Hi! Removing gcc@ , as this is not relevant to GCC development. On Fri, Jan 11, 2019 at 11:03:35AM +0800, Cao jin wrote: > (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? memcpy and __builtin_memcpy are the same thing, ditto for other builtins that have a library counterpart. The difference in between them is in 1) __builtin_* doesn't need to be prototyped 2) __builtin_* works even with -fno-builtin-memcpy or -fno-builtin Otherwise, if memcpy acts as a builtin, they do the same thing. You can control how memcpy is expanded through various command line switches, -Os affects it, on x86 e.g. -mstringop-strategy=, -mmemcpy-strategy=, -mtune= etc., on various other architectures other options. Jakub
