GCC LTO: Uncertain handling of -fcommon vs -fno-common during linking

Symptom

When compiling with -flto and -fcommon, GCC often succeeds linking multiple source files that each declare a tentative global variable. Switching to -fno-common triggers duplicate symbol errors during the LTO phase, even though standard linking without LTO honours the flags consistently.

Uncertainty

The GCC manual does not explicitly state whether -fcommon should be treated as a weak symbol during LTO, and empirical tests show that the behavior appears to vary across GCC 11, 12, and 13. This leaves developers unsure whether to depend on -fcommon when building shared libraries or static binaries that use LTO.

Questions

• Is the treatment of -fcommon as weak during LTO a documented GCC design choice, or an implementation quirk?
• Do newer GCC releases (e.g., 13 and beyond) change the default handling of -fcommon in LTO, and if so, how?
• Should developers rely on -fcommon for tentative definitions when using LTO, or is a different strategy recommended?