Why sys-devel/gcc-4.5.4 filter -Os, -O1, -O0, --param CFLAGS

[peat.psuwit]

I use those flags for workaround from gcc being killed. (My system is Samsung Galaxy Mini, ARMv6 with 280MB of ram but Android use most of it.) This is my CFLAGS set in /etc/make.conf :
CFALGS = "--param gcc-min-expand=0 --param gcc-min-heapsize=8192 -march=armv6j -mtune=arm1136j-s -mfpu=vfp -mfloat-abi=softfp -Os"
(BTW, "emerge --info" also give me this result)

But when I decide to compile gcc (Actual command : "emerge --update --newuse -v gcc"), it tell me this :
CFLAGS = "-march=armv6j -mtune=arm1136j-s -mfpu=vfp -mfloat-abi=softfp -O2"
(I also replace -Os with -O1 and -O0 but result is the same.)

I can't compile gcc because gcc get killed in 2nd stage and I can't update other package because this will be part of dependency tree, in early position.
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I'm student from Thailand.

[Genone]

Some ebuilds (esp. for toolchain packages) will filter all but a few known flags as unexpected flags often cause trouble with them. Unfortunately I don't see an easy way to override this for your case as the toolchain packages have most of their code deeply nested in eclasses.

[peat.psuwit]

Thanks to Genone for the information.

But I think filtering the lower optimisation flag to higher one sound strange and doesn't make sense for me(ex. Filter -O1 to -O2). Why?
_________________
I'm student from Thailand.