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, 6 insertions, 0 deletions

diff --git a/tvix/eval/src/opcode.rs b/tvix/eval/src/opcode.rs
index 762cff7b5282..467798177550 100644
--- a/tvix/eval/src/opcode.rs
+++ b/tvix/eval/src/opcode.rs
@@ -130,6 +130,12 @@ pub enum OpCode {
     /// of the stack is `false`.
     OpJumpIfFalse(JumpOffset),
 
+    /// Pop one stack item and jump forward in the bytecode
+    /// specified by the number of instructions in its usize
+    /// operand, *if* the value at the top of the stack is a
+    /// Value::Catchable.
+    OpJumpIfCatchable(JumpOffset),
+
     /// Jump forward in the bytecode specified by the number of
     /// instructions in its usize operand, *if* the value at the top
     /// of the stack is the internal value representing a missing