Hi Jakub, On 1/11/19 5:12 PM, Jakub Jelinek wrote: > 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. > Thanks very much! I tried the -mmemcpy-strategy=byte_loop, it seems worked! The .o used to emit call to self-defined memcpy() now has no memcpy entry in the nm output! -- Sincerely, Cao jin
