From b9e89885c41f88909bec8a66e1020921d22b90a2 Mon Sep 17 00:00:00 2001
From: Chris Fallin <chris@cfallin.org>
Date: Sat, 8 May 2021 21:48:58 -0700
Subject: [PATCH] Error checking: properly signal a crit-edge requirement
 failure (used for regalloc.rs fuzzer)

---
 src/cfg.rs     | 20 ++++++++------------
 src/ion/mod.rs |  2 +-
 src/lib.rs     |  2 ++
 3 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/src/cfg.rs b/src/cfg.rs
index 31853f0..f73ca1a 100644
--- a/src/cfg.rs
+++ b/src/cfg.rs
@@ -5,7 +5,7 @@
 
 //! Lightweight CFG analyses.
 
-use crate::{domtree, postorder, Block, Function, Inst, OperandKind, ProgPoint};
+use crate::{domtree, postorder, Block, Function, Inst, OperandKind, ProgPoint, RegAllocError};
 
 #[derive(Clone, Debug)]
 pub struct CFGInfo {
@@ -37,7 +37,7 @@ pub struct CFGInfo {
 }
 
 impl CFGInfo {
-    pub fn new<F: Function>(f: &F) -> CFGInfo {
+    pub fn new<F: Function>(f: &F) -> Result<CFGInfo, RegAllocError> {
         let postorder =
             postorder::calculate(f.blocks(), f.entry_block(), |block| f.block_succs(block));
         let domtree = domtree::calculate(
@@ -74,20 +74,16 @@ impl CFGInfo {
 
             if f.block_preds(block).len() > 1 {
                 for (i, &pred) in f.block_preds(block).iter().enumerate() {
-                    // Assert critical edge condition.
-                    assert_eq!(
-                        f.block_succs(pred).len(),
-                        1,
-                        "Edge {} -> {} is critical",
-                        pred.index(),
-                        block.index(),
-                    );
+                    // Check critical edge condition.
+                    if f.block_succs(pred).len() > 1 {
+                        return Err(RegAllocError::CritEdge(pred, block));
+                    }
                     pred_pos[pred.index()] = i;
                 }
             }
         }
 
-        CFGInfo {
+        Ok(CFGInfo {
             postorder,
             domtree,
             insn_block,
@@ -96,7 +92,7 @@ impl CFGInfo {
             block_entry,
             block_exit,
             pred_pos,
-        }
+        })
     }
 
     pub fn dominates(&self, a: Block, b: Block) -> bool {
diff --git a/src/ion/mod.rs b/src/ion/mod.rs
index 08ebf32..9d9c3c2 100644
--- a/src/ion/mod.rs
+++ b/src/ion/mod.rs
@@ -4445,7 +4445,7 @@ impl<'a, F: Function> Env<'a, F> {
 }
 
 pub fn run<F: Function>(func: &F, mach_env: &MachineEnv) -> Result<Output, RegAllocError> {
-    let cfginfo = CFGInfo::new(func);
+    let cfginfo = CFGInfo::new(func)?;
 
     let mut env = Env::new(func, mach_env, cfginfo);
     env.init()?;
diff --git a/src/lib.rs b/src/lib.rs
index 3dfc41f..6827939 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -958,6 +958,8 @@ impl Output {
 /// An error that prevents allocation.
 #[derive(Clone, Debug)]
 pub enum RegAllocError {
+    /// Critical edge is not split between given blocks.
+    CritEdge(Block, Block),
     /// Invalid SSA for given vreg at given inst: multiple defs or
     /// illegal use. `inst` may be `Inst::invalid()` if this concerns
     /// a block param.