PGO/LTO Build Fails With -O0

[ericsnowcurrently]

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 (FYI, my build toolchain is probably a bit outdated, but I'm not sure that's a significant factor.)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
• gh-128042: configure warns when CFLAGS=-O0 and --enable-optimizations #137975

[taegyunkim]

It doesn't even build on my ubuntu 22.04 with gcc 11.4.0 and libc 2.35 when CFLAGS=-O0 is set.

Last few lines from when I tried ./configure CFLAGS=-O0 --enable-optimizations --with-lto and make -s -j $(nproc)

/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c19f0): undefined reference to `.L343'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c19f8): undefined reference to `.L344'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a00): undefined reference to `.L345'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a08): undefined reference to `.L346'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a10): undefined reference to `.L347'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a18): undefined reference to `.L348'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a20): undefined reference to `.L349'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a28): undefined reference to `.L350'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a30): undefined reference to `.L351'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a38): undefined reference to `.L352'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a40): undefined reference to `.L353'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a48): undefined reference to `.L354'
/usr/bin/ld: /tmp/cc2jZgIm.ltrans1.ltrans.o:(.data.rel.local+0x1c1a50): undefined reference to `.L355'
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:1927: Programs/_freeze_module] Error 1
make[1]: *** [Makefile:1007: profile-gen-stamp] Error 2
make: *** [Makefile:1019: profile-run-stamp] Error 2

I created PRs to udpate docs as linked in the PR, and to update devguide.

[AA-Turner]

Are CFLAGS inherited from some other process here? It seems straightforward not to both enable and disable optimisations at the same time, doing so will produce unexpected results.