gh-106529: Make FOR_ITER a viable uop

[gvanrossum]

Also clean up a few nits in the code generator, and add back an important Make dependency for ceval.o.

• Issue: Branching design for Tier 2 (uops) interpreter #106529

[gvanrossum]

Hold on, need to remove merge conflicts from commit history.

[gvanrossum]

NOTE: This fix resulted in _SPECIALIZE_UNPACK_SEQUENCE becoming a viable uop. It was missing a TIER_ONE_ONLY marker; I've added it back. (The fix is needed to restore the feature that this function doesn't look inside #if TIER_ONE.)

[gvanrossum]

@brandtbucher Let me know how bad this interferes with the JIT branch. The stuff I put in _FOR_ITER_TIER_TWO probably shouldn't be compiled literally into the template. Maybe I should make part of the code at the deoptimize label into a macro? (But the setting of frame->instr_ptr must differ -- we can't use target since that's also used by error exits, since GH-112065.)

[gvanrossum]

Benchmark results:
https://github.com/faster-cpython/benchmarking-public/tree/main/results/bm-20231115-3.13.0a1+-5c5d8bd-PYTHON_UOPS

[brandtbucher]

@brandtbucher Let me know how bad this interferes with the JIT branch. The stuff I put in _FOR_ITER_TIER_TWO probably shouldn't be compiled literally into the template. Maybe I should make part of the code at the deoptimize label into a macro? (But the setting of frame->instr_ptr must differ -- we can't use target since that's also used by error exits, since GH-112065.)

So, this doesn't actually look too bad. I think with a tiny bit of reworking, this could be merged into the JIT branch with no pain:

• Move the line that sets frame->instr_ptr from the exit_trace label into the _EXIT_TRACE instruction, using a macro (like CURRENT_TARGET() or something) to access the target member.
• Remove the stuff from _FOR_ITER_TIER_TWO that updates the stats, saves the stack pointer, and decrefs the executor, and just goto exit_trace; instead after updating frame->instr_ptr.

[gvanrossum]

Okay, I think I managed to do that. I can now merge into the justin branch with only a single trivial merge conflict (we both added something to the end of ceval_macros.h). Next up of course, how would I redefine CURRENT_TARGET() in templace.c?

[brandtbucher]

#define CURRENT_TARGET() (target)

[gvanrossum]

#define CURRENT_TARGET() (target)

Yup, and I also needed to move the exit_trace: label one line down (to avoid setting frame->instr_ptr twice).

It seems to work, but unsure how to prove it (it compiles and passes tests, but it would too if the JIT was never invoked).

Anyway, we should probably wait until we've decided that making Tier 2 5% slower is a good idea.

[markshannon]

There seem to be a few unrelated changes in this PR.
Could you make another PR for the increase to the uop buffer size, extra debugging and change to UNPACK_SEQUENCE?

[markshannon]

No hacks, please 🙂

The code should look like this:

if (next == NULL) {
    if (_PyErr_Occurred(tstate)) {
        if (!_PyErr_ExceptionMatches(tstate, PyExc_StopIteration)) {
            GOTO_ERROR(error);
        }
        _PyErr_Clear(tstate);
    }
    /* iterator ended normally */
    Py_DECREF(iter);
    STACK_SHRINK(1);
    DEOPT_IF(true);
}

The trace generator can adjust the target, so it points after the END_FOR.

[gvanrossum]

Could you make another PR for the increase to the uop buffer size, extra debugging and change to UNPACK_SEQUENCE?

Sure, see gh-112214. I've merged that into main, since the tests pass, next I'll merge it into this PR and do the other thing you requested.

[gvanrossum]

Okay, here's the new version. @markshannon Want to review one more time?

[gvanrossum]

I've applied Brandt's suggestions.

I've also split off some more cleanups and debug improvements into gh-112218 and gh-112220. Once those land I'll merge main once more and this should shrink a bit more (the cases_generator tweaks will be separated out).

[gvanrossum]

Benchmark says 4% slower (with Tier 2):
https://github.com/faster-cpython/benchmarking-public/tree/main/results/bm-20231117-3.13.0a1+-14aea56-PYTHON_UOPS

• For spectral_norm, one out of five traces is still too long (that must be one hell of a trace :-)
• Also, spectral_norm no longer uses any unsupported opcodes (nice!)
• Pystats diff show about 8% more traces executed, 8% more uops executed
• Overall, FOR_ITER_GEN is now by far the most-used unsupported opcode (74k, vs. 9k for the next most-used, CALL)

[gvanrossum]

FWIW, I have a branch on top of this that makes the uop interpreter 3% faster. The approach is to let the code generator generate code that extracts oparg and operand from the instruction stream as needed (i.e., only for opcodes that use them) rather than the interpreter loop preemptively extracting them for every instruction. I'm not completely happy with it, and it probably would make life a little harder for the JIT template, which doesn't like directly referencing next_uop -- this should probably wait until we're ready to generate the JIT template code separately from the Tier 2 interpreter code.

[brandtbucher]

Honestly, as long as it’s done in macros, it shouldn’t be too bad. Macros are a great escape hatch in the short term before we start generating bespoke “JIT cases”.

[gvanrossum]

Honestly, as long as it’s done in macros, it shouldn’t be too bad. Macros are a great escape hatch in the short term before we start generating bespoke “JIT cases”.

So it could generate something like

case _FOO_UOP: {
    oparg = CURRENT_OPARG();
    operand = CURRENT_OPERAND();
    ...
}

where the default definitions for those (in ceval_macros.h) would be

#define CURRENT_OPARG() next_uop[-1].oparg
#define CURRENT_OPERAND() next_uop[-1].operand

[brandtbucher]

Yup!