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Re: How to control GCC builtin functions optimization
- From: Jakub Jelinek <jakub at redhat dot com>
- To: Cao jin <caoj dot fnst at cn dot fujitsu dot com>
- Cc: gcc-help at gcc dot gnu dot org
- Date: Fri, 11 Jan 2019 10:12:01 +0100
- Subject: Re: How to control GCC builtin functions optimization
- References: <38b466d6-f905-0fb8-e9aa-04c7aac5e3d3@cn.fujitsu.com>
- Reply-to: Jakub Jelinek <jakub at redhat dot com>
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