |  6 ++++++
 wasm_tests/test2.wat    | 12 ++++++++++++
 wasm_tests/test3.wat    | 13 +++++++++++++
 6 files changed, 43 insertions(+), 7 deletions(-)
 create mode 100644 wasm_tests/test.wat
 create mode 100644 wasm_tests/test2.wat
 create mode 100644 wasm_tests/test3.wat

diff --git a/.gitignore b/.gitignore
index e15772d..38ada36 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,3 +2,5 @@
 *~
 .*.swp
 Cargo.lock
+/*.wasm
+wasm_tests/*.wasm
diff --git a/src/backend/mod.rs b/src/backend/mod.rs
index 059df29..0f61be6 100644
--- a/src/backend/mod.rs
+++ b/src/backend/mod.rs
@@ -25,7 +25,7 @@ impl<'a> WasmBackend<'a> {
         log::trace!("RPO:\n{:?}\n", rpo);
         let trees = Trees::compute(body);
         log::trace!("Trees:\n{:?}\n", trees);
-        let ctrl = StackifyContext::new(body, &cfg, &rpo).compute();
+        let ctrl = StackifyContext::new(body, &cfg, &rpo)?.compute();
         log::trace!("Ctrl:\n{:?}\n", ctrl);
         Ok(WasmBackend {
             body,
diff --git a/src/backend/stackify.rs b/src/backend/stackify.rs
index 067b0f6..2b5917d 100644
--- a/src/backend/stackify.rs
+++ b/src/backend/stackify.rs
@@ -96,17 +96,17 @@ impl CtrlEntry {
 }
 
 impl<'a, 'b> Context<'a, 'b> {
-    pub fn new(body: &'a FunctionBody, cfg: &'b CFGInfo, rpo: &'b RPO) -> Self {
+    pub fn new(body: &'a FunctionBody, cfg: &'b CFGInfo, rpo: &'b RPO) -> anyhow::Result<Self> {
         let (merge_nodes, loop_headers) =
-            Self::compute_merge_nodes_and_loop_headers(body, cfg, rpo);
-        Self {
+            Self::compute_merge_nodes_and_loop_headers(body, cfg, rpo)?;
+        Ok(Self {
             body,
             cfg,
             rpo,
             merge_nodes,
             loop_headers,
             ctrl_stack: vec![],
-        }
+        })
     }
 
     pub fn compute(mut self) -> Vec<WasmBlock<'a>> {
@@ -119,7 +119,7 @@ impl<'a, 'b> Context<'a, 'b> {
         body: &FunctionBody,
         cfg: &CFGInfo,
         rpo: &RPO,
-    ) -> (HashSet<Block>, HashSet<Block>) {
+    ) -> anyhow::Result<(HashSet<Block>, HashSet<Block>)> {
         let mut loop_headers = HashSet::new();
         let mut branched_once = HashSet::new();
         let mut merge_nodes = HashSet::new();
@@ -128,6 +128,9 @@ impl<'a, 'b> Context<'a, 'b> {
             for &succ in cfg.succs(block) {
                 let succ_rpo = rpo.rev[succ].unwrap();
                 if succ_rpo <= block_rpo {
+                    if !cfg.dominates(succ, block) {
+                        anyhow::bail!("Irreducible control flow: edge from {} to {}", block, succ);
+                    }
                     // Backward branch.
                     loop_headers.insert(succ);
                 } else {
@@ -155,7 +158,7 @@ impl<'a, 'b> Context<'a, 'b> {
             }
         }
 
-        (merge_nodes, loop_headers)
+        Ok((merge_nodes, loop_headers))
     }
 
     fn handle_dom_subtree(&mut self, block: Block, into: &mut Vec<WasmBlock<'a>>) {
diff --git a/wasm_tests/test.wat b/wasm_tests/test.wat
new file mode 100644
index 0000000..445e7a9
--- /dev/null
+++ b/wasm_tests/test.wat
@@ -0,0 +1,6 @@
+(module
+  (func (result i32)
+        i32.const 2
+        i32.const 2
+        i32.add
+        ))
diff --git a/wasm_tests/test2.wat b/wasm_tests/test2.wat
new file mode 100644
index 0000000..68eecf6
--- /dev/null
+++ b/wasm_tests/test2.wat
@@ -0,0 +1,12 @@
+(module
+  (func (param i32) (result i32)
+        local.get 0
+        if (result i32)
+          i32.const 1
+          local.get 0
+          i32.add
+        else
+          i32.const 2
+          local.get 0
+          i32.add
+        end))
diff --git a/wasm_tests/test3.wat b/wasm_tests/test3.wat
new file mode 100644
index 0000000..bde69be
--- /dev/null
+++ b/wasm_tests/test3.wat
@@ -0,0 +1,13 @@
+(module
+  (func (param i32) (result i32)
+        (local $i i32)
+        (local.set $i (local.get 0))
+
+        (local.get $i)
+        (block $loop-break (param i32) (result i32)
+            (loop $l (param i32) (result i32)
+                  (if (i32.eq (local.get $i) (i32.const 100))
+                    (br $loop-break
+                      (local.get $i)))
+                  (local.set $i (i32.add (i32.const 10) (local.get $i)))
+                  (br $l (local.get $i))))))