How can I turn off/disable -O2/-Os optimization?

[Zhihua Che]

Hi, everyone

    I'm debugging linux kernel using kgdb/qemuï but I find that the
execution order didn't math the program order usually.  I guess it was
because the compiling optimization. So I decided to remove -O options
in Makefiles.
    First I removed -Ox in variables HOSTCFLAGS/HOSTCXXFLAGS in
<src_root_dir>/Makefile around line 248 and -Ox in variable
KBUILD_CFLAGS in <src_root_dir>/arch/x86/boot/Makefile around line 55.
But it didn't work, the step execution still didn't follow the program
order. Then I found I missed -Ox in KBUILD_CFLAGS in
<src_root_dir>/Makefile around line 562, so I removed them, but this
time, the compiling failed, complaining
    rcupdate.h:917:2: error: size of unnamed array is negative
   And finally, after grep, I find there are a load of -O2 option are
assigned in a load of Makefiles. So, How could I disable -O
optimization for debugging?

   Thanks for any tips
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