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
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3 changed files with 26 additions and 10 deletions
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@ -630,16 +630,34 @@ impl Compiler<'_> {
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self.patch_jump(final_jump);
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self.patch_jump(final_jump);
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}
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}
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/// Compile `assert` expressions using jumping instructions in the VM.
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///
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/// ```notrust
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/// ┌─────────────────────┐
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/// │ 0 [ conditional ] │
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/// │ 1 JUMP_IF_FALSE →┼─┐
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/// │ 2 [ main body ] │ │ Jump to else body if
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/// ┌┼─3─← JUMP │ │ condition is false.
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/// Jump over else body ││ 4 OP_ASSERT_FAIL ←┼─┘
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/// if condition is true.└┼─5─→ ... │
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/// └─────────────────────┘
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/// ```
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fn compile_assert(&mut self, slot: LocalIdx, node: &ast::Assert) {
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fn compile_assert(&mut self, slot: LocalIdx, node: &ast::Assert) {
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// Compile the assertion condition to leave its value on the stack.
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// Compile the assertion condition to leave its value on the stack.
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self.compile(slot, &node.condition().unwrap());
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self.compile(slot, &node.condition().unwrap());
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self.emit_force(&node.condition().unwrap());
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self.emit_force(&node.condition().unwrap());
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self.push_op(OpCode::OpAssert, &node.condition().unwrap());
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let then_idx = self.push_op(OpCode::OpJumpIfFalse(JumpOffset(0)), node);
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// The runtime will abort evaluation at this point if the
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self.push_op(OpCode::OpPop, node);
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// assertion failed, if not the body simply continues on like
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// normal.
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self.compile(slot, &node.body().unwrap());
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self.compile(slot, &node.body().unwrap());
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let else_idx = self.push_op(OpCode::OpJump(JumpOffset(0)), node);
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self.patch_jump(then_idx);
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self.push_op(OpCode::OpPop, node);
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self.push_op(OpCode::OpAssertFail, &node.condition().unwrap());
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self.patch_jump(else_idx);
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}
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}
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/// Compile conditional expressions using jumping instructions in the VM.
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/// Compile conditional expressions using jumping instructions in the VM.
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@ -121,8 +121,8 @@ pub enum OpCode {
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/// Close scopes while leaving their expression value around.
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/// Close scopes while leaving their expression value around.
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OpCloseScope(Count), // number of locals to pop
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OpCloseScope(Count), // number of locals to pop
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/// Asserts stack top is a boolean, and true.
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/// Return an error indicating that an `assert` failed
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OpAssert,
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OpAssertFail,
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// Lambdas & closures
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// Lambdas & closures
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OpCall,
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OpCall,
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@ -561,10 +561,8 @@ impl<'o> VM<'o> {
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}
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}
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}
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}
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OpCode::OpAssert => {
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OpCode::OpAssertFail => {
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if !fallible!(self, self.pop().as_bool()) {
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return Err(self.error(ErrorKind::AssertionFailed));
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return Err(self.error(ErrorKind::AssertionFailed));
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}
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}
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}
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OpCode::OpCall => {
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OpCode::OpCall => {
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