fix(tvix/eval): fix branching on catchable defaults (b/343) r/7178

This commit adds Opcode::OpJumpIfCatchable, which can be inserted
ahead of most VM operations which expect a boolean on the stack, in
order to handle catchables in branching position properly.

Other than remembering to patch the jump, no other changes should be
required.

This commit also fixes b/343 by emitting this new opcode when
compiling if-then-else.  There are probably other places where we
need to do the same thing.

Change-Id: I48de3010014c0bbeba15d34fc0d4800e0bb5a1ef
Reviewed-on: https://cl.tvl.fyi/c/depot/+/10288
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>
Autosubmit: Adam Joseph <adam@westernsemico.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/tvix/eval/src/vm/mod.rs b/tvix/eval/src/vm/mod.rs
index 6128e3a601a0..642b6317caee 100644
--- a/tvix/eval/src/vm/mod.rs
+++ b/tvix/eval/src/vm/mod.rs
@@ -562,6 +562,13 @@ impl<'o> VM<'o> {
                     }
                 }
 
+                OpCode::OpJumpIfCatchable(JumpOffset(offset)) => {
+                    debug_assert!(offset != 0);
+                    if self.stack_peek(0).is_catchable() {
+                        frame.ip += offset;
+                    }
+                }
+
                 OpCode::OpJumpIfNoFinaliseRequest(JumpOffset(offset)) => {
                     debug_assert!(offset != 0);
                     match self.stack_peek(0) {