gh-98831: Support conditional effects; use for LOAD_ATTR by gvanrossum · Pull Request #101333 · python/cpython
We can now write
inst(OP, (foo if (oparg & 1) -- bar if (oparg & 2)) {
...
}
which pops foo off the stack if oparg & 1, otherwise setting it to NULL, and pushes bar onto the stack if oparg & 2 (otherwise ignoring it).
This syntax cannot be combined with an array size on the same effect, but it can be combined with a type (untested). In addition, it is incompatible with an array size closer to the stack top, e.g. foo if (oparg&1), bar[oparg>>1] is not supported (I don't think it's used, and generating code for it would be slightly awkward, but could be done if needed).
The implementation switches to using POP() and PUSH() (from PEEK() and POKE()) for those variables that are either conditional or whose position relative to the bottom (!) of the stack depends on a condition. (See the added test for details.)
To demonstrate this, I converted LOAD_ATTR (but not its specializations).