Cranelift DFG: make inst clone deep-clone varargs lists. (#5727)

When investigating #5716, I found that rematerialization of a `call`, in
addition to blowing up for other reasons, caused aliasing of the varargs
list (the `EntityList` in the `ListPool`), such that editing the args of
the second copy of the call instruction inadvertently updated the first
as well.

This PR modifies `DataFlowGraph::clone_inst` so that it always clones
the varargs list if present. This shouldn't have any functional impact
on Cranelift today, because we don't rematerialize any instructions with
varargs; but it's important to get it right to avoid a bug later!

cranelift/codegen/src/ir/dfg.rs

@@ -907,6 +907,10 @@ impl DataFlowGraph {
     pub fn clone_inst(&mut self, inst: Inst) -> Inst {
         // First, add a clone of the InstructionData.
         let inst_data = self.insts[inst].clone();
+        // If the `inst_data` has a reference to a ValueList, clone it
+        // as well, because we can't share these (otherwise mutating
+        // one would affect the other).
+        let inst_data = inst_data.deep_clone(&mut self.value_lists);
         let new_inst = self.make_inst(inst_data);
         // Get the controlling type variable.
         let ctrl_typevar = self.ctrl_typevar(inst);
@@ -1615,4 +1619,25 @@ mod tests {
         assert_eq!(pos.func.dfg.resolve_aliases(c2), c2);
         assert_eq!(pos.func.dfg.resolve_aliases(c), c2);
     }
+
+    #[test]
+    fn cloning() {
+        use crate::ir::InstBuilder;
+
+        let mut func = Function::new();
+        let mut sig = Signature::new(crate::isa::CallConv::SystemV);
+        sig.params.push(ir::AbiParam::new(types::I32));
+        let sig = func.import_signature(sig);
+        let block0 = func.dfg.make_block();
+        let mut pos = FuncCursor::new(&mut func);
+        pos.insert_block(block0);
+        let v1 = pos.ins().iconst(types::I32, 0);
+        let v2 = pos.ins().iconst(types::I32, 1);
+        let call_inst = pos.ins().call_indirect(sig, v1, &[v1]);
+        let func = pos.func;
+
+        let call_inst_dup = func.dfg.clone_inst(call_inst);
+        func.dfg.inst_args_mut(call_inst)[0] = v2;
+        assert_eq!(v1, func.dfg.inst_args(call_inst_dup)[0]);
+    }
 }

cranelift/codegen/src/ir/instructions.rs

@@ -123,6 +123,15 @@ impl BlockCall {
     pub fn display<'a>(&self, pool: &'a ValueListPool) -> DisplayBlockCall<'a> {
         DisplayBlockCall { block: *self, pool }
     }
+
+    /// Deep-clone the underlying list in the same pool. The returned
+    /// list will have identical contents but changes to this list
+    /// will not change its contents or vice-versa.
+    pub fn deep_clone(&self, pool: &mut ValueListPool) -> Self {
+        Self {
+            values: self.values.deep_clone(pool),
+        }
+    }
 }
 
 /// Wrapper for the context needed to display a [BlockCall] value.