gh-115999: Add free-threaded specialization for SEND.

[nascheme]

No additional thread safety changes are required. Note that sending to a generator that is shared between threads is not safe in the free-threaded build.

• Issue: Make the specializing interpreter thread-safe in --disable-gil builds #115999

[ZeroIntensity]

Related: GH-126371 makes native generators thread safe for free-threading. Once this gets merged, I can finish that one up and add any needed locking for the specialization instruction.

[nascheme]

I didn't test too much but applying GH-126371 on top of this PR makes the TestRaces.test_racing_send_gentest run without generating TSAN warnings. Without it, there are warnings and CPython sometimes segfaults. So, perhaps no additional changes will be needed to the specialized opcodes to make it safe.

[corona10]

I try to organize unitest codes for specialized opcodes with more understandable function name :)

See:

cpython/Lib/test/test_opcache.py

Lines 1400 to 1418 in c112de1

	defbinary_subscr_dict():
	for_inrange(100):
	a={1: 2, 2: 3}
	self.assertEqual(a[1], 2)
	self.assertEqual(a[2], 3)
	binary_subscr_dict()
	self.assert_specialized(binary_subscr_dict, "BINARY_SUBSCR_DICT")
	self.assert_no_opcode(binary_subscr_dict, "BINARY_SUBSCR")
	defbinary_subscr_str_int():
	for_inrange(100):
	a="foobar"
	foridx, expectedinenumerate(a):
	self.assertEqual(a[idx], expected)
	binary_subscr_str_int()
	self.assert_specialized(binary_subscr_str_int, "BINARY_SUBSCR_STR_INT")
	self.assert_no_opcode(binary_subscr_str_int, "BINARY_SUBSCR")

[mpage]

LGTM. I left a small suggestion inline to make the implementation of _Py_Specialize_Send a little more concise, but feel free to ignore it if you prefer the current code structure.

I've kicked off benchmark runs for both the default and free-threaded builds for this change. I'll post the results once they're available, likely tomorrow morning.

[mpage]

Performance looks neutral on both the default and free-threaded builds. I think it's still worth merging this to stay consistent with the default build.