The compiler may optimise away globals with debug offsets

[pablogsal]

Bug report

Bug description:

When compiling with LTO and other aggressive optimization levels, is possible that the compiler will optimise away globals containing debug offsets. This is currently happening in https://buildbot.python.org/#/builders/125/builds/6895 for the new AsyncioDebug section, making the test fail because the information is not burned in the binary.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-129223: Do not allow the compiler to optimise away symbols for debug sections #129225
• gh-129223: Raise KeyError in search_map_for_section() if not found #129262

[picnixz]

(Tagging it as performance even though it's probably a tiny tiny regression)

[pablogsal]

This is the fix of #124640 (comment)

What regression are you referring to?

[picnixz]

IIUC, it's just that the compiler is not adding some additional information so, because of this additional information, the final binary changes a bit (perhaps the profile itself is changed). Hence the "tiny tiny" (ok, the wording was probably a poor wording). Sorry if I misunderstood what happened though!

[pablogsal]

IIUC, it's just that the compiler is not adding some additional information so, because of this additional information, the final binary changes a bit (perhaps the profile itself is changed). Hence the "tiny tiny" (ok, the wording was probably a poor wording). Sorry if I misunderstood what happened though!

Hummm, seems that I didn't explain myself correctly. Apologies!

So what's happening here is that we rely on a global variable compiled in a very specific way to be present in the asyncio module shared object. Unfortunately as nothing else is using the symbol that means that the compiler when using LTO is deciding that as nobody uses the symbol then is safe to remove. This means that the section is never burned into the final shared object and we cannot do the external inspection.

The fix doesn't have performance implications: is basically convincing the compiler to leave the global alone even if is not used anywhere apparently :)

[picnixz]

Oh that's not even an asyncio issue!