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How to control GCC builtin functions optimization

(pls CC me when replying because I am not subscriber)

I met an interesting phenomenon when looking into linux kernel
compilation, it can be simply summarized as following: in
arch/x86/boot/compressed, memcpy is defined as __builtin_memcpy, while
also implemented as a function. But when using memcpy, in some case GCC
optimize it to inline code, in other case GCC just emit a call to
self-defined memcpy function. This can be confirmed according to the
symbol table via `nm bluh.o`.

The compiling flags is, for example:
cmd_arch/x86/boot/compressed/pgtable_64.o := gcc
-Wp,-MD,arch/x86/boot/compressed/.pgtable_64.o.d  -nostdinc -isystem
/usr/lib/gcc/x86_64-redhat-linux/8/include -I./arch/x86/include
-I./arch/x86/include/gene    rated  -I./include
-I./arch/x86/include/uapi -I./arch/x86/include/generated/uapi
-I./include/uapi -I./include/generated/uapi -include
./include/linux/kconfig.h -include ./include/linux/compiler_types.h
-D__KERNEL__ -DCONFIG_CC_STACKPROTECTOR -m64 -O2 -fno-strict-aliasing
-fPIE -DDISABLE_BRANCH_PROFILING -mcmodel=small -mno-mmx -mno-sse
-ffreestanding -fno-stack-protector    -DKBUILD_BASENAME='"pgtable_64"'
-DKBUILD_MODNAME='"pgtable_64"' -c -o
arch/x86/boot/compressed/pgtable_64.o arch/x86/boot/compressed/pgtable_64.c

Now the questions is: from code-reading, it is kind of non-intuitive, is
there any explicit way to control the optimization behavior accurately?
Cao jin

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