diff --git a/src/ir/func.rs b/src/ir/func.rs
index 29ddbe1..826173a 100644
--- a/src/ir/func.rs
+++ b/src/ir/func.rs
@@ -135,6 +135,7 @@ impl FunctionBody {
         let cfg = crate::cfg::CFGInfo::new(self);
         crate::passes::basic_opt::gvn(self, &cfg);
         crate::passes::resolve_aliases::run(self);
+        crate::passes::ssa::run(self, &cfg);
     }
 
     pub fn convert_to_max_ssa(&mut self) {
diff --git a/src/passes.rs b/src/passes.rs
index 93de4ad..e0b36b0 100644
--- a/src/passes.rs
+++ b/src/passes.rs
@@ -6,3 +6,4 @@ pub mod empty_blocks;
 pub mod maxssa;
 pub mod resolve_aliases;
 pub mod trace;
+pub mod ssa;
diff --git a/src/passes/ssa.rs b/src/passes/ssa.rs
new file mode 100644
index 0000000..283c0b9
--- /dev/null
+++ b/src/passes/ssa.rs
@@ -0,0 +1,56 @@
+//! SSA validation.
+
+use crate::cfg::CFGInfo;
+use crate::entity::*;
+use crate::ir::*;
+
+struct DefBlocks {
+    def_block: PerEntity<Value, Block>,
+}
+impl DefBlocks {
+    fn compute(body: &FunctionBody) -> Self {
+        let mut def_block = PerEntity::default();
+        for (block, data) in body.blocks.entries() {
+            for &(_, param) in &data.params {
+                def_block[param] = block;
+            }
+            for &inst in &data.insts {
+                def_block[inst] = block;
+            }
+        }
+        DefBlocks { def_block }
+    }
+}
+
+pub fn run(body: &FunctionBody, cfg: &CFGInfo) {
+    let def_blocks = DefBlocks::compute(body);
+
+    for (block, data) in body.blocks.entries() {
+        let validate = |value| {
+            let value = body.resolve_alias(value);
+            let def_block = def_blocks.def_block[value];
+            assert!(cfg.dominates(def_block, block));
+        };
+
+        for &inst in &data.insts {
+            match &body.values[inst] {
+                &ValueDef::Operator(_, ref args, _) => {
+                    for &arg in args {
+                        validate(arg);
+                    }
+                }
+                &ValueDef::PickOutput(val, _, _) => {
+                    validate(val);
+                }
+                &ValueDef::Trace(_, ref args) => {
+                    for &arg in args {
+                        validate(arg);
+                    }
+                }
+                &ValueDef::Alias(..) => {}
+                &ValueDef::None | &ValueDef::Placeholder(_) | &ValueDef::BlockParam(..) => {}
+            }
+        }
+        data.terminator.visit_uses(|u| validate(u));
+    }
+}