Re: [PATCH 02/22] kbuild: add support for Clang LTO

[Nathan Chancellor]

Hi Sami,

On Wed, Jun 24, 2020 at 01:31:40PM -0700, 'Sami Tolvanen' via Clang Built Linux wrote:
> This change adds build system support for Clang's Link Time
> Optimization (LTO). With -flto, instead of ELF object files, Clang
> produces LLVM bitcode, which is compiled into native code at link
> time, allowing the final binary to be optimized globally. For more
> details, see:
> 
>   https://llvm.org/docs/LinkTimeOptimization.html
> 
> The Kconfig option CONFIG_LTO_CLANG is implemented as a choice,
> which defaults to LTO being disabled. To use LTO, the architecture
> must select ARCH_SUPPORTS_LTO_CLANG and support:
> 
>   - compiling with Clang,
>   - compiling inline assembly with Clang's integrated assembler,
>   - and linking with LLD.
> 
> While using full LTO results in the best runtime performance, the
> compilation is not scalable in time or memory. CONFIG_THINLTO
> enables ThinLTO, which allows parallel optimization and faster
> incremental builds. ThinLTO is used by default if the architecture
> also selects ARCH_SUPPORTS_THINLTO:
> 
>   https://clang.llvm.org/docs/ThinLTO.html
> 
> To enable LTO, LLVM tools must be used to handle bitcode files. The
> easiest way is to pass the LLVM=1 option to make:
> 
>   $ make LLVM=1 defconfig
>   $ scripts/config -e LTO_CLANG
>   $ make LLVM=1
> 
> Alternatively, at least the following LLVM tools must be used:
> 
>   CC=clang LD=ld.lld AR=llvm-ar NM=llvm-nm
> 
> To prepare for LTO support with other compilers, common parts are
> gated behind the CONFIG_LTO option, and LTO can be disabled for
> specific files by filtering out CC_FLAGS_LTO.
> 
> Note that support for DYNAMIC_FTRACE and MODVERSIONS are added in
> follow-up patches.
> 
> Signed-off-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
> ---
>  Makefile                          | 16 ++++++++
>  arch/Kconfig                      | 66 +++++++++++++++++++++++++++++++
>  include/asm-generic/vmlinux.lds.h | 11 ++++--
>  scripts/Makefile.build            |  9 ++++-
>  scripts/Makefile.modfinal         |  9 ++++-
>  scripts/Makefile.modpost          | 24 ++++++++++-
>  scripts/link-vmlinux.sh           | 32 +++++++++++----
>  7 files changed, 151 insertions(+), 16 deletions(-)
> 
> diff --git a/Makefile b/Makefile
> index ac2c61c37a73..0c7fe6fb2143 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -886,6 +886,22 @@ KBUILD_CFLAGS	+= $(CC_FLAGS_SCS)
>  export CC_FLAGS_SCS
>  endif
>  
> +ifdef CONFIG_LTO_CLANG
> +ifdef CONFIG_THINLTO
> +CC_FLAGS_LTO_CLANG := -flto=thin $(call cc-option, -fsplit-lto-unit)
> +KBUILD_LDFLAGS	+= --thinlto-cache-dir=.thinlto-cache
> +else
> +CC_FLAGS_LTO_CLANG := -flto
> +endif
> +CC_FLAGS_LTO_CLANG += -fvisibility=default
> +endif
> +
> +ifdef CONFIG_LTO
> +CC_FLAGS_LTO	:= $(CC_FLAGS_LTO_CLANG)
> +KBUILD_CFLAGS	+= $(CC_FLAGS_LTO)
> +export CC_FLAGS_LTO
> +endif
> +
>  # arch Makefile may override CC so keep this after arch Makefile is included
>  NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
>  
> diff --git a/arch/Kconfig b/arch/Kconfig
> index 8cc35dc556c7..e00b122293f8 100644
> --- a/arch/Kconfig
> +++ b/arch/Kconfig
> @@ -552,6 +552,72 @@ config SHADOW_CALL_STACK
>  	  reading and writing arbitrary memory may be able to locate them
>  	  and hijack control flow by modifying the stacks.
>  
> +config LTO
> +	bool
> +
> +config ARCH_SUPPORTS_LTO_CLANG
> +	bool
> +	help
> +	  An architecture should select this option if it supports:
> +	  - compiling with Clang,
> +	  - compiling inline assembly with Clang's integrated assembler,
> +	  - and linking with LLD.
> +
> +config ARCH_SUPPORTS_THINLTO
> +	bool
> +	help
> +	  An architecture should select this option if it supports Clang's
> +	  ThinLTO.
> +
> +config THINLTO
> +	bool "Clang ThinLTO"
> +	depends on LTO_CLANG && ARCH_SUPPORTS_THINLTO
> +	default y
> +	help
> +	  This option enables Clang's ThinLTO, which allows for parallel
> +	  optimization and faster incremental compiles. More information
> +	  can be found from Clang's documentation:
> +
> +	    https://clang.llvm.org/docs/ThinLTO.html
> +
> +choice
> +	prompt "Link Time Optimization (LTO)"
> +	default LTO_NONE
> +	help
> +	  This option enables Link Time Optimization (LTO), which allows the
> +	  compiler to optimize binaries globally.
> +
> +	  If unsure, select LTO_NONE.
> +
> +config LTO_NONE
> +	bool "None"
> +
> +config LTO_CLANG
> +	bool "Clang's Link Time Optimization (EXPERIMENTAL)"
> +	depends on CC_IS_CLANG && CLANG_VERSION >= 110000 && LD_IS_LLD

I am curious, what is the reason for gating this at clang 11.0.0?

Presumably this? https://github.com/ClangBuiltLinux/linux/issues/510

It might be nice to notate this so that we do not have to wonder :)

Cheers,
Nathan