PGO: compiler warnings for source files with no profile data

[erlend-aasland]

PGO builds do not generated profile data for all source files. This means that for a PGO build, you may end up with compiler warnings for source files with no profile data. Do we want to silence these warnings? If we do, it may be harder to spot if PGO builds fail to include profile data correctly.

Spotted while working on #117752.

Linked PRs

• gh-117804: Autoconf: silence some clang warnings for PGO builds #117796
• gh-117804: Document how to suppress PGO unprofiled source warnings #117859
• [3.12] gh-117804: Document how to suppress PGO unprofiled source warnings (GH-117859) #117912

[erlend-aasland]

Example warning:

gcc -bundle -undefined dynamic_lookup      Modules/syslogmodule.o   -o Modules/syslog.cpython-313-darwin.so
gcc  -fno-strict-overflow -Wsign-compare -Wunreachable-code -DNDEBUG -g -O3 -Wall    -std=c11 -Wextra -Wno-unused-parameter -Wno-missing-field-initializers -Wstrict-prototypes -Werror=implicit-function-declaration -fvisibility=hidden -fprofile-instr-use=/Users/erlend.aasland/src/python/build-main/code.profclangd -I../main/Include/internal -I../main/Include/internal/mimalloc -IObjects -IInclude -IPython -I. -I../main/Include     -c ../main/Modules/termios.c -o Modules/termios.o
warning: no profile data available for file "termios.c" [-Wprofile-instr-unprofiled]

[zooba]

Unless they're actually disruptive (breaking builds), I'd rather leave them there by default.

Seems easy enough to add the suppression to CFLAGS manually, yeah? Should we just doc it?

[zooba]

It's one of these cases where if we suppress them once, we'll forget about coverage entirely, which is worse than simply continuing to not fix it (someone might be motivated to fix it in the future if they see it)

[erlend-aasland]

Seems easy enough to add the suppression to CFLAGS manually, yeah? Should we just doc it?

I would be fine with that.