From 899fbdbddb93050f26236ff7c72e7ae4704d497b Mon Sep 17 00:00:00 2001 From: Griffin Smith Date: Mon, 10 Oct 2022 12:56:11 -0400 Subject: refactor(tvix/eval): Compile OpAssert using conditional jumps In order to behave nicely with tryEval, asserts need to leave the instruction pointer in a reasonable place even if they fail - whereas with the previous implementation catching a failed assert would still end up running the op for the *body* of the assert. With this change, we compile asserts much more like an `if` expression with conditional jumps rather than having an OpAssert op. Change-Id: I1b266c3be90185c84000da6b1995ac3e6fd5471b Reviewed-on: https://cl.tvl.fyi/c/depot/+/6925 Reviewed-by: tazjin Tested-by: BuildkiteCI --- tvix/eval/src/compiler/mod.rs | 26 ++++++++++++++++++++++---- 1 file changed, 22 insertions(+), 4 deletions(-) (limited to 'tvix/eval/src/compiler') diff --git a/tvix/eval/src/compiler/mod.rs b/tvix/eval/src/compiler/mod.rs index 1463bb100eae..941b945a8bb6 100644 --- a/tvix/eval/src/compiler/mod.rs +++ b/tvix/eval/src/compiler/mod.rs @@ -630,16 +630,34 @@ impl Compiler<'_> { self.patch_jump(final_jump); } + /// Compile `assert` expressions using jumping instructions in the VM. + /// + /// ```notrust + /// ┌─────────────────────┐ + /// │ 0 [ conditional ] │ + /// │ 1 JUMP_IF_FALSE →┼─┐ + /// │ 2 [ main body ] │ │ Jump to else body if + /// ┌┼─3─← JUMP │ │ condition is false. + /// Jump over else body ││ 4 OP_ASSERT_FAIL ←┼─┘ + /// if condition is true.└┼─5─→ ... │ + /// └─────────────────────┘ + /// ``` fn compile_assert(&mut self, slot: LocalIdx, node: &ast::Assert) { // Compile the assertion condition to leave its value on the stack. self.compile(slot, &node.condition().unwrap()); self.emit_force(&node.condition().unwrap()); - self.push_op(OpCode::OpAssert, &node.condition().unwrap()); + let then_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node); - // The runtime will abort evaluation at this point if the - // assertion failed, if not the body simply continues on like - // normal. + self.push_op(OpCode::OpPop, node); self.compile(slot, &node.body().unwrap()); + + let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node); + + self.patch_jump(then_idx); + self.push_op(OpCode::OpPop, node); + self.push_op(OpCode::OpAssertFail, &node.condition().unwrap()); + + self.patch_jump(else_idx); } /// Compile conditional expressions using jumping instructions in the VM. -- cgit 1.4.1