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 <tazjin@tvl.su>
Tested-by: BuildkiteCI
This commit is contained in:
Griffin Smith 2022-10-10 12:56:11 -04:00 committed by grfn
parent 1ab252470a
commit 899fbdbddb
3 changed files with 26 additions and 10 deletions

View file

@ -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.

View file

@ -121,8 +121,8 @@ pub enum OpCode {
/// Close scopes while leaving their expression value around.
OpCloseScope(Count), // number of locals to pop
/// Asserts stack top is a boolean, and true.
OpAssert,
/// Return an error indicating that an `assert` failed
OpAssertFail,
// Lambdas & closures
OpCall,

View file

@ -561,10 +561,8 @@ impl<'o> VM<'o> {
}
}
OpCode::OpAssert => {
if !fallible!(self, self.pop().as_bool()) {
return Err(self.error(ErrorKind::AssertionFailed));
}
OpCode::OpAssertFail => {
return Err(self.error(ErrorKind::AssertionFailed));
}
OpCode::OpCall => {