git: Force use -O2 when lto enabled

[Ra2-IFV]

Git doen't support lto with gcc flag -O3 and -Ofast. This is discussed in #24366.
If CONFIG_USE_LTO=y, filter -O% from CPPFLAGS and set to -O2.

Maintainer: @krant @neheb
Compile tested: aarch64 f637cf5ef73bbecaf6183e5c416e3043a5f1e202
Run tested: aarch64 f637cf5ef73bbecaf6183e5c416e3043a5f1e202 looks good

Description:

[Ra2-IFV]

🤔

[krant]

That would switch git to -O2 even for those who have TARGET_OPTIMIZATION=-Os which is the default.
Another issue - it overwrites TARGET_CPPFLAGS which could contain important bits set by build system, like here.
Overall, you put filtered-out CFLAGS into CPPFLAGS and then pass them both to git's make, full gcc cmdline must be a mess.

[Ra2-IFV]

Yeah that't not expected behavior... Closing but keep it here in case someone has the same problem.

[Ra2-IFV]

@krant Sorry for the ping, but I have some progress on this...

ifeq ($(CONFIG_USE_LTO),y)
	ifneq (,$(filter -O3 -Ofast,$(TARGET_CFLAGS)))
		TARGET_CFLAGS := $(filter-out -O%,$(TARGET_CFLAGS)) -O2
	endif
        ifneq (,$(filter -O3 -Ofast,$(TARGET_CXXFLAGS)))
		TARGET_CXXFLAGS := $(filter-out -O%,$(TARGET_CXXFLAGS)) -O2
	endif
endif

How about this one? At least it seems won't break the flags...
Or we can assume CFLAGS and CXXFLAGS have the same optimization levels (at least in most cases it does) and remove one if statements...
I was just back from learning Makefile syntax and the difference between CFLAGS, CXXFLAGS and CPPFLAGS, and, for best practices, never put flags that aren't for the C pre-processor, if you are trying to apply a flag for both C and CPP files...

[Ra2-IFV]

New update on this:
This is a reproducible bug, reported in this mailing list https://lore.kernel.org/git/CAHfWF5mjquES-nocQaK+CAEsqWgdy-_OYdGtN82heYs0eJP3eQ@mail.gmail.com/T/#t
And I was wrong, it's caused by fortify headers.