Bug report

Bug description:

fails: ./configure CFLAGS=-O0 --enable-optimizations --with-lto
succeeds: ./configure CFLAGS=-O1 --enable-optimizations --with-lto

It fails when it runs the tests to build the profile. The problematic test is TestLRUC.test_lru_recursion in Lib/test/test_functools.py. It runs out of stack at well below the expected limit. Py_C_RECURSION_LIMIT is set to 10000. The test runs a simple recursive fibonacci function starting at 2857 (limit*2//7). It keeps crashing with smaller values, down to about 450. I'm not sure why the combination of PGO and -O0 special in this regard. (FYI, my build toolchain is probably a bit outdated, but I'm not sure that's a significant factor.)

I can think of several solutions here:

• "don't do that!": document that -O0 should not be combined with --enable-optimizations (docs, maybe devguide)
• print a warning in configure with that combination
• lower Py_C_RECURSION_LIMIT with that combination

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-128042: Update docs to not use CFLAGS=-O0 with --enable-optimizations #137651