diff --git a/cranelift/filetests/regalloc/coalesce.cton b/cranelift/filetests/regalloc/coalesce.cton
index 7624eaef7c..bdddf26ef4 100644
--- a/cranelift/filetests/regalloc/coalesce.cton
+++ b/cranelift/filetests/regalloc/coalesce.cton
@@ -110,3 +110,15 @@ ebb1(v10: i32):
     v11 = iadd_imm v10, 1
     return v11
 }
+
+function %gvn_unremovable_phi(i32) native {
+ebb0(v0: i32):
+    v2 = iconst.i32 0
+    jump ebb2(v2, v0)
+
+ebb2(v3: i32, v4: i32):
+    brnz v3, ebb2(v3, v4)
+    v5 = iconst.i32 1
+    brnz v3, ebb2(v2, v5)
+    return
+}
diff --git a/lib/cretonne/src/regalloc/coalescing.rs b/lib/cretonne/src/regalloc/coalescing.rs
index 509f5a45e7..34a9b8bde0 100644
--- a/lib/cretonne/src/regalloc/coalescing.rs
+++ b/lib/cretonne/src/regalloc/coalescing.rs
@@ -450,13 +450,21 @@ impl<'a> Context<'a> {
                     return Some(a);
                 }
 
-                // The local conflict could be avoided by splitting at this predecessor, so try
-                // that. This split is not necessarily required, but it allows us to make progress.
+                // The local conflict could likely be avoided by splitting at this predecessor, so
+                // try that. This split is not necessarily required, but it allows us to make
+                // progress.
                 let new_val = self.split_pred(pred_inst, pred_ebb, argnum, pred_val);
-                assert!(
-                    self.add_class(new_val).is_ok(),
-                    "Splitting didn't resolve conflict."
-                );
+
+                // If this tiny new live range can't be merged, there is something in the already
+                // merged values that is fundamentally incompatible with `pred_inst`, and we need
+                // to start over after removing that value.
+                // TODO: It is unfortunate that we discover this *after* splitting. It would have
+                // been better if we could detect and isolate `merged` before splitting.
+                if let Err((merged, _)) = self.add_class(new_val) {
+                    dbg!("Splitting didn't help: {} interferes", merged);
+                    // We need to start over, isolating the bad value.
+                    return Some(merged);
+                }
             }
         }