How to control GCC builtin functions optimization

[Jakub Jelinek]

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